New Insight into the Crystal Chemistry of Uranium and Thorium Borates, Borophosphates and Borate-phosphates

Von der Fakultät für Georessourcen und Materialtechnik der Rheinisch-Westfälischen Technischen Hochschule Aachen

zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften

genehmigte Dissertation vorgelegt von M.Sc.

Yucheng Hao

aus (Heze, CHINA)

Berichter: Prof. Dr. rer. nat. Evgeny V. Alekseev Univ.-Prof. Dr. rer. nat. Dirk Bosbach Univ.-Prof. Dr. rer. nat. Georg Roth

Tag der mündlichen Prüfung: 06. Februar 2018

Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verfügbar © Copyright 2017 Yucheng Hao

II New Insight into the Crystal Chemistry of Uranium and Thorium Borates, Borophosphates and Borate-phosphates

Abstract

This dissertation work is devoted to a systematic study of phase formation, their structures and properties in the AI/AII-U/Th-B-(P)-O system at different conditions. This work resulted in obtaining and characterizing around thirty novel uranium and thorium phases. Among them, several compounds uncovered novel structural topologies and different physicochemical properties. As an example, a lead uranyl borate (LUBO) is the first zeolite- like polyborate material observed in the uranyl oxo-borates system. It possesses a unique boron-oxygen open framework structure with multi-intersection channels. This robust polyborate framework demonstrates high thermal stability up to 690 °C.

3- An introduction of [PO4] ortho-phosphates anions into the above mentioned oxo-borate system leads to the formation of complex novel uranium borophosphates, borate-phosphates and phosphates. Among them, three novel highly porous uranyl borophosphates with unique three dimensional open framework structures have been structurally characterized. One of these phases, namely, Cs3(UO2)3[B(PO4)4]∙(H2O)0.5 has been proven to be a promising ionic exchanger.

In the Th-B-O system, a new polymorphic modification of ThB2O5 has been isolated at ambient pressure. A further investigation of the ThB2O5 phase diagram demonstrates very unusual stability ranges of both polymorphic modifications. It has been shown that the nature of the flux plays a crucial role in the formation of both polymorphic modifications.

The structures of all materials obtained in above mentioned systems have been determined by single crystal X-ray diffraction (SXRD) with a further characterization by powder X-ray diffraction (PXRD), Infrared (IR)/Raman spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and energy dispersive X-ray spectroscopy (EDS). The computational studies of the phase formation in Th-B-O system have been performed using density functional theory (DFT).

The results of the performed study clearly demonstrate that the counter cations play a key role in the structural formation of phases in studied systems. Consequently, the structures of compounds determine their physicochemical properties.

III Neue Erkenntnisse in der Kristallchemie von Uran und Thorium Boraten, Borophosphaten und Borat-phosphaten

Abstract

Ziel dieser Arbeit war es, systematisch das AI/AII-U/Th-B-(P)-O-System im Hinblick auf Phasenbildung, Struktur sowie Eigenschaften zu untersuchen. Hierbei gelang es, 30 neue Uran- und Thorium-Phasen zu entdecken und zu charakterisieren. Einige dieser Phasen zeigen neuartige Strukturtopologien sowie verschiedene physikalisch-chemische Eigenschaften. Ein Beispiel hierfür ist ein Bleiuranylborat (LUBO), welches das erste Zeolith-artige Polyborat-Material im Uranyl-Borat System darstellt. Es besitzt eine einzigartige offene Bor-Sauerstoff Gerüststruktur mit mehrfach kreuzenden Kanälen. Dieses robuste Polyborat-Gerüst zeigt hohe thermische Stabilität bis 690 °C.

3- Die Einführung von [PO4] Orthophosphat Anionen in das oben erwähnte Oxoborat- System führt zur Bildung von komplexen neuen Uranborophosphaten, -boratphosphaten und –phospaten. In diesem System wurden unter Anderem drei neue hochporöse Uranylborophosphate mit einzigartigen offenen dreidimensionalen Gerüststrukturen strukturell charakterisiert. Eine dieser Phasen, Cs3(UO2)3[B(PO4)4]∙(H2O)0.5, zeigt hierbei vielversprechende Ionenaustausch-Eigenschaften.

Im Th-B-O-System wurde ein neues Polymorph von ThB2O5 unter Normaldruck isoliert.

Eine weitere Untersuchung des Phasendiagramms von ThB2O5 ergab ungewöhnliche Stabilitätsfelder für beide polymorphen Modifikationen. Bei der Bildung beider polymorphen Modifikationen stellte sich heraus, dass die Eigenschaften des jeweils eingesetzten Flussmittels eine zentrale Rolle spielen.

Die Strukturen aller oben erwähnten Materialien wurden mittels Röntgen- Einkristalldiffraktometrie (SXRD) bestimmt und im Anschluss wurden die Materialien weiter mit Röntgen-Pulverdiffraktometrie (PXRD), Infrarot (IR)/Raman Spektroskopie, thermogravimetrischer Analyse (TGA), dynamischer Differenzkalorimetrie (DSC) sowie energiedispersiver Röntgenspektroskopie (EDS) untersucht. Die computergestützten Berechnungen im Th-B-O-System wurden mit Hilfe von Dichtefunktionaltheorie (DFT) durchgeführt.

Die Ergebnisse dieser Arbeit zeigen eindeutig, dass der Einfluss der Ladungsausgleichs- Kationen eine elementare Rolle in der Strukturgenese der Phasen in den untersuchten Systemen spielt. Infolgedessen zeigt sich, dass die Struktur eines Materials dessen physikalisch-chemischen Eigenschaften bestimmt.

IV Contents

Contents V 1. Introduction 1 1.1 Basic structural chemistry of uranium and thorium…...….…………………...…………1 1.2 Basic structural chemistry of borates …………………………………...... 4 1.3 Basic structural chemistry of borophosphates and borate-phosphates……..…………5 1.4 Overview of the previously reported uranium and thorium borates……………………7 1.5 Motivation...... 16 2. Experimental Methods and Characterization Techniques 18 2.1 Phase syntheses….………………...... 18 2.2 Ion-exchange experiments……...... 20 2.3 Characterization techniques...... 21 3. Complex Structural Chemistry of Uranyl Borates under Ambient and Extreme Conditions 26 3.1 Introduction...…….…………………………………………………………………….26 3.2 Experiment Section...……… ……………..………….…………………….………….27 3.3 Results and Discussion..…………………….……………………………………….....29 3.4 Conclusions..…………………….……………………………………………………....49 4. Influence of Synthetic Conditions on Chemistry and Structural Properties of Alkaline Earth Uranyl Borates 51 4.1 Introduction...…….…………..………………………………………………………….51 4.2 Experiment Section...………...……………..………….…………………….………….51 4.3 Results and Discussion …………………….……………………………………….....53 4.4 Conclusions…………………..….……………………………………………………....62 5. Complexity Reseaches New Limits: A Zeolitic Uranium Borate 64 5.1 Introduction...…… …………………………………………………………………….65 5.2 Experiment Section...……….……………..………….…………………….………….65 5.3 Results and Discussion …………………….…………………………………………...66 5.4 Conclusions…………………….……………………………………………………....75 6. Highly Porous Alkali-metal Uranyl Borophosphates with Unique 3D Open Framework Structures 77 6.1 Introduction...…….…………………….……………….……………………………….77 6.2 Experiment Section..……….……………..…………………………………….……….77 6.3 Results and Discussion…………………….…………………………………………....79 6.4 Conclusions……….……………….…………………………………………………....90

V 7. Microporous Uranyl Borophosphate with Potential Ionic Exchange Properties 92 7.1 Introduction...…….………..……………………………...…………………………….92 7.2 Experiment Section...………..……………..…………..….………………….………..93 7.3 Results and Discussion …………………….………………………………………....95 7.4 Conclusions……………..……….……………………………………………………..107 8. Which Role do Counter Cation play in the Formation of Actinide Borate-Phosphates? 108 8.1 Introduction...…….……………………………………………….…………………..108 8.2 Experiment Section...……… ……………..………….………………………………108 8.3 Results and Discussion …………………….………………………………………...109 8.4 Conclusions…………………….……………………………………………………..116

9. Flux Induced Polymorphism in ThB2O5 under ambient pressure 118 9.1 Introduction...... ……………………………………………...………………………118 9.2 Experiment Section...……...……………….…….……..…..………..……………….118 9.3 Results and Discussion…..….….……………….………………….………...………120 9.4 Conclusions.....……….…….………….………………………………………………132 References………………....…….….………………………………………………….…...133 10. List of Uranyl Phosphates Phases...... 145 Conclusions and outlook………………………………..…………….……………………146 Acknowledgement……………………………………….…………………………………149 Appendix: Papers and conference…………………………….…..……………………....151

VI The list of abbreviations

 CCI- Cation-cation interaction  FBB-Fundamental building blocks  BBU-Basic building blocks  PBU-Primary building unit  CBU-Composite building unit  MRs-Membered rings  BVS-Bond valence sum  PXRD-Powder X-ray diffraction  EDS- Energy-dispersive X-ray spectroscopy  SEM- Scanning electron microscopy  TG-DSC-Thermogravimetry-Differential scanning calorimetry  DFT- Density functional theory  BPO-Borophosphates  HT/HP-High temperature/High pressure  LUBO-Lead uranyl borate  DT-Double triangle  ICSD-Inorganic Crystal Structure Database  VDP- Voronoi-Dirichlet polyhedra  MOF-Metal of organic framework  FIB-Focused ion beams

VII Chapter 1. Introduction

The 5f-block elements, namely actinides, show great diversity in their crystal chemistry due to the complexity of their electron configurations. Among them, both uranium and thorium are widely disseminated in the earth’s crust compared to the other actinides. More importantly, uranium and thorium are found with high load in mineral forms. The presented work is focused on a study of uranium and thorium phases. Uranium is considered the most important element from the actinides series, as it is widely used for energy production. Understanding the chemical behavior of uranium under all different potentially occurring conditions is paramount for the safe and responsible handling as well as a reliable nuclear safety assessment. U (VI) can be chosen as a structural substitute for higher valence state actinides, such as Np (V/VI), and Pu (V/VI). Additionally, U (IV) and Th (IV) are considered to be good structural surrogates for studying the crystal chemistry of transuranic elements in oxidation state (IV), especially those with considerable handling issues such as Np (IV) and Pu (IV). Considering these aspects, the investigation of uranium and thorium and their synthetic compounds are of great importance for the fundamental study of actinide chemistry on a molecular level. Due to the very diverse and complex structures, oxo-borate phases demonstrate various properties and can be applied for gas adsorption, catalysis, ion exchange/ separation, luminescence and nonlinear optics (NLO). The oxo-borates have been found to form hetero-anionic species with for example silicates 4- 4- 3- [SiO4] , germanates [GeO4] and phosphates [PO4] . These hetero-anionic families are named as borosilicates, borogermanates, borophosphates or borate-phosphates. Recent studies demonstrate that f- elements including actinides can react with oxo-borate species with formation of complex actinide or lanthanide borates. This can lead to immobilization and separation of f-elements with different oxidation states. The actinide borates display very complex structural chemistry which attract research groups worldwide and is a subject of the study in this dissertation work.

1.1 Basic structural chemistry of uranium and thorium 1.1.1 Basic structural chemistry of uranium Uranium was firstly discovered in 1789 as a chemical element by Martin Heinrich Klaproth in a pitchblende specimen [1]. Autunite, namely, Ca(UO2)2(PO4)2•(H2O)10.5, is the first uranium bearing compound structure reported by Beintema in 1938 [2]. Uranium chemistry attracts an attention of chemists, material scientists and mineralogists. Uranium exhibits a large variety of coordination geometries which is caused by different oxidation states, namely, +2, +3, +4, +5, and +6. The two most stable oxidation states under ambient conditions are +4 or +6. The most common forms of uranium oxides are triuranium octoxide (U3O8) and uranium dioxide UO2 [3]. Uranium in lower oxidation states (+2, +3,

1 and +4) usually demonstrates similar coordination geometry with the lanthanide series. These coordination numbers range from 6 to 12 in oxygen based phases [4]. The structures with tetravalent uranium are well studied and U (IV) demonstrates almost identical coordination behavior with its Th (IV) analogues [5]. Compared to the oxidation state of UIV, more UVI bearing inorganic compounds have been investigated and characterized to date [6]. UVI in oxygen based phases is usually presented as a uranyl group, (O=U=O)2+. This group possesses a linear or nearly linear geometry with an angle around 178°. In this case, uranium can coordinate with 4 to 6 additional oxygen atoms in the equatorial plane which is perpendicular to the uranyl group. The resulting coordination polyhedra are tetragonal, pentagonal, and hexagonal bipyramids [7] (see Figure 1.1). These polyhedra can connect with each other via vertex, edge and face-sharing, and yield isolated zero-dimensional clusters, one-dimensional chains, two-dimensional sheets, or condensed three-dimensional frameworks [7]. The valence unit of the uranyl O atom is ~1.7 v.u.. As a consequence, the uranyl O atoms can be further bonded to the other cations. In case of coordination to other uranyl group such interaction called cation-cation interaction (CCIs) and this is a specific term which is used in actinide chemistry [8] (see Figure 1.2). CCIs is a rare phenomenon and occurs only in 2% of all known U(VI) phases [9].

UO6 UO7 UO8

Figure 1.1: Square (UO6), pentagonal (UO7) and hexagonal (UO8) bipyramids polyhedra. Uranium and oxygen atoms are shown in yellow and red, respectively.

Figure 1.2: An illustration of the cation-cation interaction (CCI) of uranyl units in Sr[(UO2)5(BO3)2O2(OH)2]∙5H2O [10]. The axial uranyl bonds are shown as red and the equatorial U-O bonds are shown in orange.

2 1.1.2 Basic structural chemistry of thorium In 1815, Berzelius analyzed a rare mineral and assumed that the mineral contained a new element, which he named as thorium [11]. Thorium is found as the most abundant radioactive element in nature [12]. In

1949, the first thorium based structure, namely, Th(OH)2CrO4•(H2O) was reported by Lundgren et al.[13]. The electron configuration of thorium is [Rn]-6d27s2 and unlike uranium it has no 5f electrons. Due to the electronic configuration, the oxidation state of thorium is dominated by 4+. The absence of electrons on the 6d and 7f orbitals leads to the formation of colorless thorium (IV) bearing compounds [14]. Under air atmosphere, thorium exists in a form of thorium dioxide – ThO2, which has fluorite structure [15]. Due to the large size of the cation and comparative (to uranium 6+) lower oxidation state, thorium 4+ has a rare diverse coordination environment and exceedingly rich coordination chemistry [16] (see Figure 1.3).

ThO6 ThO7 ThO8 ThO9 ThO10

Figure 1.3: View of common thorium polyhedra, ThOx (x = 6 - 10). 6-coordinated: tetragonal bipyramid, 7-coordinated: pentagonal bipyramid, 8-coordinated: square antiprism, 9-coordinated: monocapped square antiprism, 10-coordinated: bicapped square antiprism. Thorium polyhedra are shown in yellow and O atoms are red.

- 2- 2- 2- 3- 3- Thorium compounds with complex oxo-anions, such as, ClO4 , SO4 , SO3 , SeO4 , SeO3 , BO3 and 3- PO4 etc. [17-27] have been synthesized and characterized in the past in order to investigate their physico-chemical properties. Some of them found to be useful, for example, thorium perchlorate

Th(ClO4)4·4H2O, is soluble in water and alcohols, and it is an important intermediate in the purification of 4-n thorium [17]. In aqueous solution, thorium (IV) starts to hydrolyze into hydroxide complexes Th(OH)n at around pH = 4 and micro-molar concentrations. Those thorium hydroxide complexes can further be connected into the polynuclear compounds via olation or oxolation processes. A number of polynuclear hydroxide thorium species have been isolated. They can be found in different forms from dimers 6+ 10+ 9+ Th2(OH)2 to hexamers Th6(OH)14 and Th6(OH)15 [23-24]. From recent publications, the largest 8+ thorium oligomer is the decanuclear [Th10F16O8(NH3)32] which existed within the structure of

[Th10F16O8(NH3)32](NO3)8]19.6NH3. This finding confirms a presence of large thorium polynuclear species in both aqueous solution and solid state [28].

3 1.2 Basic structural chemistry of oxo-borates Inorganic oxo-borates possess quite complex chemical compositions and structural chemistry [29-33]. Based on the different structures, they have a wide range of applications, such as in adsorption/separation, catalysis, ion exchange, crystals for laser techniques, luminescence and nonlinear optics (NLO), etc [34- 40]. In particular, borates are promising second-order NLO materials, because they have wide transparent regions, relatively high laser damage threshold, moderate birefringence, high thermal and chemical stability, etc. There are a number of efficient NLO borates materials, such as, β-BaB2O4 (BBO), LiB3O5

(LBO), CsLiB6O10 (CLBO), BiB3O6 (BiBO), La2CaB10O19 (LCB) and Li4Sr(BO3)2 [41-46]. Boron has a 2 high affinity to oxygen and form two basic building units (BBUs), namely, BO3 triangle with sp 3 hybridization and BO4 tetrahedron with sp hybridization. The two BBUs can be interconnected via 4- 3- 5- 7- vertex or edge-sharing into a series of polymeric anionic units ([B2O5] , [B3O6] , [B3O7] , [B3O8] , 6- 5- [B4O9] , [B5O10] etc.). These polymeric units are also called Fundamental Building Blocks (FBBs) [47, 48] (see Figure 1.4).

Figure 1.4: Anionic units in borates. B atoms are shown in green and O atoms are red.

A large number of borates materials have been reported where various FBBs form different topological arrangements. According to the introduction of different metal or non-metal cations/anions, borates can be divided into the following systems: main group and transitional metal borates, lanthanide and actinide borates, and organic templated borates [49-54]. The oxo-borates feature 0D clusters, 1D chains, 2D layers or condensed 3D framework structures (see Figure 1.5).

4 Figure 1.5: The four different types of oxo-borates dimensionality: from 0D cluster-1D chain-2D layer - 3D framework structure. BO3 triangles and BO4 tetrahedra are shown in green.

1.3 Basic structural chemistry of borophosphates and borate-phosphates - 3- 3- 4- 4- 4- 5- Introduction of different oxo-anions, such as NO3 , PO4 , AsO4 , SiO4 , GeO4 , SeO4 , GaO4 etc., into the oxo-borates system leads to formation of new families of mixed-oxo anionic borates, namely, nitrate- borates [55], borate-phosphates or borophosphates [53], boroarsenates [54], borosilicates [58], borogermanates [59], boroselenates [60], and galluborates [61] etc.. The introduction of mixed oxo-anions into borates system increases the complexity of structural chemistry of the phases forming in these systems. Additional oxo-anions increase the porosity of mixed anionic borates which is potentially useful for catalysis, ion exchange/separation, and sorption. Among the mixed oxo-anion borates, the borophsophates (BPOs) are of high interest due to the structural variety and advanced applications [59, 60]. So far, after a first study in 1994 by Kniep et al [64] (see Figure 1.6), a large number of anion sub-structural units featuring 0D oligomers [65-67], 1D chains [68- 73] 2D layers [74-76], and condensed 3D frameworks [77-78] have been observed. Commonly, borophosphates are denoted as intermediate compounds of the system MxOy-B2O3-P2O5 (H2O) (M = metal elements, ammonium). Additionally, “templated cations (mainly organic)” play a key role in the preparation of open-framework structures in the mentioned system.

5 Figure 1.6: Crystal structure: of Sr(BPO5) (a) and its FBB [B3P3O17] (b), Ba3(BP3O12) (c) and its FBB [BP3O13] (d). BO4 and PO4 tetrahedra are shown in green and pink, respectively.

Figure 1.7: Crystal structures of CsFe(BP3O11) (a) and Cs2Cr3(BP4O14)(P4O13) (b), with the corresponding P-O-P partial features in the structures. BO4 and PO4 tetrahedra are shown in green and pink, respectively.

Based on the previously reported works, the different structural architectures of BPOs is strongly dependent on the synthetic conditions and the nature of templating cations. For example, all BPOs containing P−O−P linkages have been prepared by conventional high-temperature solid-state synthesis. 6 PO4 tetrahedra can further condense into polyanionic groups within the structures of BPOs. This effect significantly expanded the structural diversity of BPOs family. The first examples of BPOs with P−O−P connection are CsFe(BP3O11) and Cs2Cr3(BP4O14)(P4O13) reported in [79] (see Figure 1.7). The prolonged use of nuclear energy has resulted in a considerable amount of radioactive waste [80]. Vitrification in borophosphates or borosilicates glasses is one of the methods used for immobilization of components of spent fuel. Crystalline borophoshates may form within these nuclear glasses [81-83]. Compared to the main group and transition metal BPOs, the actinide based BPOs are quite limited. In total, there are only six reported uranium and thorium bearing BPOs to date (see more details in section

1.4.2). Th2[BO4][PO4] is the first actinide BPOs, which was reported by Lipp & Burns in 2011 being 14− prepared via a high temperature flux method [84]. Its structure features two face-sharing [ThO9] and 16− 5− [ThO10] polyhedra, forming a double layer that is parallel to (100) plane. The [BO4] tetrahedra are located between the layers of Th-polyhedra within the double layer. These double layers are connected 3− through [PO4] tetrahedra (see Figure 1.8).

(a) (b)

Figure 1.8: Crystal structure of Th2[BO4][PO4] along the b-axis ( a) and face sharing [Th2O14] dimer (b). Thorium polyhedra, BO4 and PO4 tetrahedra are shown in yellow, green and pink, respectively.

1.4 Overview of the previously reported uranium and thorium oxo-borates

1.4.1 Uranium Oxo-Borates K6[UO2{B16O24(OH)8}]·12H2O [85] is the first actinide oxo-borate being prepared via a slow evaporation method by Behm in 1985. The structure of this phase is based on isolated [UO2{B16O24(OH)8}] clusters, in which a uranyl core is surrounded by a 16-borates ring as shown in Figure 1.9. Gasperin has obtained six uranyl borates, namely, UO2(B2O4), Li(UO2)BO3, Na(UO2)BO3, Ca(UO2)2(BO3)2, Mg(UO2)B2O5,

Ni7(UO2)(B4O14) through high temperature solid state method (>1000 °C) [86-91]. The oxo-borate groups

7 are only presented as BO3 triangles in all of these structures, except Ni7(UO2)(B4O14). UO2(B2O4) features a 2D layered structure composed of edge sharing 1D uranyl and 1D borate chains as shown in Figure 1.10.

The structure of Ni7(UO2)(B4O14) is based on isolated 1D uranyl borate chains following the a-axis and isolated BO3 triangles as shown in Figure 1.11.

(a) (b)

Figure 1.9: The structure of K6[UO2{B16O24(OH)8}]·12H2O (a) and 16 member oxo-borate ring cluster (b). (UO8 hexagonal bipyramids are shown in yellow, BO3 and BO4 tetrahedra in green).

Figure 1.10: The structure of UO2(B2O4) (a) and a 2D uranyl oxo-borate sheet (b). (UO8 hexagonal bipyramids are shown in yellow, BO3 triangles are in green).

Figure 1.11: The structure of Ni7(UO2)(B4O14) (a) and a 1D uranyl borate chain (b). UO6 square bipyramids are shown in yellow, BO3 triangles and BO4 tetrahedra in green).

8 In the uranyl borates ternary system (U-B-O), Wang et al [92] have synthesized four new phases using an excess of boric acid flux at 190 °C, namely, UO2B2O4, UO2[B8O11(OH)4], α, β - (UO2)2[B9O14(OH)4] and

(UO2)2[B13O20(OH)3]3·1.25H2O. These uranyl borates possess 2D layered structures constructed from

UO8 hexagonal bipyramids and BO3 triangles (see Figure 1.12).

Figure 1.12: The structure of UO2B2O4 (a) and a 2D uranyl borate sheet (b). (UO8 polyhedra are shown in yellow, BO3 triangles in green).

Hinteregger et al. [93] have reported one uranium (VI) borate UB4O8, which was synthesized via a high- temperature/high-pressure reaction (HT/HP, 5.5 GPa/1100 °C). UB4O8 is constructed from corner-sharing 4+ BO4 tetrahedra, which form 2D layers parallel the bc-plane. The U cations are located between the boron-oxygen layers (see Figure 1.13).

Figure 1.13: The structure of UB4O8 (a), a 2D borate sheet (b) and a 2D uranyl sheet. (UO10 polyhedra are shown in yellow, BO4 tetrahedra in green).

Compared to uranyl borates ternary system, more uranyl borates have been prepared and well + characterized in the quaternary systems (A -U-B-O). In Li-U-B-O system, after Li(UO2)BO3 [87], only one new lithium uranyl borate, Li[(UO2)B5O9]·H2O, was reported by Wang et al. [94]. This structure 2+ consists of linear uranyl units, UO2 , which are surrounded by BO3 triangles and BO4 tetrahedra to create

UO8 hexagonal bipyramids. The borate anions are bridging the uranyl units to form 2D sheets. Additional

BO3 triangles connect the polyborate layers together to form a 3D framework structure (see Figure 1.14).

9 Figure 1.14: The structure of Li[(UO2)B5O9]·H2O (a) and a 3D polyborate framework (b). (UO8 polyhedra are shown in yellow, BO3 triangles and BO4 tetrahedra in green).

Four new sodium uranyl borates were synthesized by Wang et al [95] using the excess of H3BO3-flux, namely, α, β-Na[(UO2)2B10O15(OH)5)], Na[(UO2)2B10O15(OH)5]3·3H2O and Na[(UO2)B6O10(OH)]3·2H2O.

These compounds share a common structural motif similar to Li[(UO2)B5O9]·H2O [94]. They are consisting of a linear uranyl cation surrounded by BO3 triangles and BO4 tetrahedra to create a UO8 hexagonal bipyramidal environment around uranium. The borate anions connect the uranyl units to create

2D sheets. Additional BO3 triangles are directed approximately perpendicular to the sheets and extend the polyborate layers. In the structure of Na[(UO2)B6O10(OH)]3·2H2O, the BO3 units link the layers together to yield its 3D networks. This forms large pores which host the Na+ cations and water molecules (see Figure 1.15).

Figure 1.15: The structure of Na6[UO2{B16O24(OH)8}]·14H2O (a), a 3D polyborate framework (b). (UO8 polyhedra are shown in yellow, BO3 triangles and BO4 tetrahedra in green).

Zhang et al. [96] have reported one nano-cluster sodium uranyl polyborate

Na6[UO2{B16O24(OH)8}]·14H2O using a slow evaporation method. The crystal structure of

Na6[UO2{B16O24(OH)8}]·14H2O consists of crown-shaped uranyl borate nano-clusters where each uranyl group is surrounded by a 16-oxo-borates ring (see Figure 1.16).

10 Figure 1.16: The structure of Na[(UO2)B6O10(OH)]3·2H2O (a), a 16-boron ring uranyl polyborate cluster (b). (UO8 polyhedra are shown in yellow, BO3 triangles and BO4 tetrahedra in green).

In the K-U-B-O system, eleven synthetic uranium borates have been synthesized via four different methods. Wang et al. have synthesized three potassium uranyl borates, K2[(UO2)2B12O19(OH)4]3·0.3H2O,

K[(UO2)2B10O15(OH)5] and K[(UO2)2B10O16(OH)3]3·0.7H2O, using a large excess of molten boric acid at 190 °C [97]. All of these compounds adopt layered structures, which shared the common structural motif with Li and Na based phases. Wu et al. has applied the HT/HP method and obtained three 2D uranyl borates [98]. In this three HT/HP phases, boron only exhibits BO3 coordination environment. This is different from other uranyl borates prepared at room temperature or quasi-hydrothermal conditions (see Figure 1.17).

Figure 1.17: The structure of K4[(UO2)5(BO3)2O4]·H2O (a), a 2D uranyl borate sheet (b). (UO7 polyhedra are shown in yellow, UO6 are in orange, BO3 triangles in green).

Stritzinger et al. has obtained two potassium uranyl borates from super critical water reactions, namely

K10[(UO2)16(B2O5)2(BO3)6O8]·7H2O and K13[(UO2)19(UO4)(B2O5)2(BO3)6(OH)2O5]·H2O [99]. Within these compounds, oxo-borates exist as isolated BO3 triangles or B2O5 dimers. The most striking feature, a

[UO4(OH)2] unit, is found in K13[(UO2)19(UO4)(B2O5)2(BO3)6(OH)2O5]·H2O. This unit contains U(V)-O

11 tetra-oxo core with oxygen and trans hydroxide anions. This compound is a rare example of a mix-valent U(VI)/U(V) oxo-phase (see Figure 1.18).

Figure 1.18: The structure of K13[(UO2)19(UO4)(B2O5)2(BO3)6(OH)2O5]·H2O (a), a 2D uranyl borate sheet (b). (UO7 polyhedra are shown in yellow, UO6 are in orange, BO3 triangles are in green).

In 2016, Wang et al. have used molten H3BO3-flux method at 250 °C. A new 3D potassium uranyl borate

K[(UO2)B6O10(OH)] [100] has been prepared, which shared the common structural motif as the other uranyl borates obtained from lower temperatures (see Figure 1.19).

Figure 1.19: The structure of K[(UO2)B6O10(OH)] (a), a uranyl borate cluster (b). (UO8 polyhedra are shown in yellow, BO3 triangles and BO4 tetrahedra are in green).

Three rubidium and three cesium uranyl borates have been reported in recent publications. One Rb-uranyl borate and one Cs-uranyl borate, Rb(UO2)(BO3) and Cs(UO2)(BO3) were obtained by Wu et el. via a new low temperature route [101]. Both of them feature a 2D structures with a new [UO5]∞ anion topology. The

FBB in these structures is based on the edge-sharing dimers comprised of (UO7)2 polyhedra (see Figure 1.20). Wang et al. have synthesized four Rb and Cs-uranyl borates with the mentioned molten boric acid method, namely, Rb2[(UO2)2B13O20(OH)5] and Rb[(UO2)2B10O16(OH)3]3·0.7H2O [97], α, β-

Cs[(UO2)2B11O16(OH)6] [10]. These compounds share the common structural motif as those described + above on Figure 5, 6 and 10. Four A metal uranyl borates Ag[(UO2)(B5O8(OH)2] [94], α, β-

Tl2[(UO2)2B11O18(OH)3], Tl[(UO2)2B10O16(OH)3] and Tl2[(UO2)2B11O19(OH)] [95] were also reported by Wang et al.. The structures of these phases are simlar to the above described alkali metal uranyl borates. 12 (a) (b)

Figure 1.20: The structure of Rb(UO2)(BO3) along the [-102] direction (a), a 2D uranyl borate sheet (b). (UO7 polyhedra are shown in yellow, BO3 triangles are in green).

3- 1.4.2 Uranium Borates with other oxo-anions [TO4 (T = P and As) and AlOx (x = 4, 5, 6)]

Wu et al. has synthesized the first three novel uranyl borophosphates in 2013 via a

H3BO3−NH4H2PO4/NH4H2AsO4 flux method [83]. Ag2(NH4)3[(UO2)2{B3O(PO4)4(PO4H)2}]·H2O features a 2D layered uranyl borophosphate structure. A new borophosphate FBB was observed in this structure [102] (see Figure 1.21). The other two silver uranyl borophosphates are isostructural and feature a 3D framework structure and possess a FBB based on two BO4 and five TO4 (T = P, As) tetrahedra (see Figure 1.22).

Figure 1.21: The structure of Ag2(NH4)3[(UO2)2{B3O(PO4)4(PO4H)2}]·H2O (a), a 2D uranyl borophosphate sheet (b). (UO7 polyhedra are shown in yellow, PO4/AsO4 tetrahedra and Ag atoms are in green, pink and blue, respectively).

The first uranium borate-phosphate, Ba5[(UO2)(PO4)3(B5O9)]·nH2O, was prepared by Wu et al. via a high temperature solid state method. This phase exhibits unprecedented complex inorganic nano-tubular fragments with an external diameter of ~2 × 2 nm. The nano-tubular aggregates are based on borate tubes.

These nano-tubes is decorated with UO2(PO4)3 moieties in the outside, forming a complex cross-section shape [103] (see Figure 1.23).

13 Figure 1.22: The structure of Ag0.57(NH4)3((UO2)2(B2P2.76As2.24O18.57(OH)1.43)) (a), a 1D borophosphate/arsenate chain (b). (UO7 polyhedra are shown in yellow, BO4, PO4/AsO4 tetrahedra and Ag atoms are in green, pink and blue, respectively).

Figure 1.23: The structure of Ba5[(UO2)(PO4)3(B5O9)]·nH2O (a), a schematic representation of fragments hierarchy nanotubules (b). (UO6 polyhedra are shown in yellow, BO3 triangles, BO4, PO4 tetrahedra and Ba atoms are in green, pink and blue, respectively).

Figure 1.24: The structure of U2[BO4][PO4] (a), the local coordination environments of BO4 (b) and PO4 tetrahedra (c). (Uranium atoms are shown in yellow, BO4 and PO4 tetrahedra are in green and pink, respectively).

14 Hinteregger et al. have reported a new uranium (IV) borate phosphate, U2[BO4][PO4], which was synthesized via a HT/HP method at 12.5 GPa and 1000 °C. The structure of U2[BO4][PO4] consists of double layers of linked uranium–oxygen polyhedra parallel to the a-axis. The borate tetrahedra are located between the uranium–oxygen layers within the double layer. The phosphate groups further linked the double layers [104] (Figure 1.24).

The first actinide aluminoborate, UO2[B3Al4O11(OH)], was obtained by Wu et al. using supercritical water as a reaction meadia at a pressure ~100 MPa and a temperature ~500 °C. UO2[B3Al4O11(OH)] features a complex 3D framework based upon BO3 triangles and three different types of AlOx (x = 4, 5, 6) polyhedra. 2+ The uranyl groups (UO2) are incorporated into the aluminoborate framework and exist as UO8 hexagonal bipyramids [105] (see Figure 1.25).

Figure 1.25: The structure of UO2[B3Al4O11(OH)] (a), the local coordination environments of UO8 hexagonal bipyramids (b). (UO8 hexagonal bipyramids are shown in yellow, BO3 triangles and AlOx tetrahedra are in green and pink, respectively).

1.4.3 Thorium oxo-borates Compared to uranium, thorium has no 5f electrons and therefore its structural chemistry is not as diverse as crystal chemistry of uranium. Up to date only a limited number of thorium borates have been reported.

In 1991 Gasperin has reported the first thorium borate, ThB2O5 [106], which was synthesized via a high temperature borax flux method. The structure of ThB2O5 features a condensed 3D thorium borate framework. Boron atoms are only coordinated by oxygen atoms to form BO3 triangles. Two of these BO3 planar trigonal units are corner sharing forming an isolated B2O5 dimer. Each BO3 triangle is corner sharing with two and edge sharing with one thorium polyhedral. Thorium cations are eight-fold coordinated and are existing as ThO8 distorted one-caped pentagonal bipyramids. Thorium polyhedra are ordered in a diamond packing mode within this structure. (see Figure 1.26). Hinteregger et al. [93] have prepared a thorium borate, ThB4O8, via a high-temperature/high-pressure method (HT/HP, 5.5

GPa/1100 °C). ThB4O8 is isostructural with previously reported uranium borate UB4O8 (see Figure 1.4).

Lipp and Burns [84] have synthesized one thorium borate-phosphate, Th2[BO4][PO4], using boron oxide 15 as a flux at 1200 °C. The structure of Th2[BO4][PO4] is isostructural with that of U2[BO4][PO4] (see Figure 1.24).

Figure 1.26: 3D thorium borate network structure (a), the local coordination environments of thorium center (b). ThO8 polyhedra and BO3 triangles are shown in yellow and green, respectively.

Wang et al. have reported a remarkable anion exchanger, [ThB5O6(OH)6][BO(OH)2]·2.5H2O (NDTB-1) [107], synthesized through a boric acid flux method. The structure of NDTB-1 is a porous super- tetrahedral 3D framework. The building blocks of this framework are twelve-coordinate Th4+ ions. These 4+ Th cations are surrounded by BO3 and BO4 anions (see Figure 1.27). NDTB-1 showed great anion - 2- 2- 4- - exchange properties with a variety of toxic anions, such as MnO4 , CrO4 , Cr2O7 , and ReO , IO3 and 2- - SeO3 . The kinetic studies of NDTB-1 (10 mg) show a rapid uptake of highly radioactive TcO4 from solution, with 72% being removed in 36 hours [108, 109].

Figure 1.27: The structure of [ThB5O6(OH)6][BO(OH)2]·2.5H2O (a), the local coordination environments of ThO12 polyhedra (b). (ThO12 polyhedra are shown in yellow, BO3 triangles and BO4 tetrahedra are in green).

1.5 Motivation This work is devoted to a systematic study of uranium and thorium based oxo-borate phases formation under different synthetic conditions and investigation of their structures and properties. An extensive

16 literature analysis shows that a number of actinide borates have been prepared and characterized in the last two decades. However, compared to the borates of main group, transition and rare earth elements, actinide borates are still quite less explored. This limits a deep understanding of actinides chemistry in oxo-borate systems. Particularly, the phases of actinide borates in combination with different oxo-anions

(PO4, AlOx etc.) are scarcely investigated. One of the main questions is how small changes in synthetic conditions can lead to striking differences in chemical compositions and structural properties of resulting compounds. To answer this question, four synthetic methods have been applied in this work: slow evaporation at room temperature, mild hydrothermal synthesis, high-temperature solid-state reaction method, and synthesis under extreme conditions of pressure and temperature (more details see Chapter 2). Another main question of this work is how the chemical composition affects structures of actinide borates and vice versa. Based on the literature analyses, our knowledge on relation between synthetic conditions and resulting crystal structures of actinide borates is limited. During this PhD research work, the following systems were investigated: 1) A1+-U-B-O system, 2) A2+-U- B-O system, 3) A1+-U-B-P-O system, 4) A2+-U-B-P-O system and 5) A1+-Th-B-O systems. Several new methodologies have been developed for the preparation of uranium/thorium borates, which can help us to understand the fundamental crystal chemistry of actinide (IV/VI) borates at different reaction conditions. 3- The introduction of [PO4 ] oxo-anion into the actninide oxo-borate systems can yield highly porous zeolite-like crystal structures. Additionally, these materials can potentially possess attractive properties, such as gas adsorption/separation or ion-exchange, etc.. Answering the questions rised above will increase our capabilities for prediction of actinide behavior in complex oxo-salt systems.

17 Chapter 2. Experimental Methods and Characterization Techniques

2.1 Phase syntheses.

Caution! The UO2(NO3)2∙6H2O used in this study contained natural uranium, nevertheless the standard precautions for handling radioactive materials must be followed.

2.1.1 Slow evaporation method This method is the easiest way for the synthesis of new crystalline compounds. However, it is very sensitive and can be affected by many factors, such as the nature of reagents and solvents, pH values, evaporation rate, crystallization temperature etc.. In this work, uranium (VI) nitrate was used as a starting material for all syntheses and distilled water was used as a solvent. The detailed steps of the crystallization procedure are shown in Figure 2.1: first, a certain amount of reagents has been weighed and placed into a clean glass vial (20 ml), second, 1.5-2.5 ml distilled water has been added into the vial to dissolve the reagents, third, the vial was covered by a lid with a hole in the middle, then the vial was placed on a stable shelf for 1-4 weeks. A size of the hole in the lid controls the evaporation rate.

Figure 2.1: Chemical representation of slow evaporation method.

2.1.2 Method of hydrothermal reaction A first report of the hydrothermal crystal growth was made by German geologist Karl Emil von Schafhäutl in 1845 [110]. With this method, a number of materials were prepared. Many of chemical compounds are insoluble at ambient conditions, but can be dissolved under elevated temperatures and pressures (~220 °C, ~2 MPa). A standard hyrothermal reactor consists from a stainless steel outer container and an internal Teflon vessel. A detailed handling procedure is shown in Figure 2.2: first, weighting in a certain amount of reagents and 0.2-2.5 ml of distilled water into the autoclave (23 ml), second, sealing the autoclaves tightly then transferring them into a programmable furnace and starting of the experiments. Usually, the used reaction temperatures were in the range of 150-220 °C. These temperatures were held for 24-48 hours, and then the reactors were slowly cooled down to room- temperature. Crystals formed on the bottom of the Teflon vessel during slow cooling process.

18 Figure 2.2: The hydrothermal method procedure.

2.1.3 High temperature solid state method The high temperature solid state reaction route is one of the most widely used methods for the preparation of crystalline or polycrystalline phases. Solids usually do not react at lower temperatures and it is necessary to heat them to higher temperatures to increase solid state reaction kinetics. The most important factors are nature of initial reagents, reaction temperatures, cooling rates, fluxes, surface area etc. [111- 112]. The detailed handling procedure is shown in Figure 2.3: first, weighing in a certain amount of chemicals, mixing and grinding thoroughly in a mortar, second, the mixture has to be placed into a platinum crucible and then transferred into the programmable furnace. In this work a temperature range from 800 to 1200 °C has been used. Materials were held at this temperature for 3-5 hours, and then slowly cooled down to room-temperature. Crystalline phases form during the cooling process.

Figure 2.3: The high temperature solid state method procedure.

2.1.4 The high-temperature high-pressure (HT/HP) solid state method Besides the conventional conditions, a method of high-temperature/high-pressure reaction has been used in this study. Using this synthetic method, a number of temperature/pressure dependent crystalline phases have been prepared. However, compounds that have been synthesized via high-temperature high-pressure solid state method are quite limited [83, 113, 114]. Sample preparation is similar with the high temperature solid state method. The difference is that the samples are pressed and sealed inside small platinum crucible. Sometimes other crucible materials can be used such as graphite, gold, palladium alloys etc.. The sealed crucible has to be placed into the HT/HP device shown on Figure 2.4. In this work only talc-pyrex piston cylinder assembly was used for pressure generation [115]. After the setting of

19 temperature and pressure program (in most cases 4 GPa, 1000 °C was used). Normally the reaction will take from 2 to 5 days.

Figure 2.4: The HT/HP solid state method device.

2.2 Ion-exchange experiments

Ion-exchange reactions were performed on polycrystalline phases (certain amount) in glass vials (20 ml) with solutions of monovalent cations: NaNO3 (aq), KNO3 (aq) and RbNO3 (aq), divalent cations:

Mg(NO3)2·6(H2O) (aq), Ca(NO3)2·4(H2O) (aq), Sr(NO3)2 (aq), Ba(NO3)2 (aq), Ni(NO3)2·6(H2O) (aq),

Co(NO3)2·6(H2O) (aq), Cu(NO3)2·6(H2O) (aq), Zn(NO3)2·6(H2O) (aq), Cd(NO3)2·6(H2O) (aq), Pb(NO3)2

(aq) and (UO2)(NO3)2·6(H2O) solutions, trivalent cations: Bi(NO3)3 (aq), [La-Lu(NO3)3·6(H2O)] (aq). The parallel experiments were kept both at room temperature and 70 °C for a certain time. The ion-exchanged samples were recovered through filtration, washed with an excess of deionized water and acetone for 1 day. After that the samples were dried in dry furnace for 24 hours (see Figure 2.5).

Figure 2.5: A series of ion exchange experiments.

20 2.3 Characterization techniques

2.3.1 Crystallographic Studies and Powder X-ray Diffraction. Diffraction data for all compounds were collected on an Agilent Technologies SuperNova diffractometer with Mo-Kα radiation (λ = 0.71073 Å) at room temperature (see Figure 2.6). All data sets were corrected for Lorentz and polarization factors as well as for absorption by the multi-scan method [116]. Structure of compounds were solved by direct method and refined by a full-matrix least-squares fitting on F2 by SHELX-97 [117]. All structures were checked for possible missing symmetry elements using PLATON with the ADDSYM algorithm [118]. X-ray powder diffraction data were measured on a Bruker-AXS D4 Endeavor diffractometer (see Figure 2.7), 40kV/40mA, in Bragg−Brentano geometry. The diffractometer is equipped with a copper X-ray tube and a primary nickel filter producing CuKα1, 2 radiation (λ = 1.54187 Å). A linear silicon strip LynxEye detector (Bruker-AXS) was used. Data were recorded in the range of 2θ = 10−80 ° with 10 s/step and a step width of 0.02 °. The aperture of the fixed divergence slit was set to 0.2 mm and the receiving slit with 8.0 mm, respectively. The discriminator of the detector was set to an interval of [0.16 - 0.25 V].

Figure 2.6: The images of SuperNova from Rigaku Oxford diffractometer (left). The inside chamber of the SuperNova diffractometer (right). 1) Sources of radiation - Copper X-ray tube (high intensity, large absorption by heavy atoms, suitable for organic compounds), Molybdenum X-ray tube (low intensity, low absorption by heavy atoms, suitable for actinide compounds). 2) Detector-CCD area detector. 3) Temperature control system - temperature ranges from 100 K (liquid nitrogen) to 500 K. 4) Camera.

21 Figure 2.7: Bruker D4 Endeavor diffractometer (left), the inside chamber of the diffractometer (middle), the sample holder (right).

2.3.2 Scanning Electron Microscopy (SEM)/Energy-dispersive X-ray Spectroscopy (EDS) Analysis. Elemental analyses, scanning electron microscopy image and energy-dispersive X-ray spectroscopy (SEM/EDS) were collected on a FEI Quanta 200F Environment Scanning Electron Microscope with a low-vacuum mode at 0.6 mbar (see Figure 2.8).

Figure 2.8: The FEI Quanta 200F Environment Scanning Electron Microscope (left) and the sample holder (right).

2.3.3 Raman and IR Spectroscopy. Unpolarized Raman spectra were recorded with a Horiba LabRAM HR spectrometer using a Peltier cooled multichannel CCD detector. An objective lens with a 50 × magnification was linked to the spectrometer, allowing the analyses of samples as small as 2 μm in diameter. The samples were in the form of single crystals. The incident radiation was produced by a He-Ne laser line at a power of 17 mW (λ = 632.8 nm). The focal length of the spectrometer was 800 mm, and an 1800 gr/mm grating was used. The spectral resolution was approximately 1 cm-1 with a slit of 100 μm. The spectrum was recorded in the range of 100−4000 cm-1. Infrared (IR) spectra were measured use polycrystalline samples mixed with KBr powder, with FT-IR spectrometer Bruker Equinox 55 under room temperature (see Figure 2.9). 22 Figure 2.9: Horiba LabRAM HR spectrometer for Raman experiments (left), Bruker Equinox spectrometer for IR experiments (right).

2.3.4 Thermal Analysis (TG-DSC Experiments). The thermal behavior of the polycrystalline phases up to 1200 °C was studied by differential scanning calorimetry (DSC) analysis coupled with thermogravimetry (TG) in air at a heating rate of 10 °C/min using a Netzsch STA 449C Jupiter apparatus (see Figure 2.10). The samples (~10-20 mg) were loaded in platinum crucibles, which were covered by platinum lids. During the measurements a constant dry air flow of 20-30 mL/min was applied.

Figure 2.10: Netzsch STA 449C Jupiter apparatus for TG-DCS experiment.

23 2.3.5 Bond-Valence Analysis. The BVS is a semi-empirical method for validating newly determined structures, it is capable for predicting many of the properties of those chemical structures that can be described by localized bonds.

Relating Vi, the oxidation state of cation i, to Sij, the valence of the bond between the cation i and the anion j follow the rule (1):

 S (1) vi  ij j

The applied equation with bond valence to interpret the bond length variation following the rule (2):  0 ij  exp(r r ) (2) sij  j B

Hereby, r0 is the bond valence parameter [119, 120]. Usually, the value of B is constant that equals to 0.37

Å [12]. But for Uranium (VI), the r0 and B is variable according to the different coordination numbers.

For UO6 polyhedral, r0 = 2.074 Å, B = 0.554 Å, for UO7 polyhedral, r0 = 2.045 Å, B = 0.510 Å and for

UO8 polyhedral, r0 = 2.042 Å, B = 0.506 Å, given by Peter C. Burns [121].

2.3.6 ICP-MS analysis. ICP-MS is a type of mass spectroscopy which is capable of detecting metals and several non-metals at concentrations as low as one part in 1015 (part per quadrillion, ppq) on non-interfered low-background isotopes. This is achieved by ionizing the sample with inductively coupled plasma and then using a mass spectrometer to separate and quantify those ions. ICP-MS analyses were performed on Perkin Elmer Sciex ELAN 6100 ICP/MS instrumentation (see Figure 2.11). The ion-exchanged solution samples were diluted to lower the concentrations below 100 ppb for ICP-MS.

24 Figure 2.11: A Perkin Elmer Sciex ELAN 6100 ICP/MS instrumentation.

25 Chapter 3. Complex Structural Chemistry of Uranyl Borates under Ambient and Extreme Conditions

3.1 Introduction According to the different counter cations, borates can be divided into three groups: main group and transition metal borates, lanthanide and actinide borates, and organic complex templated borates [122-126] 4- 4- 3- 5- Moreover, borates can combine with a series of mixed oxo-anions, such as SiO4 , GeO4 , PO4 , GaO4 . etc., give rise to new families of materials [58, 60, 127-129] From current literature, compared to the 3d- and 4f- transition metal oxo-borates, actinides oxo-borates are relatively less explored [130] The main reason for this is that actinides are radioactive elements and are very difficult to handle appropriately [131] Over the past decade, uranium borates have attracted considerable attention because of their diverse structural architectures and importance in the nuclear waste management area [95].

Wang has used hydrothermal methods with excess of H3BO3 as flux at around 200 °C. The resultant 2+ structures shared a common structural motif, consisting of a linear uranyl core (UO2) surrounded by BO3 triangles and BO4 tetrahedra, forming a UO8 hexagonal bipyramid, for example, Li[UO2B5O9](H2O)3,

Na[(UO2)(B6O10(OH)](H2O)2, Na[(UO2)2(B10O15(OH)5], K2[(UO2)2B12O19(OH)4)](H2O)0.3,

Rb[(UO2)2B10O16(OH)5], Cs[(UO2)2B11O16(OH)6]. etc [130] Compared to Gasperin and Wang, Wu has prepared a series of potassium uranyl borates, K4[(UO2)5(BO3)2O4](H2O). etc. using more extreme high- temperature/high-pressure conditions (HT/HP, 650 ± 5 °C and 200 ± 10 MPa) [98]. The first example of a mixed-valent U(VI)/U(V) borate, K13[(UO2)19(UO4)(B2O5)2(BO3)6(OH)2O5](H2O) [132] was prepared by Stritzinger et al. from a supercritical water reaction. The most striking feature in this structure is the tetraoxo core unit, [UO4(OH)2], that contains U(V) with trans hydroxide anions. A series of alkaline earth uranyl borates were synthesized via different methods, which have completely different structural architectures and physiochemical properties than the alkali metal uranyl borates similar conditions [10]. In this chapter, new uranyl borates will be described in terms of their synthesis, structural characteristics and physiochemical properties. Herein, we report the syntheses of nine novel uranyl borates, namely,

(H3O)(UO2)(BO3) (1), Li(UO2)(BO3)·(H2O) (2), α-K4[(UO2)5(BO3)2O4] (3), β-K4[(UO2)5(BO3)2O4] (4),

K2(UO2)5(BO3)2O3·(H2O)4 (5). A6[(UO2)12(BO3)8O3]·(H2O)6 (A = Rb and Cs) (6 and 7),

Rb3[(UO2)3(BO3)2O(OH)]·(H2O) (8) and K4Sr4[(UO2)13(B2O5)2(BO3)2O12] (9), their complex structures, thermal stability and spectroscopic properties.

26 3.2 Experiment Section

3.2.1 Materials and Methods. Uranyl nitrate UO2(NO3)2∙6H2O (International Bioanalytical Industries,

Inc.), lead nitrate Pb(NO3)2 (Alfa-Aesar, 99.9%), potassium nitrate KNO3 (Alfa-Aesar, 99.9%), lithium tetraborate Li2B4O7 (Alfa-Aesar, 99.9%), potassium tetraborate K2B4O7∙4(H2O) (Alfa-Aesar, 99.9%), potassium nitrate KNO3 (Alfa-Aesar, 99.9%), strontium carbonate SrCO3 (Alfa-Aesar, 99.9%) rubidium hydroxide RbOH (50% wt. in aq. Soln., Alfa-Aesar, 99.6%), phosphorous acid H3PO3 (Alfa-Aesar,

99.9%), rubidium nitrate RbNO3 (Alfa-Aesar, 99.9%), cesium nitrate CsNO3(Alfa-Aesar, 99.9%) and boric acid H3BO3 (Alfa-Aesar, 97%) were all used as received.

3.2.1.1 Syntheses of 1 and 2. Both compounds were prepared by a hydrothermal method. The mixtures of UO2(NO3)2∙6H2O (0.0516 g, 0.10 mmol), Pb(NO3)2 (0.0332 g, 0.10 mmol), Li2B4O7 (0.0426 g, 0.25 mmol) and deionized water (0.5 ml), in a ratio of U : Pb : B : Li = 1 : 1 : 10 : 5 for 1, UO2(NO3)2∙6H2O

(0.0522 g, 0.10 mmol), Pb(NO3)2 (0.0336 g, 0.10 mmol), Li2B4O7 (0.0682 g, 0.40 mmol) and deionized water (0.5 ml) in a ratio of U : Pb : B : Li = 1 : 1 : 16 : 8 for 2, were sealed into Teflon-lined stainless steel autoclaves (23 ml) and then transferred into a programmable furnace, heated up to 220 °C and holding for 24 hours, then slowly cooled down to room temperature with a rate of 3 °C/h. The resulting products were washed with hot water and then rinsed with ethanol. Yellowish thin needle shaped crystals (1 and 2) were obtained. Good quality crystals were collected for further analyses. 3.3.1.2 Synthesis of 3. Compound 3 was obtained from high temperature solid state synthesis method. A mixture of UO2(NO3)2∙6H2O (0.0522 g, 0.1102 mmol), K(NO3)2 (0.0366 g, 0.2156 mmol), K2B4O7(H2O)4

(0.0628 g, 0.1968 mmol) and H3BO3 (0.0643 g, 1.0399 mmol) in a ratio of U : K : B = 1 : 6 : 18. All the reactants were thoroughly ground in an agate mortar and then transferred to a platinum crucible. The reaction mixtures were heated up to 980 °C for 5 hours in a box furnace and then cooled down to 450 °C at a cooling rate of 5 °C/h, then switched off the furnace. Yellow plate crystals 3 were obtained. Pure polycrystalline samples of 3 were synthesized quantitatively by the reaction of a mixture of UO2(NO3)2 ∙

6H2O (0.2028 g, 0.4015 mmol), K2CO3 (0.0697 g, 0.5011 mmol), H3BO3 (0.0372 g, 0.60 mmol) with a molar ratio of 4 : 5 : 4 at 900 °C for 2 days. The product obtained was characterized using laboratory X- ray powder diffraction (XRD) which indicated 3 was obtained with high purity. EDS elemental analyses on several single crystals of this compound gave an average molar ratio of U : K = 5 : 4.16, which is in good agreement with the single crystal structure compositions. 3.2.1.3 Synthesis of 4. Compound 4 was synthesized via high temperature/high pressure method. The starting chemicals of U3O8 (0.0536 g, 0.0636 mmol), KNO3 (0.0336 g, 0.2102 mmol) and B2O3 (0.0698 g, 1.0072 mmol) with a ratio of U : K : B = 1 : 3 : 16 for 4. All the reagents were thoroughly ground in an agate mortar and then transferred to a small platinum crucible. The mixtures were pressed and then sealed 27 the platinum crucible. A pressure of 4.0 GPa was applied within 0.5 hour and the reaction mixtures were kept 4.0 GPa for the whole experimental run. The temperature was increased up to 1000 °C in 0.5 hour, after the pressure was stable at 4.0 GPa. The temperature held at 1000 °C for 6 hours then slowly cooled down to 700 °C with a rate of 5 °C/h, cooled down to 450 °C at a rate of 10 °C/h followed by quenching to room temperature. The pressure was released in half an hour. For extracting the resulting products, the platinum crucible was crushed. The yellow pallet shaped crystals of 4 were isolated. EDS elemental analyses on several single crystals of 4 gave an average molar ratio of U : K = 5 : 3.89, which is in good agreement with the proposed chemical compositions. 3.2.1.4 Synthesis of 5. Compound 5 was prepared through a hydrothermal method. The mixtures of

UO2(NO3)2∙6H2O (0.0516 g, 0.1015 mmol), Zn(NO3)2(H2O)6 (0.0632 g, 0.2124 mmol), K2B4O7(H2O)4 (0.1226 g, 0.3928 mmol) and deionized water (0.5 ml), in a ratio of U : Zn : B : K = 1 : 2 : 8 : 16 for 5, was sealed into teflon-lined stainless steel autoclaves (23 ml) and then transferred into a box furnace, heated up to 220 °C and holding for 24 hours, then slowly cooled down to room temperature with a rate of 3 °C/h. The resulting products were washed with hot water and then rinsed with ethanol. Yellow cubic block shaped crystals 5 were obtained. Fine crystals were collected for further analyses. EDS analysis on several single crystals gave an average molar ratio of U : K = 5 : 1.96, which is in good agreement with its proposed chemical compositions.

3.2.1.5 Syntheses of 6 and 7. The compounds A6[(UO2)12(BO3)8O3](H2O)6 (A = Rb and Cs) were synthesized via high temperature solid state methods. The starting chemicals of UO2(NO3)2(H2O)6

(0.1026 g, 0.2044 mmol), RbNO3 (0.1166 g, 0.8208 mmol) and B2O3 (0.1398 g, 2.0472 mmol) were mixed with a ratio of U : Rb : B = 1 : 4 : 10. All the mixtures were thoroughly ground in an agate mortar and then transferred to a platinum crucible in a box furnace. The temperature was increased to 1000 °C holding for 5 hours, and then slowly cooled down to 600 °C with a rate of 5 °C/h. In a second step it was cooled down to 450 °C at a rate of 10 °C/h followed by quenching to room temperature. Yellow pallet shaped crystals of Rb6[(UO2)12(BO3)8O3](H2O)6 were isolated. EDS elemental analyses on several single crystals of Rb6[(UO2)12(BO3)8O3](H2O)6 gave an average molar ratio of U : Rb = 2.0 : 1.13, which is in good agreement with the proposed chemical compositions of the single crystal structure. The replacement of the reactant RbNO3 by CsNO3, whilst the other experimental conditions were kept the same with the synthesis route of Rb6[(UO2)12(BO3)8O3](H2O)6, the isostructural compound Cs6[(UO2)12(BO3)8O3](H2O)6 was produced. An average molar ratio of U : Cs = 2.0 : 1.09 based on the EDS elemental analyses on the single crystals of Cs6[(UO2)12(BO3)8O3](H2O)6 was obtained, which is in good agreement with the single crystal refinement structure.

3.2.1.6 Synthesis of 8. The phase Rb3[(UO2)3(BO3)2O(OH)](H2O) was obtained from a hydrothermal method. UO2(NO3)2(H2O)6 (0.0516 g, 0.1106 mmol), H3PO3 (0.0336 g, 0.4056 mmol), RbOH (0.4 ml, 28 1.7206mmol), H3BO3 (0.0943 g, 1.5339 mmol) and 0.5 ml of deionized water were mixed in a ratio of U :

Rb : B : H2O = 1 : 17 : 15 : 27. All the chemicals were sealed into a teflon-lined stainless steel autoclave (23 ml). The autoclave was transferred into a box furnace and heated up to 220 °C for 24 hours and then slowly cooled down to 160 °C at a cooling rate of 3 °C/h and further cooling down to room temperature in 24 hours before the furnace was switched off. The resulting products were washed with hot water and filtered. Yellow block shaped crystals Rb3[(UO2)3(BO3)2O(OH)](H2O) were obtained. The yield of

Rb3[(UO2)3(BO3)2O(OH)](H2O) is around 46% based on U content. EDS elemental analysis on selected single crystals of Rb3[(UO2)3(BO3)2O(OH)](H2O) showed an average molar ratio of U : Rb = 1.0 : 1.06, which is in good agreement with the proposed chemical compositions of its single crystal structure solution.

3.2.1.7 Synthesis of 9. K4Sr4[(UO2)13(B2O5)2(BO3)2O12] was prepared via a high temperature solid state method. UO2(NO3)2(H2O)6 (0.1016 g, 0.20 mmol), SrCO3 (0.0298 g, 0.20 mmol), K2B4O7(H2O)4 (0.1241 g, 0.40 mmol) and H3BO3 (0.1282, 2.07 mmol), were mixed in a ratio of U : Sr : K : B = 1 : 1 : 2 : 5. The reactants were ground in an agate mortar thoroughly, and then transferred to a platinum crucible. The reaction mixtures were then heated up to 980 °C in a box furnace. The holding time was 5 hours. Then the sample was cooled down to 550 °C at a cooling rate of 5 °C/h before the furnace was switched off.

Yellow tablet shaped crystals K4Sr4[(UO2)13(B2O5)2(BO3)2O12] were obtained. Pure polycrystalline samples of K4Sr4[(UO2)13(B2O5)2(BO3)2O12] were synthesized quantitatively by the reaction in a mixture of UO2(NO3)2(H2O)6 (0.3035 g, 0.6046 mmol), K2CO3 (0.0131 g, 0.0936 mmol), SrCO3 (0.0278 g, 0.1875 mmol), H3BO3 (0.0177 g, 0.2801 mmol) with a molar ratio of 13 : 2 : 4 : 6 at 900 °C for 2 days. The product obtained was characterized using laboratory powder X-ray powder diffraction (PXRD) which indicated a high purity. EDS elemental analyses on several single crystals of this compound gave an average molar ratio of U : K : Sr = 13.0 : 4.06 : 4.13, which is in good agreement with the proposed chemical compositions.

3.2.2 Instrumental studies including Single Crystal (Table in all text of the work) and Powder XRD, SEM/EDS Analysis, Thermal Analysis and Raman Spectroscopy have been performed as described in Chapter 2. Additionally Bond-Valence Analysis was done according to the method described in Chapter 2.

3.3 Results and Discussion 3.3.1 Syntheses. Three different methods were applied for the preparation of novel uranyl borates in this part of the work. Among the nine compounds, 1, 2, 5 and 8 were prepared via hydrothermal synthetic method with different mineralizers, Pb(NO3)2 was used for 1 and 2, whereas Zn(NO3)2 is for 5. For the preparation of 8 no additional mineralizer was used. Noting that, all the boron source of 1, 2 and 5 is from

29 2- the tetraborate (B4O7) (Li2B4O7 for 1, 2 and K2B4O7 for 5). Interestingly, all five compounds adopted 2D layered structural architectures based on the different reaction conditions. From the current literature

[130], a number of alkali metal uranyl borates were isolated from mild temperature (ca. 200 °C) H3BO3 flux reactions. The synthesis of 8 was performed using the starting ratio of UO2(NO3)2(H2O)6 : H3PO3 :

RbOH : H3BO3 : H2O of 1 : 4 : 17 : 15 : 27. Excluding H3PO3 from the reaction with the starting ratios of 1 : 10 : 15 : 27, 1 : 17 : 15 : 27 and 1 : 20 : 15 : 27, the preparations of 8 was unsuccessful. We supposed that H3PO3 is a very important pH value controlling medium in the synthetic process. Compared to Wang’s boric acid flux method [130], we have used more water as reaction medium in hydrothermal synthesis. The final structural motif of 8 is totally different with the phases that prepared by Wang.5 The structural topology of 8 is more resemble to the structures that obtained from solid state synthesis. This probably indicates that the reaction medium (water) is dominated over polymerization of borate units from boric acid flux reaction conditions [130]. Compound 3 was prepared from the typical high temperature solid state synthesis (980 °C) with tetraborates as the reaction medium like

Sr[(UO2)2(B2O5)O]. Interestingly, not like the 3D framework structure of Sr[(UO2)2(B2O5)O], 3 has a 2D layered structure. We presumed that a nature of cations have played a key role in the formation of these two compounds. 4 was obtained from HT/HP (4GPa, 1000 °C) method, which is a more extreme synthetic condition than that was used Wu et al. (~200MPa, ~650 °C) for synthesis of

K4[(UO2)5(BO3)2O4]·H2O [98]. However, both compounds possess identical 2D layered structures but crystallized in different space groups.

Table 3.1a. Crystal Data and Structure Refinements for the compounds 1-5.

Compound 1 2 3 4 5 FW 344.84 351.78 1688.17 1688.17 1641.97 Space group Pbam Cmca C2/c Pbam Pbam a (Å) 11.3498(9) 6.8683(13) 15.714(3) 13.3612(3) 13.3853(5) b (Å) 14.5959(18) 15.459(4) 11.7415(10) 11.8625(3) 23.9949(8) c (Å) 6.8790(5) 11.231(3) 13.339(2) 6.8466(2) 6.9698(5) α (deg) 90 90 90 90 90 β (deg) 90 90 123.690(15) 90 90 γ (deg) 90 90 90 90 90 V (Å3) 1139.58(19) 1192.5(5) 2047.8(5) 1085.17(5) 2238.6(2) Z 8 8 4 2 4 λ( Å) 0.71073 0.71073 0.71073 0.71073 0.71073 F(000) 1160 1184 2824 1412 2736 -3 Dc(g cm ) 4.020 3.919 5.476 5.167 4.872 GOOF on F2 1.125 1.131 0.862 1.164 1.067 R1 0.0680 0.0652 0.0346 0.0382 0.0446 wR2 0.1708 0.1317 0.0954 0.1087 0.0951 2 2 2 2 2 ½ R1 = Fo -Fc/Fo, wR2 = {w[(Fo) - (Fc) ] /w[(Fo) ] } 30 Table 3.1b. Crystal Data and Structure Refinements for the compounds 6-9.

Compound 6 8 9 FW 4367.66 1232.12 4530.13 Space group Pnma P21/c P-1 a (Å) 26.3286(7) 18.5598(7) 6.7530(5) b (Å) 14.3683(7) 12.6453(3) 13.1869(11) c (Å) 16.1841(4) 13.3695(7) 15.9242(12) α (deg) 90 90 76.024(7) β (deg) 90 101.140(4) 87.248(6) γ (deg) 90 90 75.401(7) V (Å3) 6122.4(4) 3078.6(2) 1331.54(18) Z 4 8 1 λ( Å) 0.71073 0.71073 0.71073 F(000) 7288 4136 1886 -3 Dc(g cm ) 4.738 5.317 5.649 GOOF on F2 1.067 1.142 0.956 R1 0.0446 0.0521 0.0374 wR2 0.0951 0.1023 0.1002 2 2 2 2 2 ½ R1 = Fo -Fc/Fo, wR2 = {w[(Fo) - (Fc) ] /w[(Fo) ] }

Phases of 6, 7 and 9 were prepared from high temperature solid state methods at around 1000 °C with

K2B4O7(H2O)4 as a flux. We presumed that the reaction medium K2B4O7(H2O)4 is of great importance for the structure formation of K4Sr4[(UO2)13(B2O5)2(BO3)2O12]. We suggest that counter cations with larger radii have a higher tendency to be introduced into the final structure. For the synthesis of 6 and 7 the starting chemicals of UO2(NO3)2∙6H2O, RbNO3/CsNO3 and B2O3 with a ratio of U : Rb : B = 1 : 4 : 5 were used. The substitution of RbNO3/CsNO3 by LiNO3, NaNO3 and KNO3 with keeping the elements I ratio (U : M : B = 1 : 4 : 5) and synthetic conditions, leads to a formation of early described M(UO2)(BO3) (M = Li, Na, K) phases [101]. It is clear that the size of counter cations is playing a key role for the final composition of the material and its structure architecture.

3.3.2 Description of Crystal Structures and Topologies

Structures of 1 and 2. 1 and 2 are stoichiometrically identical with the phases A(UO2)(BO3) (A = Li, Na, K, Rb, Cs) reported previously [101], but they are not isostructural with them. 1 crystallizes in the + centrosymmetric space group Pbam. There are two hydronium cations (H3O) , two boron and two uranium atoms in the asymmetric unit. 2 crystallizes in the space group of Cmca, with one Li atom, one B and one U atom in the asymmetric unit. 1 and 2 possess identical topology of layers. This topology is well-known for inorganic uranium phases and often called as uranophane anion topology. Same topology has been observed in the structure of layers in K(UO2)(BO3) [101]. The structure is based on 2D anionic 31 + + corrugated uranyl borate layers, which are arranged along the c-axis and H3O or Li cations reside in the interlayer space (see Figure 3.1b, 3.1c). Besides, water molecules are also in the interlayer space of 2. The presence of water molecules was confirmed by Raman spectroscopy (see Figure 3.7b). Accordingly, the layered distances for 2 are ca. 7.73 Å larger than that for 1 with ca. 7.15 Å. Uranium atoms are seven-fold coordinated as UO7 pentagonal bipyramids, which is edge sharing connected into 1D uranyl chains along the a-axis. The BO3 triangles are corner or edge sharing linked to the uranyl chains, forming 2D uranyl borate layers of parallel to the ac-plane (see Figure 3.1a, 3.1e). The anion-topology of 2 is shown in Figure 1d. The axial U=O bond distances for 1 are in the range from 1.74(3) Å to 1.80(3) Å. The equatorial U-O bond lengths are in the range of [2.30(3) - 2.47(2) Å]. The axial U=O bond lengths for 2 range from 1.73(2) Å to 1.73(3) Å and the equatorial U-O bond distances are in the range of [2.36(3) - 2.282(18) Å]. Bond valence sums (BVS) suggest that the U cation is 6+ in both structures, the U(1) BVS of 5.97 v.u. and U(2) of 6.02 v.u. in 1. BVS of U(1) is 6.07 v.u. in 2. The B cation is 3+ with B(1) and B(2) BVS of 2.98 and 3.06 v.u. for 1 and B(1) BVS of 3.03 v.u. in 2. The BVS of Li is 1.13 v.u., which indicate that the oxidation state of Li is 1+.

Figure 3.1. (a) A basic 2D uranyl borate sheet of compounds 1 and 2, (b) view of the structure 1 along the c-axis and (c) 2 along the a-axis, (d) anion topology observed of the uranyl borate sheet for 1 and 2, (e) a FBB [(UO2)(BO3)] of the 2D UB sheet of 1 and 2 with ball-and-stick mode. Uranyl polyhedra, [(UO2)O5] pentagon in anion topology and BO3 triangles are shown in yellow, gray and green, lithium and oxygen are shown in blue and red, respectively.

The Li+ cations are four-fold coordinated. They occupy the interlayer space for charge balancing the anionic layers. The Li-O bond lengths are in the range 1.92(8) - 2.06(6) Å. The LiO4 tetrahedra are vertex sharing along the a-axis, forming 1D Li-O chains, with Li-Li bond distances of ca. 3.43 Å. The

32 introduction of Li+ cations results in the increase of the interlayer distance, but with no change of the topology of the layers.

It is noteworthy that in the family of A(UO2)(BO3) (A = H3O, Li, Na, K, Rb, Cs), H3O, Li, Na and K- compounds share a common layered structural motif. However, Rb and Cs-ones have a different and more complex layered topology, which implied that cations’ radii have played a key role for the layered structural architectures. Noting that Li, Na and K-compounds possess the same sheet topology as 1 and 2, but the layered distances of 1 and 2 are much larger than that in Li- (ca. 5.5 Å), Na- (ca. 5.7 Å) and K- phases (ca. 6.1 Å). It is presumed that with an increase of counter cations radii, the interlayer distance + increases in a direct proportion. However, H3O and H2O made the layered distances larger than that observed in Li, Na and K-phases. We presume that this is an effect of the week van der Waals interaction between these molecules in the interlayer space. The specific positions of water molecules in the structure of 2 (Cmca) increase the symmetry of this phase compared to Li(UO2)(BO3) (P21/c). Structures of 3 and 4. The compounds 3 and 4 have identical chemical compositions but crystallized in different space groups (C2/c and Pbam). Both phases possess layered structures with identical uranyl borate sheet topology. The 2D uranyl borate layers of 3 are based on three crystallographically independent uranium atoms and one boron atom. Two of the uranium atoms, U(1) and U(3) are seven- fold coordinated. They exist as distorted pentagonal bipyramids. The U(2) cation has contact with two O 3– atoms from the [BO3] polyanions. U(1)O7 and U(3)O7 pentagonal bipyramids are connected alternately through sharing edges and vertex. One U(2)O4 tetragonal bipyramid shares common vertex with U(1)O7 and common edge with U(3)O7, enclosed forming the U(1)-U(3)-U(1)-U(3)-U(2) 5-MRs in the 2D uranyl sheets on bc-plane. BO3 triangles are edge sharing with U(3)O7 and vertex sharing with U(3)O7 within the

2D layers. U(2)O6 were observed in the uranium 5-MRs and created the 2D uranyl borate layers parallel to the (100) plane. O(1), O(3), O(7), O(9) and O(10) are μ3-oxygens shared by three uranyl groups or two uranyl units with one BO3 planar triangle (see Figure 3.2).

33 Figure 3.2. (a) A basic 2D uranyl borate sheet of compound 4 on bc-plane, (b) view of the structure 4 along the b-axis, (c) a FBB [(UO2)5(BO3)2] of the 2D UB sheet of 4 and the ball-and-stick mode (d). Uranyl polyhedra and BO3 triangles are shown in yellow and green, potassium and oxygen are shown in blue and red, respectively.

In order to see the 2D uranyl borate layered structure more clearly, the FBB of the 2D sheets together with the anion topology were provided on Figure 3.2c, 3.2d and Figure 3.3. U(1)O7 and U(3)O7 polyhedra shared the edge to form a (U2O12) dimer. BO3 triangle is further edge sharing with this U2O12 dimer forming a (BU2O13) uranyl borate group. Two centrosymmetric (BU2O13) groups further bridged by

U(2)O6 polyhedron constructing the FBB of this structure (see Figure 3.2c). In compound 4, distances of the axile U=O bonds are observed to be 1.803(14)-1.81(2) Å, whereas the equatorial U-Oeq bond lengths of U-O are in the range of 2.25(2) - 2.450(16) Å. The B-O bond distances in BO3 triangles are from 1.31(3) Å to 1.38(3) Å and the bond angles of O-B-O are in the range of [116(2)° - 123(2)°]. The BVS of U(1), U(2), U(3) and B(1) are 5.98 v.u., 5.96 v.u., 6.02 and 3.03 v.u., respectively. This indicates that the U cations are 6+ and B cations are 3+. For 3, the U=O axile bond lengths are in a larger range than that of compound 4 and ranging from 1.793(10) Å to 1.829(10) Å. The bond distances of U-Oeq are observed for [2.170(10)-2.437(10) Å], which are shorter than that in compound 4. The B-O bond lengths are from

1.349(19) to 1.394(19) Å, which is a longer average B-O distances than that in 4. The BO3 groups are nearly regular planar triangles with bond angles from 117.2(13)° to 121.2(13)°. The BVS results of U(1), U(2), U(3) and B(1) are 6.01 v.u., 5.97 v.u., 6.04 v.u. and 2.98 v.u., respectively. These imply that the oxidation states of U cations are 6+ and B cations are 3+ [119-120]. K+ cations are resided in the interlayer space of both structures for balance a charge of the anionic layers.

It is worth to compare the structure of 3, 4 and K4[(UO2)5(BO3)2O4](H2O) [98], because they have the identical uranyl borate layeres. However, there are still some obvious differences, such as space groups,

34 unit cell volumes and interlayer distances. In K4[(UO2)5(BO3)2O4](H2O), the H2O molecules which are in the interlayer space cause a crystallization in a lower symmetry than 4.

Figure 3.3. Anion topology of layers in 3 and 4. U(2)O2O4 squares, U(1)O2O5, U(3)O2O5 pentagons and BO3 triangles are shown in gray, dark grey and green, respectively.

Structure of 5. 5 crystallizes in the orthorhombic space group of Pbam, with a more complex structural topology than the four compounds described above. It contains five unique U6+ sites U(1)-U(5) and two independent boron atoms B(1)-B(2) in the asymmetric unit. It has to be noted that the compounds 3, 4 and 5 have the same U/B molar ratio of 5 : 2 in an symmetric unit. However, 5 has different topology of layered structure compared to 3 and 4. All uranium sites in 5 are existing as UO7 pentagonal bipyramids.

Both boron atoms (B1 and B2) adopted three-fold oxygen coordination, exhibiting as coplanar BO3 triangles. The FBB of the uranyl borate layers have the same stoichiometric ratio with 3 and 4 (see Figure

3.4a, 3.4c). U(1) and U(5) share the common edge created a U2O12 dimer. The B(1)O3 triangle linked to the dimer U(1)-U(5) and produced a uranyl borate group [B(UO2)2]. The U(4)O7 polyhedra further bridged two centrosymmetric [B(UO2)2] groups via sharing edges and corners, and forming the FBB. O(2) are μ4-oxygens shared by three uranyl groups and planar BO3 triangles, which can be observed in

K15[(UO2)18(BO3)7O15] [98] (see Figure 3.4d). However, O(8), O(9), O(10), O(11), O(12), O(14), O(15) and Ow1 are μ3-oxygens. The axial U=O bond distances are from 1.759(18) Å to 1.790(14) Å. The bond lengths of U-Oeq are in the range of [2.189(16)-2.861(14) Å]. The B-O bond lengths are observed for 1.34(3) to 1.38(3) Å, and the O-B-O bond angles are from 116(2)° to 124(2)°. A new 4U-MRs in the 2D uranyl sheets of 5 can be observed. Additionally, the structure consists of 5U- MRs, which are similar to that in 3 and 4. The distances between the uranyl borate layers are ca. 6.98 Å and are larger than that in 3 and 4. K+ cations are located between the layers as well as the water molecules (see Figure 3.4b). The K+ cations are six or seven-fold coordinated with K-O bond distances from 2.776(14) to 2.986(19) Å. We presumed that the larger size of K+ cations and the existence of

35 disordered water molecules make the distance between the layers larger than that in the structure of 3 and 4.

Figure 3.4. (a) A 2D uranyl borate sheet of compound 5 on ab-plane, (b) the structure 5 along the a-axis, (c) the FBB [(UO2)5(BO3)2] of the 2D UB sheet of 5 and the ball-and-stick fashion (d).

The new topology of 2D layers in 5 was observed for the first time. To describe the new topology, we 2- simplified the [(UO2)5(BO3)2O3] layers of 5 by omitting the oxygen anions and kept the connectivity between the B-U cations. The simplified U-B net was observed as a new 7-nodal net topological type with 5 5 6 6 3 7 7 7 a point symbol of {3 . 4 }2{3 . 4 .5 }3{3 . 4 . 5 }2 [133-135]. The topology of the 2D sheets can also be depicted using the anion topology method [136]. The 2D anion sheets of 5 are constructed by pseudo P chains. Here one pentagon linkes to two other pentagons, denoted as Ps chains and are shown in dark grey. P chains are shown in grey and to be bridged with modified R chains. The R chains are based on two isolated BO3 triangles, and denoted as Rb chains which are shown in green in Figure 3.5. This shows that compound 5 has a complex distorted sayrite (Pb2(UO2)5O6(OH)2·4H2O) topology compared to that in 3 and 4. This anion topology is built on P chains, alternating pentagon units in the Ps chains where the positions of squares are substituted by two sorts of BO3 triangles within the R chains with the following sequence.…PRbPsPRbPsPRbPs…[137]

36 Figure 3.5. Anion topology observed in 5. U(2)O2O5, U(3)O2O5 pentagons, U(1)O2O5, U(4)O2O5 U(5)O2O5 pentagons and BO3 triangles are shown in gray, dark grey and green, respectively.

Structure of 6 and 7. Compounds 6 and 7 were obtained from high temperature solid state method. The structure of 7 is not publishable, due to the nonmerohedral twinning, but this one is a full analogous of 6. We have picked several crystals from different syntheses, unfortunately, all of the thin pallet crystals contained twinning components with the refinement of R1 value approximately 10%. 6 crystallizes in the orthorhombic Pnma space group and adopts a novel uranyl borate layered structure. It contains six crystallographically unique U and four B atoms in the asymmetric unit. Uranium atoms exhibit two different oxygen coordination environments, which are distorted UO6 tetragonal and UO7 pentagonal bipyramids. The axial U=O bond distances in the UO7 pentagonal bipyramids are from 1.75(2) Å to

1.784(19) Å. The Ueq-O bond lengths are in the range of [2.170(18)-2.579(14) Å]. The O=U=O bond angles range from 177.0(8) ° to 178.7(8) °. Whereas for the UO6 tetragonal bipyramids, the U=O bond lengths are slightly shorter than that in UO7 and range from 1.75(2) to 1.78(2) Å. The Ueq-O bond distances are in the range of [2.170(18)-2.419(16) Å] and the O=U=O bond angles are 177.5(9) °. The B- O bond lengths are observed from 1.32(3) to 1.42(3) Å and the O-B-O bond angles are from 111(2) ° to 131(3) °. The Rb-O bond distances are in the range of [2.70(4)-3.47(2) Å]. The BVS of U(1)-U(6), B(1)- B(4) and Rb(1)-Rb(8) are 6.11 v.u., 5.99 v.u., 5.96 v.u., 5.91 v.u., 6.01 v.u., 6.06 v.u., 3.07 v.u., 3.10 v.u., 2.95 v.u., 2.98 v.u. and 1.13 v.u., 0.96 v.u., 0.97 v.u., 1.05 v.u., 0.98 v.u., 1.09 v.u., 1.06 v.u., and 0.99 v.u., respectively, which implied that the U cations are 6+, B cations are 3+ and Rb cations are 1+ oxidation states. 6- The FBB, namely [(UO2)12(BO3)8O3] , of the negatively charged uranyl borate layers is shown in Figure

3.1c, 3.1d. U(1)O7-U(6)O7 polyhedra shared corners or edges forming a U6-hexamer, in which a 5U-MR can be observed on the ac-plane. The B(2)O3 triangles are reside in the 5U-rings via shared edges or corners with U(3)O7, U(4)O6 and U(6)O7 polyhedra. B(1)O3, B(3)O3 and B(4)O3 triangles are further linked with U(1)O7, U(2)O7, U(3)O7, U(4)O6 and U(5)O7 polyhedra through sharing edges or corners, and creating the FBB. Each FBB shares vertexes or edges with seven neighboring ones, and polymerized into

37 6- + the unique 2D uranyl borate sheets {[(UO2)12(BO3)8O3] }n parallel to ac-plane (see Figure 3.6a). The Rb cations are located in the interlayer space as well as the water molecules. We supposed that water molecules residing in the interlayer space are absorbed from air during slow cooling process. This assumption is based on the synthetic method that we applied (the temperature of the synthesis was 980 °C). It is obvious that water molecules can’t exist in the structure at such a high temperature and ambient pressure. Therefore the only way is water absorption to enter the interlayer space during the process of crystal cooling.16 The presence of water in molecular form was confirmed by the broad weak vibrational peak around 3000 cm-1 in the Raman spectrum (see Figure 3.6).

Figure 3.6. (a) A 2D uranyl borate sheet of compound Rb6[(UO2)12(BO3)8O3]·(H2O)6 on ac-plane, (b) view of the structure down the b-axis, (c) the FBB [(UO2)6(BO3)4] of the 2D UB sheet of Rb6[(UO2)12(BO3)8O3]·(H2O)6 and the ball-and-stick fashion (d). Uranyl polyhedra and BO3 triangles are shown in yellow and green, rubidium and oxygen are shown in blue and red, respectively.

The topology of Rb6[(UO2)12(BO3)8O3]·(H2O)6 is relatively complex, which can be described using the anion topology method proposed by Burns [136]. Two basic different fragments can be separated within the 2D anion uranyl borate sheets as shown in Figure 3.7a, 3.7b. The first fragment is composed of two centrosymmetric S-shaped finite P-chains as shown in Figure 3.7c, where each chain contains five pentagons (shown as dark gray). Two BO3 triangles are encircled by the two P-chains (denoted as P1, P2 chains). The second fragments can be designated as Rt clusters (shown as two gray squares shared two

BO3 triangles and linked four additional ones in the rest corners). The stacking mode of

Rb6[(UO2)12(BO3)8O3]·(H2O)6 can be described as the anion topology of [P1P2Rt]n (see Figure 3.7c). In order to see the new topology of the uranyl borate sheets more clearly, we simplified the 2D layers of

Rb6[(UO2)12(BO3)8O3]·(H2O)6 by omitting the oxygen anions and kept the B and U links. The simplified

38 2 3 3 3 4 5 4 2 5 5 5 5 5 6 6 3 7 7 7 cationic net is a 10-nodal net topology (3 .4 .5){(3 .4 )(3 .4 .5 .6 )(3 .4 .5 )(3 .4 )2(3 .4 .5 )(3 .4 .5 )3. From the point symbol view, the topology of this uranyl borate is quite complex and comparable with the reported phases previously.

Figure 3.7. Anion topology observed in Rb6[(UO2)12(BO3)8O3]·(H2O)6. UO2O5 pentagons, UO2O4 and BO3 triangles are shown in dark grey, grey and green, respectively.

Structure of 8. Rb3[(UO2)3(BO3)2O(OH)]·(H2O) crystallizes in the monoclinic centrosymmetric space group P21/c. There are three uranium, two boron and three rubidium atoms in the asymmetric unit, all of them show standard crystallographic environment. The uranyl U=O bond distances range from 1.771(19) to 1.85(2) Å for all the distorted pentagonal bipyramids, whereas for the equatorial interactions range from 2.224(16) to 2.544(17) Å. The O=U=O bond angles are in the range of [175.5(8)-178.4(8) °]. The B-

O bond lengths for the BO3 triangles are in the range of [1.24(4)-1.40(3) Å] and the O-B-O bond angles are from 108(2) ° to 128(3) °. The Rb-O bond distances are observed range from 2.78(2) to 3.62(2) Å. The BVS of U(1)-U(6), B(1)-B(4) and Rb(1)-Rb(6) are 6.09 v.u., 5.94 v.u., 6.12 v.u., 5.98 v.u., 5.91 v.u., 6.01 v.u., 3.02 v.u., 3.06 v.u., 2.93 v.u., 2.97 v.u. and 1.07 v.u., 0.97 v.u., 0.99 v.u., 1.02 v.u., 0.91 v.u., 1.05 v.u., respectively, indicated that the U cations are 6+, B cations are 3+ and Rb cations are 1+ oxidation states. 4- The structure is based on a novel 2D uranyl borate layers {[(UO2)3(BO3)2O2] }n, with a U/B ratio 3 : 2, 4- composed of UO7 pentagonal bipyramids and BO3 triangles. The FBB, [(UO2)3(BO3)2O2] of

Rb3[(UO2)3(BO3)2O(OH)]·(H2O) is provided in Figure 3.8c, and it is not as complex as that observed in

Rb6[(UO2)12(BO3)8O3]·(H2O)6. U(1)O7, U(4)O7 and U(6)O7 or U(2)O7, U(3)O7 and U(5)O7 pentagonal bipyramids shared common edges or corners with each other connected into U3 trimers (see Figure 3.8c).

Two BO3 triangles further bridged the U3 trimers via shared edges or corners within the unique FBBs. The FBB is connected with the six neighboring ones, further polymerized into the 2D infinite uranyl borate layers paralleled to the ab-plane. Noting that, O(20), O(22), O(23), O(26), O(27), O(28), O(29),

39 O(36) and O(38) are µ3-Os, the rest are µ2-Os and no µ4-Os compared to K4Sr4[(UO2)13(B2O5)2(BO3)2O12]. Rb+ cations are resided in the interlayer space for charge balance.

Figure 3.8. (a) A basic 2D uranyl borate sheet of compound Rb3[(UO2)3(BO3)2O(OH)]·(H2O). on ab- plane, (b) view of the structure along the b-axis, (c) a FBB [(UO2)3(B2O7)O2] of the 2D UB sheet and the ball-and-stick mode (d). Uranyl polyhedra and BO3 triangles are shown in yellow and green, rubidium and oxygen are shown in blue and red, respectively.

One can separate the 2D uranyl borate layers of Rb3[(UO2)3(BO3)2O(OH)]·(H2O) in two different fragments using anion topology method [136], as shown in Figure 3.9. The first fragments are typical P- chains composed of edge sharing linked UO7 pentagonal bipyramids (shown as dark grey pentagon). The second fragments can be designated as quasi-U(D) chains (denoted as Ut or Dt chains, shown as one triangle linked each pentagon in both sides of U or D chains). The stacking mode of

Rb3[(UO2)3(BO3)2O(OH)]·(H2O) can be described as distorted protasite [138] Ba(UO2)3O3(OH)2·(H2O)3 sheet anion topology with additional BO3 triangle linkers on U(D) chains, i.e., …PUt PDtPUt PDt…. (see Figure 3.9). For the purpose of describing the new topology of the sheets more clearly, we simplified the uranyl borate layers of Rb3[(UO2)3(BO3)2O(OH)](H2O) by omitting the oxygen anions and only kept the connectivity between the B-U cations. The simplified B-U net was observed as a 10-nodal net topology 2 3 2 2 2 3 2 3 2 3 2 3 3 3 5 4 4 2 5 5 5 7 7 7 (3 .4.5 )(3 .4 .5 )(3 .4 .5 .6 )2(3 .4 .5)2(3 .4 .5 .6) (3 .4 .5 .6 )(3 .4 .5 )(3 .4 .5 ). From the point symbol view, we can observe that the cationic topology of Rb3[(UO2)3(BO3)2O(OH)]·(H2O) is not as complex as that in the structure of Rb6[(UO2)12(BO3)8O3]·(H2O)6 and K4Sr4[(UO2)13(B2O5)2(BO3)2O12]. It is worth to compare the structural relationships of the rubidium uranyl borates family. There are only three known synthetic phases, Rb2[(UO2)2B13O20(OH)5], Rb[(UO2)2B10O16(OH)3]·(H2O)0.7 and

Rb(UO2)(BO3) [97, 101]. The first two compounds were obtained from a low temperature with excess boric acid flux method, in which uranium only exist as one independent UO8 hexagonal bipyramid in the 40 structure. The UO8 polyhedra are surrounded by three BO3 triangles and six BO4 tetrahedra forming the common uranyl borate structural motif [130]. Whereas the uranium atoms in both Rb(UO2)(BO3) and

Rb3[(UO2)3(BO3)2O(OH)]·(H2O) displayed as UO7 pentagonal bipyramids. But only one crystallographically unique U-site in Rb(UO2)(BO3) encircled by four equal BO3 triangles, and three unique U-sites in Rb3[(UO2)3(BO3)2O(OH)]·(H2O), corner or edge sharing linked with three different planar BO3 triangles. These facts make the anion topology of Rb3[(UO2)3(BO3)2O(OH)]·(H2O) is the most complex one in this uranyl borates series.

Figure 3.9. Anion topology observed in Rb3[(UO2)3(BO3)2O(OH)]·(H2O). UO2O5 pentagons and BO3 triangles are shown in dark grey and green, respectively.

Structure of 9. The compound K4Sr4(UO2)13(B2O5)2(BO3)2O12 crystallizes in the low-symmetry triclinic P-1 space group. This is the first example of mixed alkali and alkali-earth metal uranyl borate, which has a completely different structural arrangement compared to previously reported phases [10, 83, 101, 130] .

Structure of K4Sr4(UO2)13(B2O5)2(BO3)2O12 is based upon seven uranium, three boron, two potassium and three strontium atoms in an asymmetric unit. The axial bond lengths of U=O for the uranyl polyhedra are in the range of [1.802(11)-1.847(10) Å], whereas for the equatorial U-O bond distances range from 2.189(10) Å to 2.993(7) Å. The O=U=O bond angles are in the range of [174.9(5)-179.999(1) °]. The B-O bond lengths for the BO3 triangles are in the range of [1.33(2)-1.40(2) Å] and the O-B-O bond angles are from 117.4(14) ° to 126.5(14) °. K-O bond distances range from 2.661(12) Å to 3.190(12) Å, whereas for Sr-O bond lengths are in the range of [2.578(13)- 2.967(12) Å]. The BVS of U(1)-U(7) and B(1)-B(3) are 5.96 v.u., 5.93 v.u., 6.05 v.u., 5.91 v.u., 5.99 v.u., 6.07 v.u., 5.95 v.u., and 3.05 v.u., 2.97 v.u., 3.03 v.u., respectively, suggested that the U cations are 6+ and B cations are 3+ oxidation states [121].

K4Sr4[(UO2)13(B2O5)2(BO3)2O12] features a complex 2D layered structural topology, which is based on the uranyl borate layers parallel to the (111) plane (see Figure 3.10a, 3.10b). The uranyl borate sheets are composed of distorted UO8 hexagonal, UO7 pentagonal bipyramids and co-planar BO3 triangles. In order 12- to present the layered structure more clearly, the FBB, namely, [(UO2)13(B2O5)2(BO3)2O12] of its uranyl borate sheet was provided in Figure 4.5c. Within the FBB, U(6)O8 polyhedra is the inversion center was 41 7- surrounded by two centrosymmetric subgroups [(UO2)6(B2O5)(BO3)O5] (see Figure 3.10c). The subgroups are constructed by six UO7 polyhedra, one B2O5 dimer and one B(1)O3 triangle shared corners or edges with each other. The two subunits are further bridged by the U(6)O8 hexagonal bipyramid via edges sharing. The FBBs polymerized, forming the unique 2D infinite uranyl borate layers 12- [(UO2)13(B2O5)2(BO3)2O12] paralleled to the (111) plane (see Figure 3.10c, 3.10d). Noting that, O(6), + 2+ O(9), O(13), O(16), O(20), O(21), O(22) and O(25) are µ3-Os, whereas O(24) is µ4-O. K and Sr cations are located in the interlayer space for charging balance.

Figure 3.10. (a) A basic 2D uranyl borate sheet of compounds K4Sr4[(UO2)13(B2O5)2(BO3)2O12], (b) view of the structure along the [1-1 0] direction, (c) FBB: [(UO2)13(B2O5)2(BO3)2] of the 2D UB sheet with polyhedra (c) and ball-and-stick (d) fations. Uranyl polyhedra, [(UO2)O6] hexagons and [(UO2)O5] pentagons and BO3 triangles in anion topology are shown in cyan, yellow and green, potassium, strontium and oxygen are shown in blue, light blue and red, respectively.

The topology of the 2D layers in K4Sr4[(UO2)13(B2O5)2(BO3)2O12]was isolated firstly in the uranyl borates family. The topology of the 2D uranyl borate sheets also can be described using the anion topology method proposed by Burns et al. (1996) [136] (see Figure 3.11a, 3.11c). The sheet anion topology of

K4Sr4[(UO2)13(B2O5)2(BO3)2O12] is exceedingly complex. It can be separated into two different fragments based on pentagon and hexagon parts, as shown in Figure 3.11c. The first fragment is based on distorted and corrugated infinite pentagon chains from edge or corner-shared UO7 pentagonal bipyramids (shown in gray in Figure 3.11a). These chains can be designated as Pd-chains according to Burns et al. The hexagon fragment is based on infinite edge or corner-shared UO8 hexagonal bipyramids, which can be designated as Ht chains (shown as dark gray in Figure 3.11a). These two fragments are edge sharing with

42 each other and further linked with BO3 triangles or B2O5 dimers, can be described as … Pd Ht Pd Ht Pd

Ht…[137]. For the purpose of analyzing the new topology, the simplified cationic net of 12- [(UO2)13(B2O5)2(BO3)2O12] by omitting the oxygen anions and only the links between B and U cations are kept. The simplified net was observed as a new 10-nodal net topological type with a point symbol of 10 10 7 3 3 5 5 6 4 6 6 3 6 6 6 3 7 7 6 7 7 7 (3 .4 .5 .6)(3 .4 )2(3 .4 )2(3 .4 )2(3 .4 .5 )6(3 .4 .5 .6 )2(3 .4 .5 .6)2(3 .4 .5 )2 .

It is interesting to compare the structure of K4Sr4[(UO2)13(B2O5)2(BO3)2O12] with structures of

K4[(UO2)5(BO3)2O4]∙(H2O) [98] and Sr[(UO2)5(BO3)2O2(OH)2]∙5H2O [10], because all of them are layered uranyl borates and have close U/B ratios and chemical compositions. Two different coordination geometries of U atoms can be observed in all three phases but these are UO6 and UO7 polyhedra in

K4[(UO2)5(BO3)2O4]∙(H2O) and Sr[(UO2)5(BO3)2O2(OH)2]∙5H2O, and UO7 and UO8 polyhedra in

K4Sr4(UO2)13(B2O5)2(BO3)2O12. Besides, in K4Sr4[(UO2)13(B2O5)2(BO3)2O12] borate groups are not only presented by isolated BO3 triangles, but B2O5 dimers are also existing in its structure. The various coordination environments of uranium atoms, and complex borate groups in

K4Sr4[(UO2)13(B2O5)2(BO3)2O12], making its 2D uranyl borate sheet topology much more complex than that in K4[(UO2)5(BO3)2O4]∙(H2O) and Sr[(UO2)5(BO3)2O2(OH)2]∙5H2O.

Figure 3.11. Anion topology representation of the layer observed in K4Sr4[(UO2)13(B2O5)2(BO3)2O12]. UO2O6 hexagons, UO2O5 pentagons and BO3 triangles are shown in dark grey, grey and green, respectively.

43 3.3.3 Comparison of the Structural Complexity in the 2D Uranyl Borates Family Based on the FBBs. Uranyl borates present extremely rich structural chemistry based on the variety of uranium coordination chemistry and borates groups [6, 10, 92]. However, due to the equatorial coordination nature of the uranyl 2+ unit (UO2) , uranyl borates mostly adopt the lower condensed 2D layered structures. All the borates units that reported in this work exhibited planar BO3 triangles. Thus, the coordination chemistry of uranyl polyhedra has dominated the structural complexity of the 2D layered topology. Here, we derive topologically and metrically possible finite clusters (FBBs) to describe the complexity of the infinite uranyl borate layers.

After comparison, we can observe that compounds UO2B2O4 and M(UO2)(BO3)(M = H3O, Li, Na, K, Rb, - Cs) series have the simplest topologies of the layer with FBB [(UO2)(BO3)] , in which only contain one 4- uranyl polyhedra and one borates group. The FBBs of 3 and 4 are same as [(UO2)5(BO3)2O4] , which is 4- composed of one UO6, four UO7 polyhedra and two BO3 triangles. FBB [(UO2)5(BO3)2O4] can be observed in the structure of K4[(UO2)5(BO3)2O4]∙(H2O) [98] and Sr[(UO2)5(BO3)2O2(OH)2]∙5H2O [10]. 2- [(UO2)5(BO3)2O3] is the FBB of compound 5. The U/B ratios in the FBB of 3, 4 and 5 are the same as 5 :

2, but their geometry are totally different. In the FBB of 3 and 4, the substitution of UO6 by UO7 polyhedra in the center, give rise to the FBB of 5. The FBB of 5 is observed for the first time in the 2D 4- uranyl borates family [6, 10]. The FBB of 8 is [(UO2)3(BO3)2O2] , which has the second simplest topological geometries based upon three uranyl polyhedra and two borates units. Two different local groups can be observed in this structure. First one is based on a single UO7 pentagonal bipyramid and three BO3 triangles, and the second one is based on one UO7 and two BO3 groups. The above mentioned local configurations can be also found in the structures of Sr[(UO2)5(BO3)2O2(OH)2]∙5H2O [10] and 6- K12[(UO2)19(UO4)(B2O5)2(BO3)6(BO2OH)O10]·nH2O [132] The FBB [(UO2)6(BO3)4O3] of

Rb6[(UO2)12(BO3)8O3](H2O)6 is as complex as uranyl borates of K4[(UO2)5(BO3)2O4]  H2O,

K10[(UO2)16(B2O5)2(BO3)6O8]  7H2O [132] and Sr[(UO2)5(BO3)2O2(OH)2]5H2O [10]. Their FBBs are composed of three different BBUs, which are UO7, UO6 polyhedra and BO3 triangles. As shown in Figure 6- S6c, the uranium configurations of [(UO2)6(BO3)4O3] can be observed in the structure of

Rb3[(UO2)3(BO3)2O(OH)](H2O), but absent in K4Sr4[(UO2)13(B2O5)2(BO3)2O12]. Notably, clusters based on four groups (one UO7 pentagonal bipyramid and three BO3 triangles) are uncovered firstly in the 2D uranyl borates family [6, 10].

The FBB of K4Sr4(UO2)13(B2O5)2(BO3)2O12 is one of the most complex one compared to

K13[(UO2)19(UO4)(B4O10)(BO3)6(OH)2O5]·H2O [132] and K15[(UO2)18(BO3)7O15] [98] (see Table 3.2). In 12- total, seven different geometries can be observed in [(UO2)13(B2O5)2(BO3)2O12] . The geometry of U(1) is the simplest (one UO7 polyhedra and one BO3 triangle). Such geometry also can be found in the

44 structures of K12[(UO2)19(UO4)(B2O5)2(BO3)6(BO2OH)O10]·H2O and K4[(UO2)5(BO3)2O4]·H2O. The geometry that composed of two groups (one UO7 pentagonal bipyramid and one B2O5 dimer) is uncovered for the first time [6, 10]. Clusters based on four groups (UO7, UO8 polyhedra, BO3 triangles and B2O5 dimers) occur in five different configurations. To the best of our knowledge, these configurations are uncovered firstly and have not been observed in known inorganic uranium compounds [6, 10]. The rich local geometrical configurations of the uranium centers observed in

K4Sr4[(UO2)13(B2O5)2(BO3)2O12], making its structural topology very rare, which is one of the most complex one in the 2D uranyl borates family to date.

It is noteworthy to compare the structural topology of Rb6[(UO2)12(BO3)8O3](H2O)6 and

Rb3[(UO2)3(BO3)2O(OH)](H2O) with roubaulite, Cu2[(UO2)3(CO3)2O2(OH)2](H2O)4 [139], due to same molar ratios of U/B and U/C and similar triangular geometries of BO3 and CO3 groups. The FBB of 6- roubaulite is [(UO2)3(CO3)2O4] and it contains two uranyl pentagonal and one hexagonal bipyramids and two carbonate triangles. These FBBs are further connect each other by edge sharing and forming a 1D structural topology in Cu2[(UO2)3(CO3)2O2(OH)2](H2O)4 (see Figure S8). In contrast, the FBB in 6- Rb6[(UO2)12(BO3)8O3](H2O)6 is [(UO2)6(BO3)4O3] . It is composed from five UO7, one UO6 polyhedra and four BO3 triangles. These FBBs are further edge and vertex sharing with the neighboring ones, connected into the infinite 2D layered structures. The FBB in Rb3[(UO2)3(BO3)2O(OH)](H2O) is 4- [(UO2)3(BO3)2O2] and it is based upon three UO7 polyhedra and two BO3 triangles. These FBBs are further interlinked into a 2D infinite uranyl borates layers. A comparison of FBBs and their linkage in studied borates and roubaulite indicates that structural topology of uranyl borates is more complex than in carbonates. However, It has to be noted that a number of complex uranyl carbonates is limited compared to the recent achievements in uranyl borates chemistry.

3.3.4 Thermal Analyses. The thermogravimetric (TG) and differential scanning calorimetry (DSC) curves of 3 and 5 polycrystalline in the temperature range of 50 to 1200 ºC are shown in Figure 3.7. There is no obvious weight loss from the TG curve of 3. The DSC curve shows a strong endothermic peak at around 943 ºC, which corresponds to the melting of the compound (see Figure 3.12a). TG analysis shows that 5 has a weight loss around 200 °C, which corresponds to the eliminating of 4.0 mol water molecules per formula unit, where a endothermic peak at 255 °C is observed in the DSC measurement. The mass loss observed from the TG curve 4.15% is in agreement with the calculated one (4.38 %). The endothermic peak at 1112 °C and 1161 °C correspond to the melting and decomposition of the dehydrated product (see Figure 3.12b). There is no obvious weight loss from the TG curve. The DSC curve shows a strong endothermic peak at around 1130 ºC, which corresponds to the melting of the compound (see Figure 3.12c). A large 45 endothermic peak located at around 1170 °C corresponds to the sample decomposition. In the TG curve over the temperature range around 1170 °C, there is a small amount of weight loss, which implied an incongruent melting [10]. For confirming this, we have calcinated the powder sample at 1190 °C for 10 h. The measured PXRD powder patterns for the calcinated sample are different from those of the unheated one. Hence, K4Sr4(UO2)13(B2O5)2(BO3)2O12 melt incongruently. After calcination,

K4Sr4(UO2)13(B2O5)2(BO3)2O12 became amorphous and the residuals were not characterized further.

(c) Figure 3.12. TG-DSC curves of 3, 5 and 9.

3.3.5 Raman Analyses. The Raman spectra of the 1, 2 and 5 were measured in a range of 100-4000 cm-1 and for 3 and 4 in the range of 100-1600 cm-1. It is obvious that the low frequency part between 100 and 300 cm−1 where the modes are contributed from the vibrations of the “lattice skeleton”. The Raman spectra of 1 and 2 as shown in Figure 3.13a, 3.13b, reflecting the iso-typic layered structure relationship of these, only with

46 little difference can be observed. There are more scattering peaks with stronger intensity in the section (100-1600cm-1), which is dominated by the vibration modes of the uranium polyhedra. For these two compounds, the Raman spectra show strong and sharp bands around 800 cm-1 due to the symmetric vibration υ1 mode of the uranyl ion with short uranyl U-O bond lengths around 1.80 Å [96]. Raman bands with very weak peaks around 1293 and 971 cm-1 have been assigned to the asymmetric and symmetric + stretching υ1, υ3 modes of the B–O bonds in BO3. The vibration modes from H3O cation with weak peaks are observed at around 2950 and 3054 cm-1, whereas for peaks around 3540 cm-1 are originating from the vibration of coordinated water molecules in 2. The Raman spectra of isotypes layered structure 3 and 4 are shown in Figure 3.13c, 3.13d. The peaks in the range of 730-860 cm-1 could be assigned to the symmetric stretching υ1 mode of the uranyl groups in both compounds. The Raman bands within 980- -1 1200 cm can be attributed to the asymmetric and symmetric stretching modes of the B–O bonds in BO3 triangles. The doubly degenerated in-plane O–B–O bending mode at around 530–700 cm−1 and the doubly degenerated asymmetrical stretching modes are in the range 1250–1500 cm−1. The Raman spectrum of 5 -1 shows a strong and sharp peak at 836 cm can be assigned to the symmetric vibration υ1 mode of the uranyl group. The Raman bands in the range of 480–680 cm−1 are attributed to the doubly degenerated O– −1 B–O bending mode in BO3 triangles. The weak peaks between 900 and 1150 cm are caused from the −1 symmetrical stretching mode of BO3 units. The strong Raman bands in the range of 1270–1500 cm can be assigned to the doubly degenerated asymmetrical stretching mode of BO3 groups. The vibrational modes of coordinated water molecules are located at the bands of 1600 -1800cm-1 and 3040cm-1 (see Figure 3.13e). These assignments are in consistent with the previously reported works [10, 96]. The Raman spectra of the four new uranyl borates 6-9 were measured in a range of 100-4000 cm-1 as 2+ shown in Figure 3.14. All four compounds are constructed from linear uranyl groups (UO2) and BO3 triangles or B2O5 dimers, thus their vibrational modes are quite close with each other as observed in the Figures. More vibrational modes can be observed in the first section compare to the second one, which is dominated by the vibration of uranium polyhedra. Raman bands in the lower frequencies from 190-210 -1 cm are originated from the contribution of the uranyl υ2 bending mode. The doubly degenerated in-plane −1 O–B–O bending υ4 mode in the BO3 triangles are at around 510–700 cm and the doubly degenerated asymmetrical stretching mode is in the range of 1250–1500 cm−1, the corresponding B-O bond lengths from 1.32(3) to 1.42(3) Å. The Raman spectra of all phases displayed strong and sharp peaks in the range -1 2+ of 810-840 cm . This can be attributed to the symmetric vibration υ1 mode of the uranyl (UO2) cation with short uranyl U-O bond lengths around 1.80 Å. Whilst, the relatively weak peaks at 3543 cm-1 (see Figure 3.14b), 2800-3000 cm-1 (see Figure 3.14c), and 3156 cm-1 (see Figure 3.14d) come from the vibrational modes of coordinated water molecules in the structures of Rb3[(UO2)3(BO3)2O(OH)](H2O) and A6(UO2)12(BO3)8O3((H2O)6 (A = Rb, Cs). Raman bands with very weak peaks around the range of 47 -1 990-1200 cm have been attributed to the asymmetric and symmetric stretching (υ1, υ3) modes of the B–

O–B bonds in the BO3 triangles, with bond lengths around 1.30 Å. These assignments are in consistent with the previously reported works [10, 96].

Figure 3.13. Raman shifts of compounds 1-5.

Figure 3.14. Raman shifts of compounds 6-9. 48 3.4 Conclusions

A series of uranyl borates, namely, (H3O)(UO2)(BO3) (1), Li(UO2)(BO3)·(H2O) (2), α-K4[(UO2)5(BO3)2O4]

(3), β-K4[(UO2)5(BO3)2O4] (4), K2(UO2)5(BO3)2O3·(H2O)4 (5) A6[(UO2)12(BO3)8O3]·(H2O)6 (A = Rb and

Cs) (6 and 7), Rb3[(UO2)3(BO3)2O(OH)]·(H2O) (8) and K4Sr4[(UO2)13(B2O5)2(BO3)2O12] (9) have been obtained from different conditions and well characterized. 1 and 2 are stoichiometrically identical and - comprise 2D [(UO2)(BO3)] units isolated by hydronium or lithium cations. It is interesting to note that a slight change of the initial reagents and the constant use of the same mineralizer, leads to no change of the structural topology from 1 to 2. However, changing the mineralizer from Pb(NO3)2 to Zn(NO3)2, give rise 2- to the most complicated 2D layered structure [(UO2)5(BO3)2O3] of 5 among the first five uranyl borates, due to the various local geometrical configurations of uranyl cations. This fact demonstrates that the mineralizer plays a key role to receive the final topology structure of uranyl borates. The simplified U-B net of 5 was observed as a new 7-nodal net topological type with a point symbol of 5 5 6 6 3 7 7 7 {3 .4 }2{3 .4 .5 }3{3 .4 .5 }2. Two polymorphs of K4[(UO2)5(BO3)2O4] (3 and 4) compared to 5 have shown the structural flexibility of the layered potassium uranyl borates. 3 and 4 own the identical 2D 4- uranyl borate anionic topology layers of [(UO2)5(BO3)2O4] , but 4 has a much higher symmetry than 3, which can be explained by the HT/HP conditions. Compounds 6, 7 and 9 were obtained from high temperature solid state method, whereas 8 was obtained from a mild hydrothermal synthesis. The preparation of 8 has a similar reaction temperature with Wang’s syntheses [130], but we have used more water medium in the reactions. The final structure is totally different from the uranyl borates reported by Wang et al. However, the structure feature of 8 is resemble to the phases obtained from solid state syntheses. This demonstrated that water medium has played a key role for the final structure of uranyl borates in the hydrothermal synthesis. It seems that the increase of water content in the reaction is resulted in formation of 2D layered structures similar to that prepared from solid state syntheses. Compound 9 is the first mixed alkali-alkaline earth metal uranyl borate. Its FBB 12- [(UO2)13(B2O5)2(BO3)2O12] is the most complex one to the previously reported 2D uranyl borates, in which UO8 hexagonal and UO7 pentagonal bipyramids connected by BO3 triangles and B2O5 dimers. The various uranium local configurations resulted in the most complex 2D layered uranyl borate topology to date. We presumed that the mixed cations (K1+ and Sr2+) make a vital contribution to the overall structure. Both 6 and 8 have an identical U/B molar ratio of 3 : 2, however, they possess the different FBBs within 4- their layered structures. The FBB of 8 is [(UO2)3(BO3)2O2] , composed of UO7 polyhedra and isolated 6- BO3 triangles, whereas the FBB for 6 is [(UO2)6(BO3)4O3] , consisted of UO6, UO7 polyhedra and BO3 triangles. The rich coordination chemistry of uranium in 6 makes its structure more complex than that of 8. In this study, several new FBBs of uranyl borates have been found in the A-U-B-O system for the first time. For comparison, the FBBs of the high temperature solid state phases are more complex than that 49 from hydrothermal synthesis. We supposed that the coordination chemistry of uranium favored to be more complex in the high temperature melts. This study also shed light on the structural flexibility and variety of uranyl borates at different reaction conditions.

50 Chapter 4. Influence of Synthetic Conditions on Chemistry and Structural Properties of Alkaline Earth Uranyl Borates

4.1 Introduction

Chemistry and structural properties of alkali metal uranyl borates have been widely characterized. As it has been demonstrated in chapter 1, roughly 30 Phases have been studied. In the previous chapter, more information was added for better understanding of chemistry in this system. However, to date, only two alkaline earth metal uranyl borates have been reported, namely, Mg(UO2)B2O5 and Ca[(UO2)2(B2O5)O]

[86, 88]. These phases have been prepared via synthesis in high temperature B2O3 flux. The structures of these are significantly different compared to the alkali metal uranyl borates. In order to extend our knowledge in this system, a systematic study of phase formation and their structures has been performed. Here we report the synthesis of four novel alkaline earth metal uranyl borates, namely,

A[(UO2)5(BO3)2O2(OH)2] 5H2O (1, 2) and A[(UO2)2(B2O5)O] (3, 4) (A = Sr, Ba). These have been structurally characterized and future investigated using, thermogravimetric and differential scanning calorimetry as well as Raman spectroscopy.

4.2 Experimental Section

4.2.1 Materials and Methods. Uranyl nitrate UO2(NO3)2∙6H2O (International Bioanalytical Industries,

Inc.), strontium nitrate Sr(NO3)2 (Alfa-Aesar, 99.9%), strontium carbonate SrCO3 (Alfa-Aesar, 99.9%), barium nitrate Ba(NO3)2 (Alfa-Aesar, 99.9%), lithium tetraborate Li2B4O7 (Alfa-Aesar,99.9%), and boric acid H3BO3 (Alfa-Aesar, 97%) were all used as received. 4.2.1.1 Syntheses of 1 and 2. Both compounds were synthesized by a hydrothermal method. The compositions UO2(NO3)2∙6H2O (0.0526 g, 0.10 mmol), Sr(NO3)2 (0.0212 g, 0.10 mmol), H3BO3 (0.0621 g, 1.05 mmol), Li2B4O7 (0.0426 g, 0.25 mmol) and deionized water (2 ml), in a ratio of U : Sr : B = 1 : 1 :

20 for 1, UO2(NO3)2 ∙ 6H2O (0.0526 g, 0.10 mmol), Ba(NO3)2 (0.0262 g, 0.10 mmol), H3BO3 (0.0496 g,

0.80 mmol), Li2B4O7 (0.0426 g, 0.25 mmol) and deionized water (2 ml) in a ratio of U : Ba : B = 1 : 1 : 18 for 2, were sealed into teflon-lined stainless steel autoclaves (23 ml) and then transferred into a box furnace, heated to 220 °C, held 24 hours and then cooled to room temperature with a rate of 3 °C/h. The resulting products were washed with hot water and then rinsed with ethanol to remove excess boric acid. Yellowish parallelogram and triangular pallet shaped crystals (1 and 2) were obtained as well as unknown orange needle shaped crystals with bad qualities. The yields of (1 and 2) are 52% and 47%, respectively, based on U content. Fine crystals were collected for further analyses. Due to the nonmerohedral twinning of crystals 2, as well as, partial occupation of barium positions, the refinement is not as good as for other

51 studied phases. Energy dispersive X-ray spectroscopy (EDS) elemental analysis on several single crystals of both compounds gave average molar ratios of Sr : U = 1 : 4.57 and Ba : U = 1 : 4.72, respectively for 1 and 2, which are in good agreement with those obtained from single crystal X-ray diffraction studies.

4.2.1.2 Synthesis of 3. The starting composition of UO2(NO3)2∙6H2O (0.0528 g, 0.10 mmol), Sr(NO3)2

(0.0216 g, 0.10 mmol), H3BO3 (0.0619 g, 1.05 mmol) and Li2B4O7 (0.0253 g, 0.15 mmol) with a ratio of U : Sr : B = 1 : 1 : 16 for 3. All the reactants were thoroughly ground in an agate mortar and then transferred to a platinum crucible. The reaction mixtures were heated up to 965 °C for 10 hours in a box furnace and then cooled down to 450 °C at a cooling rate of 5 °C/h before the furnace was switched off. Orange block crystals, 3, were obtained, along with yellow plate-shaped crystals which were determined 7 to be LiUBO5 . Pure polycrystalline samples of 3 were synthesized quantitatively by the reaction of a mixture of UO2(NO3)2∙6H2O (0.2028 g, 0.40 mmol), SrCO3 (0.0297 g, 0.20 mmol), H3BO3 (0.0247 g, 0.40 mmol) with a molar ratio of 2 : 1 : 2 at 900 °C for 2 days. The obtained product was characterized with X- ray powder diffraction (XRD) and Rietveld refinement. It indicates that 3 was obtained with a high purity and the yield is over 96% (see Figure 6). EDS elemental analyses on several single crystals of this compound gave an average molar ratio of Sr : U = 1 : 1.89, which is in good agreement with the proposed chemical compositions.

4.2.1.3 Synthesis of 4. A mixture of UO2(NO3)2∙6H2O (0.0532 g, 0.12 mmol), Ba(NO3)2 (0.0266 g, 0.11 mmol), H3BO3 (0.0316 g, 0.52 mmol), Li2B4O7 (0.0852 g, 0.51 mmol) and deionized water (1.5 ml) in a ratio of U : Ba : B = 1 : 1 : 25 was sealed into a teflon-lined stainless steel autoclave (23 ml) and transferred into a programmable furnace, heated to 220 °C held 36 hours and then cooled to room temperature with a rate of 4 °C/h. The product was washed with boiling water to remove excess boric acid before being rinsed with ethanol. Orange block shaped crystals were collected. The yield of 4 is a little bit lower compare to the other phases described above, which is around 39%, based on U content. EDS analysis on several single crystals gave an average molar ratio of Ba : U = 1 : 1.91, which is in good agreement with its proposed chemical compositions.

4.2.2 Instrumental studies including Single Crystal (Table in all text of the work) and Powder XRD, SEM/EDS Analysis, Thermal Analysis and Raman Spectroscopy have been performed as described in Chapter 2. Additionally Bond-Valence Analysis was done according to the method described in Chapter 2.

52 Table 4.1. Crystal Data and Structure Refinements for 1, 2, 3 and 4. a

Compound 1 2 3 4 FW 1711.47 1761.17 745.30 795.01 Space group C2/c C2/c C2/m C2/m a (Å) 18.2199(6) 18.3785(15 ) 16.7901(5) 16.8063(9) b (Å) 8.9303(2) 8.8531(3) 8.3533(2) 8.3444(4) c (Å) 16.1231(6) 17.9181(11) 6.6557(2) 6.6524(3) α (deg) 90 90 90 90 β (deg) 117.502(4) 123.823(10) 98.032(2) 98.084(5) γ (deg) 90 90 90 90 V (Å3) 2325.95(13) 2425.3(3) 924.32(4) 923.66(8) Z 4 4 4 4 λ( Å) 0.71073 0.71073 0.71073 0.71073 F(000) 2832 3059 1248 1288 -3 Dc(g cm ) 4.807 5.232 5.356 5.784 GOOF on F2 1.072 1.175 0.796 1.100 R1 0.0468 0.0661 0.0259 0.0536 wR2 0.1321 0.1687 0.0699 0.1297 a 2 2 2 2 2 ½ R1 = Fo -Fc/Fo, wR2 = {w[(Fo) - (Fc) ] /w[(Fo) ] }

4.3 Results and Discussion 4.3.1 Syntheses. Exploration of the A-U-B-O (A = Sr, Ba) system, uncovered four novel alkaline earth metal uranyl borates 1, 2, 3, and 4. The compounds 1, 2 and 4 were obtained through mild hydrothermal synthesis at 220 °C. It is noteworthy that the H3BO3 played the role of reagent and flux in these syntheses as previously described by Wang et al. [130]. The importance of the starting ratio of UO2(NO3)2∙6H2O :

Sr(NO3)2 : H3BO3 : Li2B4O7 was examined and reactions with the ratios 1 : 1 : 4 : 2.5, 1 : 1 : 10 : 2.5 and 1: 1 : 16 : 2.5 were investigated, however only with the ratio 1 : 1 : 10 : 2.5 was 1 isolated. It is interesting to note that when the reagent ratio of UO2(NO3)2∙6H2O : Ba(NO3)2 : H3BO3 : Li2B4O7 was changed from 1 : 1 : 8 : 2.5 to 1 : 1 : 5 : 5, a second polymorph of 4 was obtained that has a 3D network rather than the 2D arrangement seen in 2. 3 was synthesized by traditional high temperature solid state synthesis using a reagent ratio of UO2(NO3)2 ∙ 6H2O : Sr(NO3)2 : H3BO3 : Li2B4O7 of 1 : 1 : 10 : 1.5. We performed a synthesis of SrUBO-2 without Li2B4O7, with initial ratios of UO2(NO3)2∙6(H2O) : Sr(NO3)2 : H3BO3 of 1: 1 : 8, 1: 1 :12 and 1: 1 :18 at 965 °C for four days. The only crystalline phase was a strontium uranate

SrU2O7. Thus, we presumed that Li2B4O7 is an important boron source in the reaction for preparation of SrUBO-2. We attempted to obtain 4 via a high temperature reaction replacing strontium nitrate by barium nitrate, however, this was unsuccessful. Thus it is presumed that both H3BO3 and Li2B4O7 are necessary for the synthesis, where they act as both reagents and reaction medium. The preparation of uranyl borates by traditional hydrothermal syntheses is known to be difficult due to the higher affinity of water, over borate, to coordinate to metal centers [130]. A feature of Wang’s 53 investigations [92, 94, 95, 97] is the use of a large excess of boric acid as flux (B/U ＞ 10) but with relatively small amounts of water (~ 0.05 mL), to prepare alkali metal uranyl borates using low- temperature (180 to 220 °C) flux methods. We have explored a similar experimental protocol for the alkaline earth metal uranyl borates system however our attempts, to date, have been unsuccessful. When

Li2B4O7 and much more water (＞1.5 ml) were used as initial reagents and medium in the present work, three alkaline earth metal uranyl borates were synthesized, for the first time, by mild hydrothermal methods at temperatures of around 220 °C. In these compounds the boron exclusively forms BO3 triangles, whereas, both BO3 triangles and BO4 tetrahedra are present in the alkali metal uranyl borates [130]. This indicates that boron tends to lower its coordination number when the relative amount of water is increased in the initial reaction mixture. Wu et al. studied the reactivity of several systems in high- temperature/high pressure (HT/HP) hydrothermal conditions (650 ± 5 °C and 200 ± 10 MPa), with minimal dilute KOH solution as medium inside gold capsules. This resulted in the synthesis of the first uranyl aluminoborate UO2[B3Al4O11(OH)] [105] and three potassium uranyl borates [98], in which further evidence for the stabilization of U (VI) within a tetraoxo core (UO4O2) was confirmed. This indicates that under hydrothermal conditions, reducing the relative amount of water to borate favors the generation of uranyl borate systems. 4.3.2 Structure Description. 1 and 2 are isostructural and crystallized in the monoclinic space group C2/c. The structure, illustrated for 1 in Figure 1, is based on 2D uranyl borate layers on the (-101) plane (Figure 4.1b), the molar ratio of U/B within these layers exceeds 1 with U : B = 5 : 2. It is tempting to ascribe this to the charge of the alkaline earth metal cation (Sr or Ba) since in the alkali metal uranyl borates described by Wang et al. [85-99], the U/B molar ratios are invariably less than 1. Most of alkali 2+ metal uranyl borates feature a common structural motif consisting of linear uranyl, UO2 , cations surrounded by BO3 triangles and BO4 tetrahedra to create UO8 hexagonal bipyramids. The borate anions bridge the uranyl units to create a layered structure. There are three crystallographically independent uranium atoms in the 1 structure. Two of these, U(2) and U(3) have a distorted pentagonal bipyramid 2+ geometry, while the U(1) cation displays the common linear uranyl U(1)O2 geometry involving two O 3– atoms from [BO3] polyanions. The tetragonal bipyramid coordination of U(1) is completed with two μ3- oxogroups to form a tetragonal bipyramid (Figure 4.1b). The corners and edges of the U(1), U(2) and U(3) polyhedra sharing with each other form five member rings (MRs) on (-101) plane. Isolated BO3 triangles, contained in the five MRs, share an edge with the U(3) pentagonal bipyramids, corners with the U(2) pentagonal and U(1) tetragonal bipyramids, forming a 2D uranyl layer parallel to (-101). The U–O bond lengths range from 1.782(5) to 1.806(10) Å for the axial O atoms and from 2.306(8) to 2.540(8) Å for the

54 equatorial oxygen atoms. Within the BO3 triangles, the B-O bond lengths are in the range of 1.354(15)– 1.409(15) Å and O–B–O bond angles range from 114.3(11)° to 125.4(11)°.

Figure 4.1. Coordination environments for the uranium and boron sites within 1 (a), a 2D uranyl borate layer along [1, 0, -1] (b) , view of the structure along the b-axis (c). Uranyl polyhedra and BO3 triangles are shown in yellow and green, strontium and oxygen are shown in blue and red, respectively.

BVS calculations indicate that three uranium cations are hexavalent with values of 5.86, 5.66 and 5.99 for U(1), U(2) and U(3) respectively. The BVS values suggest that the B cation is 3+ with the B(1) BVS of 2.97, and those for O(1) to O(10) are 1.81, 1.91, 1.90, 1.61, 1.96, 2.11, 1.84, 2.04, 1.82 and 2.05, respectively [121]. According to the BVS values and the coordination mode of oxygen atoms, together with the charge balance of the structure, we conclude that O(4) atoms are present as hydroxyl (OH-) groups and all others are O2-. The Sr/Ba cations are nine-fold coordinated and occupies the interlayer space, balancing the charge of the layers. The Sr-O and Ba-O bond distances are in the range 2.508(1)-2.773(10) Å and 2.61(6)-3.035(19) Å, respectively. It is further observed that increasing the size of alkaline earth cation results in an increase in the interlayer distance in these compounds from ~7.210 Å to ~7.681 Å in 1 and 2, respectively. Thus, the A site cations presumably do not influence the connectivity within the layers, however their size does determine the interlayer dimension. Along with the cations the interlayer space is also occupied by disordered water molecules. The presence of water is confirmed by Raman spectroscopy (see Figure 4.7).

55 2- In order to reveal the topology of the layers, we simplified the [(UO2)5(BO3)2O2(OH)2] sheets of 1 by removal of anions whilst retaining the connectivity between the cations. This simplified net can be 5 5 6 6 3 7 7 7 described as a new 4-nodal net topological type with a point symbol of {3 .4 }2{3 .4 .5 }3{3 .4 .5 }2 [133-135]. The topology of the compounds can also be described using the anion topology method proposed by Burns et al.[136]. In this the 2D anion sheet of 1 are depicted as pseudo P chains (with edge- sharing pentagons and squares, denoted here as Ps chains, shown in gray and dark grey) connected with modified R chains (based on isolated BO3 triangles, denoted here as Rt chains shown in green) in Figure

4.2. This demonstrates the present compounds have a distorted sayrite [Pb2(UO2)5O6(OH)2·4(H2O)] topology built on alternating square units in the P chains and BO3 triangles, instead of squares, in the R chains, i.e.…PsRtPsRtPsRt…[137]

Figure 4.2. Anion topology observed in 1. U(1)O2O4 squares, U(2)O2O5, U(3)O2O5 pentagons and BO3 triangles are shown in gray, dark grey and green, respectively.

It is worth comparing the structure of Sr[(UO2)5(BO3)2O2(OH)2]∙5H2O (1) with that of

K4[(UO2)5(BO3)2O4]∙(H2O) [98], which also has a U/B molar ratio of 5 : 2. K4[(UO2)5(BO3)2O4]∙(H2O) was obtained by a HT/HP hydrothermal method, and crystalized in the centrosymmetric space group

P21/c (No. 14). Like 1 it has a 2D layered structure consisting of two types of uranium polyhedra, namely

UO2O4 square bipyramids and UO2O5 pentagonal bipyramids. These polyhedra are connected with each other in a quite similar manner to that in 1, forming uniform five MRs along the a-axis which are + occupied by BO3 triangles via a corner or edge sharing motif. The six, seven-fold coordinated K cations are located in the interlayer space with bond lengths ranging from 2.639(3) Å to 3.227(6) Å. The 2D layers in the two structures are built on similar moieties, in K4[(UO2)5(BO3)2O4]∙(H2O) these are 4- 2- [(UO2)5(BO3)2O4] polyanions whereas in 1 they are [(UO2)5(BO3)2O2(OH)2] . However, their 2D layered anionic topology and interlayer distance are different. The K+ cations, that occupy the interlay space, have coordination numbers of six or seven in the latter and the Sr2+ cations are nine-coordinate in 1. The effective ionic radius of seven-coordinate K+ is 1.46 Å and 9-coordinate Sr2+is 1.31 Å. Despite this, 56 the interlayer distance in 1 is larger than that in K4[(UO2)5(BO3)2O4]∙(H2O), 7.210 vs 6.801 Å. This suggests that changes in the bonding of the interlayer cation are important and it is postulated that the K+ or Sr2+ cations drive changes in the anionic topology that are reflected it the interlayer separation.

3 and 4 are iso-structural with previously reported Ca[(UO2)2(B2O5)O] [86], and crystallize in space group C2/m. This crystal structure is based on a 3D framework that contains uranyl units with cation-cation interactions (CCIs). The CCIs allow one uranyl unit that is coordinated with another uranium metal center by an oxo-atom, to form 1D chain, 2D layer or 3D framework without the need for subordinate ligands. 2+ To date, more than 15 different types of CCIs among uranyl cations (UO2 ) have been recognized, demonstrating that CCIs play an important role in determining structural motifs in actinide compounds19.

There are three crystallographically unique uranyl units in 3, U(1)O6 and U(3)O6 forming tetragonal bipyramids and U(2)O7 having a pentagonal bipyramid geometry (Figure 4.3a). Two edge-sharing U(2)O7 polyhedra further connect with U(1)O6 tetragonal bipyramids through edge sharing (O6-O4), forming a

1D chain along the c-axis. The U(3)O6 tetragonal bipyramids connect with U(1)-U(2) chains via corner sharing (O7/O8) along the b-axis, forming a 2D uranyl layer (here noted as L1) on the bc-plane. The CCIs effectively turn uranyl polyhedra perpendicular to one another creating corrugated 2D uranyl layers L1 within the structure, as illustrated in Figure 4.3b. Note that the axial O(7)/O(8) oxygen atoms of the

U(1)O6 tetragonal bipyramids belong to one U(3)O7 pentagonal bipyramid, which means each U(1)O6 polyhedra is connected to one U(3)O7 polyhedra by a cation-cation bond, displaying a two-centered cation-cation bond feature (Figure 4.4). B(1)O3 and B(2)O3 triangles connect with each other forming

B2O5 dimers through corner sharing involving (O2) (Figure 4.3a). The B-O bond distances are in the range 1.325(18)-1.409(14) Å. On the ac-plane, B2O5 dimers corner share with U(1)-U(2) chains through equatorial oxygen atoms (O1, O4, O11) and corner share with U(3)O6 tetragonal bipyramids via axial oxygen atoms (O5), forming a 2D uranyl borate layer (here noted as L2) (Figure 4.3c). L1 and L2 vertically connect with each other through B2O5 dimers resulting in the 3D uranyl borate framework 2- [(UO2)2(B2O5)O] . Strontium cations are contained within the voids of the 3D framework structure (see Figure 4.3d). The bond lengths of Sr-O are in the range of [2.488(10) - 2.666(8) Å].

57 Figure 4.3. Coordination environments for the uranium sites and a B2O5 dimer in 3 (a), a 2D uranyl layer L1 along the a-axis (b), a 2D uranyl borate layer L2 along the b-axis (c) view of the structure along the c- axis (d). Uranyl polyhedra and BO3 triangles are shown in yellow and green, strontium and oxygen atoms are shown in blue and red, respectively.

The uranyl axial U-O distances for U(2) and U(3) within the L1 layers are normal and range from 1.786(10)-1.809(8) Å. The equatorial U-O distances range from 2.189(9)-2.450(9) Å. The uranyl unit for U(1) has slightly longer U-O bond lengths of 1.871(6)-1.879(9) Å. These U-O distances are not typical for uranyl oxo-atoms (O7/O8) contacts under CCIs18. The two atoms that are bridging to the uranyl units containing U(3) have typical bridging distances of 2.300(1) and 2.302(5) Å (see Figure 4.4). BVS calculations indicate that these three uranium cations are hexavalent with values of 6.02, 6.03 and 5.96 for U(1), U(2) and U(3), respectively. The BVS values for B1, B2 are 3.02 and 2.96 and of the oxygen anions O1 to O9 are 1.89, 1.96, 2.01, 1.98, 2.05, 1.96, 1.92, 2.12 and 2.06, are typical of the expected valence states of + 3 and -2 respectively [119-121].

Figure 4.4. An illustration of the CCI in the structure of 3. The axial uranyl bonds are shown as red cylinders and the equatorial coordinating uranium oxygen linkages are shown as orange cylinders. 58 4.3.3 Local geometrical configurations of boron and uranium centers. The diversity of the uranium coordination leads to the structural complexity observed in the uranyl borates [6]. Boron atoms are three fold coordinated in both structures with planar BO3 triangles. Consequently the local configuration of uranium is the main source of the difference between 1 and 3. Both these compounds have three crystallographically distinct uranyl units, which exist as UO6 tetragonal bipyramids and UO7 pentagonal bipyramids. The geometries found in these two structures are shown in Figure 5.5. There are six different geometries in total across the two 1 and 3 structures whereas in the alkali metal uranyl borates, obtained by Wang et al., [130] there is only one pattern of borate environment for the uranium atoms. The first three geometries of the present compounds (Figure 4.5a-c), are based on three groups (one uranyl polyhedron and two borate triangles), and are present in the structure of 1. These resemble that observed in the structure of K4[(UO2)5(BO3)2O4](H2O) [98], but the geometry presented in Figure 4.5c is absent leading to a different 2D anion topology (see Figure 4.2). The coordination type shown in Figure 5.5c can also be found in K12((UO2)19(UO4)(B2O5)2(BO3)6(BO2H)O10)(H2O)9.5 [98]. The geometry types for 3, shown in Figure 5d-f, are quite rare for uranyl borates compounds. All of them include B2O5 dimer groups and a single uranyl group. The configuration of Figure 6f shows the uranyl group projected parallel to the same plane as the two B2O5 dimers, which is uncommon in inorganic uranyl compounds.

2+ Figure 4.5. The geometries of uranyl group (UO2) center with BO3 or B2O5 groups in 1 (a, b, c) and 3 (d, e, f).

59 4.4.4 XRD Analysis. The structure of SrUBO-2 was refined against a monoclinic structure in the space group C2/m (see Figure 4.6). The refined values of the lattice parameters and atomic coordinates for the uranium and strontium sites were found to agree well with the single crystal diffraction solution (Table 4.1). The somewhat larger than typical atomic displacement parameters are believed to be a consequence of use of the domed sample holder, necessary to contain the radioactive U cations. A small number of weak unindexed peaks were observed at low 2θ angles, nevertheless the XRD analysis demonstrated the bulk sample was predominantly 3.

Figure. 4.6 Observed (black markers), calculated (red line) and difference (green line) profile for 3 recorded using XRD. The lower vertical markers indicate the allowed Bragg reflections according to the space group. For clarity the background has been removed.

4.3.5 Thermal Analysis. The Thermogravimetric (TG) and Differential scanning calorimetry (DSC) curves of 3 powders in the temperature range of 300 to 1200 ºC are illustrated in Figure 5.7. The DSC measurements show a strong endothermic peak at around 1115 ºC, which corresponds to the melting of the compound. A large peak located at around 1156 °C corresponds to sample decomposition. There is a small amount of weight loss in the TG curve over this temperature range suggesting incongruent melting [58]. To confirm this is a powder sample of 3 was calcinated at 1170 ºC for 10 h, resulting in the formation of a black solid. The XRD powder pattern for the calcinated sample is different from that unheated materials and reveals the formation of U3O8.

60 Figure 4.7. TG and DSC curves for compound 3.

4.3.6 Raman Analysis. 2+ The characteristic vibrations of uranyl (UO2) in aqueous solution are three normal modes, namely the υ1 -1 -1 symmetric stretch (approximately 860–880 cm ), υ2 bending mode (around 199-210 cm ) and υ3 -1 antisymmetrical stretch (from 930 to 960 cm ) [140-141]. The BO3 unit has four internal modes, the −1 symmetrical stretching (ν1) mode at around 900–1000 cm , the B–O bending (ν2) mode at around 650– −1 −1 800 cm , the doubly degenerated asymmetrical stretching (ν3) mode in the range 1250–1450 cm and the −1 doubly degenerated in-plane O–B–O bending (ν4) mode at around 590–680 cm [142-143]. Raman spectra of the new uranium borates were measured in a range of 100 – 4000 cm-1. For convenience, we can divide the spectra into two sections, a low frequency part 100 – 1000 cm-1 and a high-frequency region 1000 – 4000 cm-1. There are more scattering peaks with stronger intensity in the first section (100- 1000 cm-1), which is dominated by contributions from the modes of the uranium polyhedra (Figure 4.8), only small variations can be observed between the Raman spectra of 1 and 2 as shown in Figure 4.8, reflecting the iso-structural relationship of these. For these two compounds, the Raman spectra show -1 strong and sharp bands around 840 cm due to the symmetric vibration υ1 mode of the uranyl ion with short uranyl U-O bond lengths of 1.782(4)-1.806(6) Å [96]. Raman bands with very weak peaks around -1 1309 and 974 cm have been assigned to the asymmetric and symmetric stretching υ1, υ3 modes of the B–

O bonds in BO3. The vibrational modes from coordinated water molecules with weak peaks are observed at around 3500 and 3000 cm-1, respectively. For the iso-structural compounds 3 and 4, the peaks in the -1 range of 720-890 cm could be assigned to the symmetric stretching υ1 mode of the uranyl ion with the CCIs units inside. The Raman bands within 970-1200 cm-1 can be attributed to asymmetric and symmetric

61 stretching υ1, υ3 modes of the B–O–B bonds in B2O5 dimers. These assignments are consistent with previously reported works [96].

Figure. 4.8 Raman spectra of the studied compounds in 4000-100 cm-1 region.

4.4 Conclusions

The first four examples of strontium and barium uranyl borates, namely, A[(UO2)5(BO3)2O2(OH)2]∙5H2O

(1, 2) and A[(UO2)2(B2O5)O] (3, 4) (A = Sr, Ba) have been synthesized and structurally characterized. 1, 2 and 4 are the first examples of alkaline earth metal uranyl borates synthesized by mild hydrothermal methods. 3 was obtained through high temperature solid state synthesis. We suggest that this approach will allow the preparation of more alkaline earth metal uranyl borates. 1 and 2 are iso-structural compounds with a novel layered structure, compared to structures reported for alkali metal uranyl borates. The 2D cationic layer in 1 and 2 is a new 4-nodal net topological type with a point symbol of 5 5 6 6 3 7 7 7 {3 .4 }2{3 .4 .5 }3{3 .4 .5 }2. Changing the synthetic conditions resulted in the formation of another two new compounds that feature a 3D framework constructed with two vertical layers, containing CCIs groups, uranyl layers U(2)-U(1)=U(3) L1 and B2O5 dimers U(1)-U(2)-B(1)(B(2)-U(3) uranyl borate layers

L2. L1 and L2 are further vertically connected with each other through bridging B2O5 dimers, leading to a 2- 3D uranyl borate framework with the formula {[(UO2)2(B2O5)O] }n. This indicates that the ability of CCIs in increasing the dimensions of uranyl borates structure is exceptional. Additionally, it is

62 noteworthy to consider which role has water in inducing the formation of such crystal structures that may contain important radionuclides, for instance Sr90 and Ba137m.

63 Chapter 5. Complexity Reaches New Limits: A Zeolitic Uranium Borate

5.1 Introduction Inorganic open-framework materials and zeolites (microporous aluminosilicates) have attracted a considerable attention of scientists in the last few decades, because they have been widely used in many industrial areas, such as catalysis, ion-exchange/sequestration, gas adsorption and separation [144-147]. Actinide-bearing open-framework materials also have a potential relevance to the subject of long-term disposal for the spent nuclear fuel [130, 149, 150]. According to the current literature, aluminates, silicates, germanates and phosphates are typical representatives of these porous materials [151-155].

Boron, as one of the most widely studied elements due to its various connection types between BO3 triangle and BO4 tetrahedron units, has been broadly used for synthesizing inorganic porous materials [156-158]. To date, more than 100 borate mineral crystal structures and more than 1000 synthetic inorganic borates have been characterized [6,82, 159, 160]. In the borate family, the addition of extra oxo- anions, numerous classes of ternary borate compounds were produced, such as aluminoborates, borosilicates, borogermanates and borophosphates [57, 58, 161, 162]. However, borates with boron- oxygen multi-directional open framework structures are extremely rare to date.

PbB4O7, a non-linear optical material of lead tetraborate, possesses a 3D boron-oxygen open framework structure, in which only BO4 tetrahedra are present, assembling into a simple corner-linked tetrahedral network [163]. The boron network of PbB4O7 has one 6-MR open tunnels along the a, b-axis, with a diameter of ~4.5 × 4.5 Å. Actinide borates AnO2[B8O11(OH)4] (An = U, Np) [164] crystallize into non- centrosymmetric crystals with Cc space group, in which four crystallographic unique BO4 tetrahedra and

BO3 triangles share corners to form a borate framework with large 9-MRs pores. The borate framework also has three types of channels, but helical nature of polyborate chains and twisting interlayer borate groups tackle migration through these channels. Roughly three decades have passed since the open-framework uranyl minerals have been discovered

[165]. (C4N2H12)U2O4F6 is the first open framework actinide material was obtained with piperazine as a structure-directing agent by Halasyamani in 1999 [148]. After that, a series of organic templated actinide porous materials have been reported, such as [NC4H12]2[(UO2)6(H2O)2(SO4)7] [166],

(NH4)4[(UO2)5(MoO4)7](H2O)5, and some other uranyl sulfate or selenite with 18-crown-6 template [167] etc. Due to subtle nature of the organic molecules used during the synthesis, thermal stabilities and expenses of these microporous materials have limited their potential applications in industrial processes.

A remarkable inorganic cationic framework [ThB5O6(OH)6][BO(OH)2]2.5H2O was prepared by Wang et al. [108] through a low-temperature flux method. This thorium borate compound has a supertetrahedral

64 cationic framework and possesses rare extra framework borate anions, which are readily accessible for 2– 4– replacing by tetrahedral anions, such as chromate CrO4 or pertechentate TcO4 . In this chapter post-transitional metal (Pb) was used to substitute alkali or alkaline earth metal into the uranyl borate system. The novel phase is a zeolite like uranyl borate, LUBO, which contains multidimensional intersecting channels in its 3D open framework structure. The synthetic route, zeolite- type structural topology, thermal analysis and vibrational spectroscopic properties were investigated in detail.

5.2 Experimental Section

5.2.1 Materials and Methods. Uranyl nitrate UO2(NO3)2∙6H2O (International Bioanalytical Industries,

Inc.), lead nitrate Pb(NO3)2 (Alfa-Aesar, 99.5%), and lithium tetraborate Li2B4O7 (VWR chemicals, 99.0%). 5.2.1.1 Syntheses of LUBO: Compound LUBO was obtained from a typical hydrothermal reaction.

UO2(NO3)2∙6H2O (0.0516 g, 0.10 mmol), Pb(NO3)2 (0.0332 g, 0.10 mmol), Li2B4O7 (0.0515 g, 0.30 mmol) and deionized water (0.5 ml), in a ratio of U : Pb : B = 1 : 1 : 12, were sealed into a teflon-lined stainless steel autoclave (23 ml) and then transferred into a box furnace, heated up to 220 °C, held 36 hours and then cooled down to 160 °C at a rate of 3 °C/h and then with a cooling rate of 6 °C/h cooled down to room temperature. The resulting products were washed with hot water and then rinsed with ethanol. Large yellow sphere-shaped crystals of LUBO were obtained. The crystals photograph is shown in Figure 1b, the largest one with a diameter of as large as 0.2 mm. Fine crystals were collected for further analyses. Energy dispersive X-ray spectroscopy (EDS) elemental analysis on several single crystals gave average molar ratios of Pb : U = 1.02 : 1.00 for LUBO, which are in good agreement with those obtained from single crystal X-ray diffraction data analysis.

5.2.2 Instrumental studies including Single Crystal (Table in all text of the work) and Powder XRD, SEM/EDS Analysis, Thermal Analysis and Raman Spectroscopy have been performed as described in Chapter 2. Additionally Bond-Valence Analysis was done according to the method described in Chapter 2.

65 Table 5.1. Crystal Data and Structure Refinements for LUBO.

Formular unit (H2O)Pb3(UO2)3B14O27 (LUBO) Formula weight /g mol-1 2033.04

Space group P63/m a (Å) 10.9331(2) c (Å) 11.9337(3) γ (deg) 120 V (Å3) 1235.36(4) Formula units/cell (Z) 2 λ( Å) 0.71073 F(000) 1728 -3 Dc(g cm ) 5.460 GOOF on F2 1.06 R1 0.0482 wR2 0.1077 2 2 2 2 2 ½ R1 = Fo -Fc/Fo, wR2 = {w[(Fo) - (Fc) ] /w[(Fo) ] }

5.3 Results and Discussion

5.3.1 Synthesis. While investigating the PbO-UO3-B2O3 system under hydrothermal conditions, we obtained a novel lead uranyl borate with zeolite-like structure. For the reaction, the initial molar ratio of the reagents equal to 1 : 1 : 3 for UO2(NO3)2∙6H2O, Pb(NO3)2, and Li2B4O7, respectively, was used with addition of ~0.5 ml of water, afforded single crystals of LUBO. Noteworthy that the reactions in similar synthetic conditions with slightly altered initial molar ratios, viz. 1:1:1, 1:1:2, and 1:1:4, did not lead to the formation of this compound. The highest yield of the reaction, which was achieved by increasing of reaction time up to four days, is about 55% (based on U). The purity of the resulting phase was confirmed by means of powder XRD diffraction. Further increase of the reaction time does not provide any reasonable improvement of the yield. It is noteworthy that we have used quite similar method in the alkaline earth metal uranyl borates system, however, alkaline earth metal cations substituted by Pb2+ has led to the novel porous zeolite like structure of LUBO. We suppose that the Pb2+ cation with lone electron pairs has played a template role for the forming of this zeolite typed structure, akin to organic templates in synthesis of zeolites or polymicroporous structures [166-167]. As mentioned earlier, the method in the present work is unlike low-temperature boric acid flux method in the work of Wang et al. [130], Wu et al.’s high-temperature/high-pressure (HT/HP) hydrothermal syntheses [98] or slow evaporation method under Zhang et al. [96]. Compared to them, we have provided another new facile route to prepare uranyl borates.

66 5.3.2 Crystal Structure and Topology Description. LUBO crystallizes in the hexagonal space group

P63/m (No. 176). The asymmetric unit contains single unique U and Pb atoms, three independent boron 12– and seven O atoms. The main structural unit in this compound is a novel (B14O27) oxo-borate open framework that incorporates uranyl and lead cations. The uranium atoms have UO8 coordination 12– polyhedra geometry in the shape of hexagonal bipyramid. 3D anionic borate framework, (B14O27) , contains both BO3 triangles and BO4 tetrahedra (see Figure 5.1).

Figure 5.1. View the structure of LUBO along the c-axis. Uranyl polyhedra, BO3 triangles and BO4 tetrahedra, lead cations, water molecules are shown as yellow, green, blue and pink, respectively.

The uranyl borates with alkaline and alkaline earth cations reported so far generally contain molecular, chain, or layer structures, with only few exceptions of 3D frameworks [6, 10]. In LUBO, the borate framework can be described as tubules connected through triangular borate groups BO3. The tubule consists of 6-MRs formed by six BO4 groups (Figure 5.2a), each ring is linked then to the successive one by three diborate groups B2O7 (see Figure 5.2b). The tubules are extending along the translation c. The connection between the tubules is provided by the triangular borate groups, which share an oxygen atom with the bridging B2O7 dimers (see Figure 5.2c and 5.2d). The framework therefore contains channels with 6-MR windows along the translation c, and three 8-MR channels along the [100], [010], and [110] directions. A simplified anionic representation of the borate framework is shown on the Figure 5.2a'–d'.

BO3 groups have the shape of almost regular triangle with B-O bond lengths of 1.371(8) Å and O–B–O bond angles of 119.98(6)°.

67 12- Figure 5.2. Construction of the anionic borate framework [(B14O27) ]. (a) a B6O24 cluster, (b) B6O24 cluster is connected by B2O7 dimers into a tubule extending along [001] direction, (c) BO3 triangles 12- connect to the tubules (d) a view on 3D anionic borate framework [(B14O27) ] along the c-axis, (a'-d') corresponding representations of anion topology.

In the structure of LUBO, the BO3 triangles are parallel and arranged by the covalent framework, so that there are pairs of stacked BO3 groups. Although the exact nature of these contacts is unclear, the presence of an interaction between the boron atoms of the BO3 group pairs is supported by both Hirshfeld and Voronoi-Dirichlet polyhedra (VDP) methods (see Figure 5.3). The VDP solid angle corresponding to the B···B contact is equal to 6.5% (for comparison, the faces that correspond to the B-O bonds are 28.7% each), the distance between the boron atoms is ~2.96 Å, which is ~0.86 Å lesser than the doubled boron van der Waals radius according to Alvarez [168]. The remaining two crystallographic sorts of boron atoms, B(1) and B(2), form slightly distorted BO4 tetrahedra with B-O distances in the range of 1.452(15)-1.509(15) Å and O–B–O angles varying from 101.6(9)° to 114.0(10)°, consistent with the previously reported borate structures [6, 10, 130]. Calculated bond valence sum values for B(1), B(2) and B(3) are ca. 3.02, 2.96 and 3.03, respectively, and agree well with the formal +3 charge of boron atoms.

68 Figure 5.3. Voronoi-Dirichlet polyhedron of the boron atom of a BO3 group in the structure of LUBO. Boron cations and oxygens are shown as dark red and red, respectively.

A clear and precise representation of the borate framework may be given using the description which is usually employed in zeolite structural chemistry. Although a typical zeolite structure usually contains two types of cations, i.e. Al-Si, Al-B, B-Si, B-Ge, Ga-Ge, etc. [151-155], the borate framework in LUBO has 12– only single sort of boron cations. (B14O27) framework can be considered as built from a novel quasi d6r [366283] composite building unit (CBU) shown in Figure 5.4a, which is comparable to Cancrinite [169]. structure’s [466263] CBU. The face symbols given in the square brackets represent the topology of a CBU. For example, [366283] CBU is a cage containing six triangular faces, two hexagonal, and three octagonal faces. In LUBO, each [366283] CBU is connected to six other via three double triangles (DTs) [63] units (Figure 5.4b, 5.4c, 5.4d) in two directions, defining a layer containing 6- and 8-MRs in the ab-plane. The connectivity in the perpendicular direction is achieved by sharing the 6-MR windows by [366283] CBUs 12- (see Figure 5.4e). The simplified 3-nodal net of (B14O27) framework was attributed to 4 2 2 3 (3.6.8 )3(3 .6.7 .8)3(6 ) point symbol [133-135] (see Figure 5.4f, 5.4g). The simplified net exhibits all the features of the initial framework, i.e. there are 6- and 8-MRs corresponding to the channels in the initial borate framework.

69 12- Figure 5.4. Construction of the borate framework [(B14O27) ] under cationic topology view. (a) A new [366283] CBU unit, (b) a three DTs [63] unit, (c), (d) the connection modes of [366283] CBU and DTs [63] unit. (e) the connection way of [366283] CBU units along c-axis, (f), (g) view of the 3D cation network along the a, c-axis.

The formation of the hexagonal channel segment in LUBO from cationic topology view is diagrammatically shown in Figure 5.5. The fundamental building block (FBB) of the hexagonal channels 14- is (B4O13) (B4) tetramer (see Figure 5.5). This fragment is based on four BO4 tetrahedra (4□):<3□>□ according to the borate classification system proposed by Burns [170]. The FBB has been known only in natural uralborite [171], but not found in any other actinide borates up to now [6, 10, 130]. These B4 tetramer groups via corner-sharing polymerization forms a hypothetical 2D boron layer with the structure shown in Figure 5.5b. The layer has 8-membered holes, which are different with those observed in actinide borates described by Wang and Wu [98, 130]. The hexagonal tunnels are result from these layers simply rolled up (see Figure 5.5). The Pb2+ attached water molecules residing inside the tunnels center axle exactly as mentioned earlier. The resulting borate hexagonal channels are external enclosed by

[(UO2)(BO3)2] (UB2) clusters via edge or corner sharing for uranyl units and corner sharing for BO3 triangles, respectively (see Figure 5.5). Those [(UO2)(BO3)2] clusters are in alignment with each other along the c-axis shifted by 0.5c and the angle are 60° between the adjacent groups. This shift has led to a linear shape along the c-axis. The resulting complex hexagonal channels have a six-point star topology. Otherwise, it is an extremely rare borate-based hexagonal channel, which was attached with stick shape

UB2 groups outside (see Figure 5.5).

70 Figure 5.5. A schematic representation of segments hierarchy in LUBO hexagonal channels. (a) A fundamental building block (FBB) of the channel, (b) corresponding unfold version of the hexagonal 2 2 2 channel cationic topology representation [(3.8 )(3 .8 )], (c) a [(UO2)(BO3)2] cluster topology representation, (d) a hexagonal channel topology representation along the c-axis, (e) view of coordinated environments for the hexagonal channel along the c-axis.

It is interesting to compare the borate open framework of LUBO with the actinide free lead tetraborate

PbB4O7 [163], because both of them have the hexagonal tunnels in their 3D oxo-borate frameworks. The

3D oxo-borate framework in PbB4O7 is based on the B3O9 trimer (FBB). The B(2)O4 tetrahedra share corners to form a chain along the c-axis, whereas B(1)2O7 dimers link the boron chains, completing the

3D borate open framework of PbB4O7. Both of the 6-MR B6O6 boron-rings in LUBO and PbB4O7 are plotted in Figure S4. The 6-MR in LUBO is a regular hexagon with a B-B bond distance of ~2.5 Å and

B-B-B angle of 119.07°, whereas in PbB4O7 the channels are slightly distorted, and B-B bond distances are ca. 2.5 Å, 2.7 Å and 2.6 Å, B-B-B angles are 123.04, 108.56 and 120.31°, characteristic of an irregular hexagon. We presume that the uranyl group has played an important template role for forming the regular hexagon in LUBO framework. The 6-MR channels form equilateral triangular windows with the edge size of ~3.4 Å, measured as the distance between oxo atoms in the vertices of the triangle as shown on Figure 5c. The diameter of the 6- MR channel is ~3.4 Å × 4.2 Å, which is slightly smaller than that of the 6-MR channels observed in beta- Eucryptite [172]. We suggest that, after a proper activation of the channels and removing the water molecules, this compound may be a good candidate for using as a molecular sieve which is able to transport some small ions and molecules (such as H2, O2, N2 and NH3). The walls of the 6-MR channels contain 8- and 3-MR windows, in which the 8-MR windows are composed of eight BO4 tetrahedra, perpendicular to the 6-MRs (Figure 5.6a). The edge size of triangular 3-MR windows is ~2.4 Å (Figure

5.6e). The hexagonal LUBO has a 63 screw axis, that generates three symmetrically equivalent 8-MR 71 trapezoid channels along [100], [010], and [110] with ~3.6 Å × 4.6 Å size occupied by Pb atoms (see Figure 5.6f).

Figure 5.6. (a) A six-MRs channel along c-axis, (b) the six-MRs channel showing with tiling mode along c-axis, (c) a six-MRs ring on ab plane, (d) an 8-MR a long c-axis, (e) a three-MRs window on the wall the 6-MRs channel, (f) an eight-MRs window along the [100], [010] or [110] direction.

BO3 groups linking the tubules into a framework form 8-MR channels that are occupied by uranyl cations. The channels have rectangular shape with oxo atoms in the vertices, and the size is ~2.8 × 4.0 Å (Figure 5.6d). Uranium atoms occupy a centrosymmetric position in the center of these 8-MRs and have hexagonal bipyramidal environments, sharing the equatorial edges with four different BO4 groups (see Figure 5.6a). Due to the symmetry, the uranyl cation is linear, whereas its equatorial environment, being predetermined by the borate framework, is corrugated. Indeed, the deviation of the oxo atoms from the uranyl equatorial plane varies from 0.25 to 0.29 Å. The bond lengths of the axial U=O bonds are 1.793(10) Å and the equatorial U-O distances are in the range of 2.358(8)-2.521(8) Å, which are consistent with previously reported uranyl borates [6]. Despite the deviation of the equatorial oxo atoms from the plane, the Voronoi-Dirichlet polyhedron volume of the uranium is equal to 9.42 Å3 and agrees well with the average value of 9.3(4) Å3. BVS calculations indicate that the uranium cations are U(VI) with a value of ca. 5.97. The uranium center coordination geometry type has a few differences compare to those alkali and alkaline earth metal uranyl borates reported previously [130]. In Na6(UO2(B16O24(OH)8))(H2O)14 [96], the UO8 hexagonal bipyramids are located in the eight boron rings as well, however, its 8-MRs are formed through eight corner sharing BO4 tetrahedra, with no BO3 triangles inside. In Wang et al’s investigations [130], a series of M+ (M = Li, Na, K Rb, Cs, Ag, Tl) uranyl borates share a common structural motif consisting of 72 + a linear uranyl have been reported. The isolated uranyl cations of M uranyl borates existed as UO8 hexagonal bipyramids surrounded by six BO4 tetrahedra and three BO3 triangles, as B9O9 rings, whereas they are B8O8 rings in the structure of LUBO. For the alkaline earth uranyl borates family, the uranium center geometries are simpler as uranyl groups are linked with BO3 triangles [10]. Pb2+ cations reside in the side 8-MR of the tubules, completing their walls. Each Pb is eight-coordinated as determined by the method of intersecting spheres [173]. The Pb2+ cations form coordination polyhedra in the shape of bicapped trigonal prism, with the vertices occupied by oxo atoms of two edge- and one vertex-sharing BO4 groups, two uranyl oxo atoms, and single water molecule, which constricts three lead cations. Although the planar fragment Pb3O alludes to an oxo-cluster, the oxygen atom most likely belongs to a water molecule. The presence of a water molecule in LUBO is supported by Raman spectroscopy and differential scanning calorimetry along with crystallographic data, i. e. Pb–O (~2.78 Å) bonds in Pb3(H2O) fragment is elongated as compared to the μ3-OPb3 oxo-clusters reported in CSD and ICSD so far (2.15-2.22 Å) [174-175]. Pb-O bond lengths are in a wide range of 2.426(9) to 3.000(5) Å and the VDP solid angles corresponding to Pb-O bonds are in a range of 9.15–13.91%. Pb atoms have a stereochemically active electron lone pair, which is supported by the DA value shows the displacement of an atoms from the center of its VDP of 0.25 Å corresponding to Pb atoms, consistent with the average 0.3(2) Å found for Pb atoms in oxo environment [176].

Figure 5.7. (a) Construction of the anionic borate framework using natural tiling, (b) a new larger cage [36•63•89], (c) a new cage of [36•62•83], (d) a [63] t-kah unit.

The channels and cavities system of the boron framework in LUBO can be clearly illustrated by natural tiling [177] (see Figure 5.7). The boron framework is constructed by three types of tiles with [36•63•89], [63], and [36•62•83] face symbols. [36•62•83] cages connected with each other by a propeller shaped [63] t-

73 kah unit (see Figure 5.3b, 5.7d), which is then linked to a larger cage [36•63•89] (Figure 5.7b). The six and eight-MR channels can be seen more clearly with the construction process of tiling. Uranyl cations reside in the six of nine 8-memebered rings. The remaining three 8-membered rings, which are shared with [36•62•83] tiles, are occupied by Pb2+ cations. 5.3.3 Thermal Analysis. Thermogravimetric (TG) and differential scanning calorimetry (DSC) analysis were performed in the range of 50 - 1200 °C and summarized on Figure 5.8. The TG curve (black solid line) exhibits three prominent weight losses. The first one (0.65%) occurs at ca. 200 °C, and attributed to the loss of water (calculated 0.9 %). The water loses at the temperature of ca. 200 °C, which implicates they are attached water molecules and reside in the hexagonal channels. The second weight loss is accompanied by a strong endothermic peak at 696 °C on the DSC curve (blue solid line), which corresponds to a partial decomposition of the uranyl borate framework in LUBO. Taking into account the fact that overall weight loss is rather small, we can conclude that this compound and its boron framework in particular are very thermally stable and robust at least up to 696 °C. To confirm this polycrystalline sample of LUBO was calcinated at 690 °C for 10 h, resulting in the formation of a yellow solid. The PXRD pattern for the calcinated sample is identical with that unheated materials and reveals the polyborate framework is retained.

Figure 5.8. Plotting of TG-DSC curves of LUBO.

5.3.4 Vibrational Spectroscopy. The assignment of the peaks in the Raman and IR spectra of LUBO was performed using the literature -1 data [10, 98]. The Raman spectrum of LUBO contains a strong and sharp peak at 824 cm due to the v1

74 2+ -1 symmetric stretch mode of uranyl (UO2) units (see Figure 5.9a). The peaks in the range of 175-210 cm −1 could be assigned to the v2 mode of the uranyl ion. Raman bands in a region of 358–500 cm are attributed to O-B-O symmetric bending ν2 mode in BO4, whereas v4 borate group bending vibration correspond to the peaks in a range of 500-790 cm−1. The Raman spectra show very weak peaks at around the range of (980–1200 cm−1) have been assigned to O-B-O triply degenerated asymmetric mode −1 stretching ν3 mode in BO4 tetrahedra. The weak peak at 1363 cm has been attributed to the asymmetrical stretching ν3 mode in BO3 triangles group. Weak peaks corresponding to vibrational modes of water molecules are located at 3530 cm-1. As shown in the IR spectrum on Figure 5.9b, the strong peaks in 3435 -1 -1 -1 cm and 1629 cm demonstrated the presence of H2O in the structure. Bands between 1110 cm and -1 1390 cm can be assigned to the antisymmetric stretching vibrations of the BO3 groups. The peaks -1 -1 associated with the BO4 groups appeared at around 1024 cm . The peaks in the range of 877-1000 cm 2+ are attributed to v3 (symmetric stretching mode) of uranyl group (UO2) . The absorption peaks at 400- -1 800 cm can be assigned to the bending vibrations of the BO3 and BO4 units.

Figure 5.9. Raman shift (a) and FT-IR (b) spectra of LUBO.

5.4 Conclusions A unique actinide borate LUBO containing an exceptional 3D oxo-borate open framework structure was 12- obtained in mild hydrothermal conditions. Its anionic borate framework (B14O27) is built of tubules, which are linked through parallel triangular BO3 groups, resulting in multi-directional intersecting 12- channels along the three axes. The simplified net corresponding to the framework of (B14O27) is a new 3, 4 2 2 3 4-coordinated 3-nodal net with a point symbol of {3.6.8 }3{3 .6.7 .8}3{6 }. The topology of the covalent pure boron-oxygen framework is quite complex and rare, compare to the previous reported borate network structures. The thermal behavior and calcinated PXRD pattern suggest that its oxo-borate framework is quite robust and can be stable up to 696 °C. The Pb2+ cations reside on the side of the 2+ tubules, completing its walls with its lone electron pairs outside. Uranyl groups UO2 are fitted into 75 windows in the borate framework and have slightly distorted hexagonal bipyramidal environments. The water molecules are located on the axes of the 6-MRs channels. The removal of water from the 6-MRs tubules, a potential molecular sieve produced for small molecules. It is remarkable that the introduction of Pb2+ cations, leads to the formation of novel oxo-borate framework with a lack of previously reported analogues. This demonstrated that the Pb2+ cations with lone electron pairs have played a key template 12- role for the construction of the (B14O27) 3D open oxo-borate framework. Moreover, the isolation of LUBO has provided a new route for synthesis the actinide borates with 3D open framework structures. Based on the synthetic condition of LUBO, our future work will focus on the designing and synthesizing actinide borates materials through introducing different polyvalent cations, such as Zn2+, Cd2+, Hg2+, Bi3+ etc. which could generate the high porosity 3D open framework structures.

76 Chapter 6. Highly Porous Alkali-metal Uranyl Borophosphates with Unique Three Dimensional Open Framework Structures

6.1 Introduction The design and synthesis of novel inorganic crystalline materials, which possess advanced functional properties has been the subject of numerous studies for many decades [37-39, 178]. The archetype zeolite structure exemplifies this, with its prolific application in several areas including acid catalysis, luminescence, ionic exchange/sequestration, conductivity, gas adsorption and separation [58, 144-147]. A plethora of structural derivatives have been found through modification of the zeolite, Al-Si-O, form through a variety of synthetic approaches, extending from traditional solid state methods towards the more recent successful application of organic templating agents [179-180]. Further novel functional properties have been ascertained by advancing the zeolite form towards the inclusion, or exchange of further cations within the structural type such as in the case of Al-Ge-O, Al-P-O, Ga-P-O and Ge-Si-O systems [181-185]. Interest in crystalline borate derived materials is currently undergoing a resurgence of interest, as despite the number of known structures being dwindled to that of the zeolite form, several of these show remarkable application towards areas traditionally targeted with zeolites [186]. However, in comparison to the aforementioned zeolite materials or mono-anionic groups of borates or phosphates, structural systems derived from anionic B-P-O system has received considerably less attention [62, 187-189]. Pertinently, borophosphate materials have found considerable interest in relation to their potential application for nuclear waste immobilization through vitrification [6, 10, 130]. It subsequently becomes necessary to establish the physiochemical and structural properties of borophosphate phases, as glass, but also in crystalline forms, particularly those which may form in the nuclear waste vitrification process. Several actinide borophosphate phases have been recently synthesized and studied in details (see Chapter 1). However, this family of compounds is still under investigated. Herein, two novel alkali metal uranyl borophosphates, KUPB1 and KUPB2 have been synthesized and characterized. Both were found to be exceedingly porous materials, which are based on novel 3D open framework structures. The unique structures of these phases have been analyzed with single crystal X-ray diffraction, thermogravimetric and differential scanning calorimetry as well as with Raman spectroscopic analysis. The structural properties of these phases are discussed in relation to their chemical composition.

6.2 Experimental Section

6.2.1 Materials and Methods. Uranyl nitrate UO2(NO3)2∙6H2O (International BioanalyticalIndustries,

Inc.), Potassium hydroxide KOH (Alfa-Aesar, 99.8%), Phosphorous acid H3PO3 (Alfa-Aesar, 99.5%),

77 Boric axid H3BO3 (Alfa-Aesar, 99.5%) and Ammonium dihydrogen phosphate NH4H2PO4 (Alfa-Aesar, 99.5%).

6.2.1.1 Synthesis of KUPB1: KUPB1 was obtained via a hydrothermal method. UO2(NO3)2∙6H2O

(0.0506 g, 0.10 mmol), KOH (0.0221 g, 0.40 mmol), H3PO3 (0.0332 g, 0.40 mmol), H3BO3 ( 0.0381g, 0.60mmol) and deionized water (0.5 ml), in a ratio of U : K : P : B = 1 : 4 : 4 : 6, were mixed thoroughly in an agate mortar and sealed into a teflon-lined stainless steel autoclave (23 ml) and then transferred into a box furnace. The furnace was heated up to 220 oC, held for 36 hours, it was then cooled to 150 oC at a rate of 3 oC/h, before being further cooled to room temperature with a cooling rate of 5 oC/h. The resulting products were washed with hot water and filtered. Greenish block shaped crystals of KUPB1 were obtained with a yield of ~41% based on U content, of which several were collected analyses. A high purity phase of KUPB1 was obtained by mechanically separating crystals of which the purity was confirmed by laboratory X-ray powder diffraction (XRD). The separated high purity phase sample was used for thermophysical property measurements. Energy dispersive X-ray spectroscopy (EDS) elemental analysis on selected single crystals presented an average molar ratio of K : U : P = 5.02 : 2.05 : 6.11 for KUPB1, which are in good agreement with those obtained from single crystal X-ray diffraction studies. 6.2.1.2 Synthesis of KUPB2: KUPB2 was prepared using a hydrothermal method. The initial compositions of UO2(NO3)2∙6H2O (0.0528 g, 0.10 mmol), KOH (0.0225 g, 0.40 mmol), H3PO3 (0.0332 g,

0.40 mmol), and H3BO3 (0.0929 g, 1.50 mmol) and deionized water (~0.2 ml) with a ratio of U : K : P : B = 1 : 4 : 4 : 15 for KUPB2. All the chemicals were mixed thoroughly in an agate mortar and then sealed into a Teflon-lined stainless steel autoclave (23 ml). The autoclave was placed into a box furnace and heated up to 220 °C for 24 hours and then cooled down to 160 °C at a cooling rate of 3 °C/h and further cooling down to room temperature with a rate of 5 °C/h before the furnace was switched off. The resulting products were washed with hot water and filtered. Yellowish needle shaped crystals, KUPB2, were obtained. The yield of KUPB2 is greater than KUPB1 at ~46% based on U content. A high purity phase of KUPB1 was obtained by mechanically separating crystals of which the purity was confirmed by XRD analysis. The separated high purity phase sample was used for thermophysical property measurements. EDS elemental analyses on selected single crystals of KUPB2 gave an average molar ratio of U : K : P = 12.15 : 2.06 : 11.97, which is in good agreement with the proposed chemical compositions of the structure.

78 Table 6.1. Crystal Data and Structure Refinements for compounds KUPB1 and KUPB2.

Compound KUPB1 KUPB2 FW 1428.62 4546.05 Space group P21 I-42m a (Å) 6.7623(2) 21.8747(3) b (Å) 19.5584(7) 21.8747(3) c (Å) 11.0110(4) 7.0652(2) α (deg) 90 90 β (deg) 95.579(3) 90 γ (deg) 90 90 V (Å3) 1449.42(8) 3380.72(12) Z 2 2 λ( Å) 0.71073 0.71073 F(000) 1306 3864 -3 Dc(g cm ) 3.273 4.446 GOOF on F2 1.021 1.164 R1 0.0245 0.0293 wR2 0.0645 0.0659 2 2 2 2 2 ½ R1 = Fo -Fc/Fo, wR2 = {w[(Fo) - (Fc) ] /w[(Fo) ] }

6.2.2 Instrumental studies including Single Crystal (Table in all text of the work) and Powder XRD, SEM/EDS Analysis, Thermal Analysis and Raman Spectroscopy have been performed as described in Chapter 2. Additionally Bond-Valence Analysis was done according to the method described in Chapter 2. 6.3 Results and Discussion

6.3.1 Syntheses. Phosphorous acid (H3PO3) has a low melting point of 73.6 °C. This is a potential reactive flux and reagent similar to well-known flux of boric acid. However, as a reagent, phosphorous acid (phosphonic acid, IUPAC systematic name) is more active in terms of its redox behavior than boric acid. H3PO3 is a good reducing agent and simultaneously it can be oxidized into H3PO4 around 200 °C. Therefore, a mixed-flux of boric acid and phosphorous acid could be a unique reactive medium for preparation of borophosphates compounds. The investigation of the A-U-P-B-O (A = Alkali metal) system, yield two unprecedented open framework alkali metal uranyl borophosphates, KUPB1 and KUPB2, at mild hydrothermal conditions. KUPB1 was obtained through the usage of UO2(NO3)2(H2O)6 : H3BO3 :

H3PO3 : KOH in a ratio of 1 : 6 : 4 : 4 together with 0.5 mL deionized water. Interestingly, KUPB2 was obtained by increasing the amount of H3BO3 fourfold holding all other reactants constant and using less water as a reaction medium, the other conditions are with little difference. We postulate that H3BO3 has played the role of both flux and reaction medium for the formation of the KUPB2 structure and crystals.

Probably, excess of H3BO3 leads to better solubility of uranium and its reaction with phosphorus. It, speculatively, can be a reason why KUPB2 has more U compared to KUPB1. In comparison the first three actinide borophosphates Ag2(NH4)3(UO2)2B3O(PO4)4(PO4H)2H2O,

79 Ag0.74(NH4)3((UO2)2B2P5O18.74(OH)1.26 and Ag0.57(NH4)3(UO2)2B2P2.76As2.24O18.57(OH)1.43 prepared with a

H3BO3−NH4H2PO4 flux method at around 200 °C using considerably less water (50 μL) [83]. For comparison with the syntheses of the first series of uranyl borophosphates made by Wu et al., we attempt to synthesize KUPB1 and KUPB2 through changing H3PO3 to NH4H2PO4, however all syntheses were unsuccessful. Substitution of H3PO3 to H3PO4 in the synthesis also not leads to a formation of KUPB1 and

KUPB2. We may suggest that H3PO3, a simultaneous reagent and low temperature flux, has played a key role in structure formation of uranium borophosphates. 6.3.2.1 Structure of KUPB1. The compound KUPB1 is the first alkali metal uranyl borophosphate, which belongs to a new structural type and crystallizes in a chiral space group of P21 (No. 4). In an asymmetric unit, KUPB1 consists of five unique K, two U, four B and six P atoms. It features a 3D open 5- framework structure, which is composed of corner sharing 1D anionic BP-chains of [B2P3O13] and UP- 7- chains of [(UO2)(PO4)3] as shown in Figure 1c and 1d. A pair of BO4 tetrahedra shared corner (O1/O17), forming a B2O7 dimer. The B2O7 dimers and PO4 tetrahedra, corner share forming 1D anionic chains, 5- [B2P3O13] , along the a-axis, which can be viewed as the Fundamental Building Block (FBB) written as

(5□:<3□>=<3□>□) following Burns et al.’s designations [47]. The FBB of [B2P3O13] is uncovered firstly in the actinide borophosphates family [83], but can be found in a few actinide free borophsophates, such 5- as Na5[B2P3O13]11, Rb3[B2P3O11(OH)2] etc. [190-191]. Two chiral symmetric [B2P3O13] chains are contained in the structure of KUPB1 as shown in Figure S1d and S1e. The bond lengths in the BO4 tetrahedra are in the range of [1.384(11)-1.516(11) Å], the bond lengths for P-O in PO4 tetrahedra are in the range of [1.483(8)-1.575(6) Å]. The Bond angles of B-O-B are 112.7(7) ° and B-O-P are in the range of [112.1(7) °-133.8(6) °] in the borophosphate chains. From BVS calculations, the valences for B cations are suggested to be 3+ with values for the B1, B2, B3 and B4 at ca. 3.11, 3.12, 3.08 and 2.95, respectively. The calculations of BVS for P1 to P6 are ca. 5.17, 4.97, 4.95, 5.13, 5.07 and 5.03, which strongly suggests the as expected P valence of 5+.

Uranium atoms are seven-fold coordinate to oxygen, forming isolated UO7 pentagonal bipyramids. The 2+ collinear uranyl moieties, (O=U=O) , are five-fold coordinated to PO4 tetrahedra through vertex sharing 7- forming 1D uranyl phosphates chains, [(UO2)(PO4)3] , along the c-axis (see Figure 6.1c). This type of

UP5 coordination environment is similar with that observed in

Ag2(NH4)3[(UO2)2{B3O(PO4)4(PO4H)2}]H2O and the layered uranyl arsenates α, β-

Rb[UO2(AsO3OH)(AsO2(OH)2)] [192] with UAs5 moieties. The uranyl axial U=O bond lengths are in the range of [1.779(5)-1.786(6) Å], whereas the equatorial bond lengths are of [2.307(6)-2.400(6) Å]. The 5- 7- two anionic chains of [B2P3O13] and [(UO2)(PO4)3] are bridged by the PO4 tetrahedra, forming a 3D 3- uranyl borophosphate open framework [(UO2)(B2P3O13)] (see Figure 6.1a and 6.1b). The BVS calculations of U1 and U2 are ca. 6.08 and 5.72, which suggest that the valence of the U cations is 6+. 80 Figure 6.1. (a) View of the structure KUPB1 along the c-axis and (b) a-axis, (c) a 1D uranyl phosphate (UP) chain along the c-axis, and (d) a 1D borophopahte (BP) chain along the a-axis. K atoms, UO7 polyhedra, BO4 and PO4 tetrahedra are shown as blue, yellow, green and pink color, respectively.

Framework Topology Studies. KUPB1 has a new zeolite-like topology, with multi-intersecting channels system as shown in Figure 6.3 and 6.4. Two 8-R channels 2(U-P-B-P) exist along the a-axis with diameters of ca. 6.2 × 6.4 Å and ca. 5.6 Å × 6.6 Å (distances are based on two opposite O atoms), respectively, in which two helical chains left- and right-hand running along the 21-axis parallel to the a- axis (see Figure 6.2). One 8-R channel 2(U-P-B-P) is along the b-axis with a diameter of ca. 5.3 Å × 6.5 Å. The largest 11-R channel [2(B-P-U-P)-B-P-B)] has a diameter of ca. 7.0 Å × 8.8 Å along the c-axis (see Figure 7.3d-7.3g). These tunnels are occupied by the K+ cations as well as the water molecules. In order to reveal the complex zeolite-like topological network, we simplify the anionic uranyl 3- borophosphate framework, [(UO2)(B2P3O13)] , of KUPB1 by removal of the oxygen anions, whilst the

B2O7 dimers were viewed as single nodes. As shown in Figure 6.3, the simplified anionic net of KUPB1 can be described as a new 4-nodal net topological type with a point symbol of 4 2 3 2 3 2 2 {4.8 .10}{4 .6}2{4 .6 .8 .10 }{8 .10} (see Figure 6.3a-6.3c). Based on its cationic network, the framework density of porous KUPB1 is ~16.4 M atoms, where M is the framework forming cations, that is B, P and U here. This relative lower framework density of KUPB1 is comparable with those zeolite materials reported previously [193-194].

81 Figure 6.2. View of the framework of KUPB1 along the a-axis (a), the cross-section (b) and the profile of a left-handed helical chain running along the 21 axis parallel to the a axis formed by corner-sharing UO7, PO4 and BO4 polyhedra (d), the cross-section (c) and the profile of a right-handed helical chain running along the 21 axis.

Figure 6.3. View of the framework of KUPB1 with their topology representations along the a, b and c- axes (a), (b) and (c), two 8-Rs along the a-axis (d), (e) one 8-Rs along the b-axis (f), one 11-Rs along the c-axis.

82 Figure 6.4. The 3D framework of KUPB1 with sticks, yellow sticks show the 3D interconnecting channel system.

Natural tiling is an efficient approach to represent a network proposed by Blatov et al. [177], which can be used for illustrating the channel system and cavities by tracing the colors of the tiles clearly as shown in Figure 6.5. The framework of KUPB1 is built from a novel composite building unit (CBU) [32∙42∙84∙112], with the intersecting of 8, 8, 8 and 11-R tunnels. Each [32∙42∙84∙112] CBU connects to four other neighboring ones, via their 4 or 8-Rs defining a layer on ab-plane. Each layer is further connected to its adjacent ones through additional two linkers of [32∙42] and [4∙112], which shows that the structure of KUPB1 is constructed from [32∙42∙84∙112]+[32∙42]+[4∙112] tiles. K+ cations are eight, nine and eleven-fold coordinated with oxygen having K-O bond distances are in the range of [2.650(11) Å-3.412(9) Å]. These K+ cation polyhedra corner or edge share with each other forming 2D wave shaped layers on the bc-plane. Simplifying the K-O-K layer via omission of oxygen anions, the 2D K-sheet is observed to form with a new 3-nodal net topology, having a point symbol of 2 2 2 2 2 2 + {4.5 }2{4 .5 .6.7}2{4 .5 .7 }. The valence of K cation is suggested to be as expected +1, according to the calculated BVS values for K1 to K5 at 1.12, 1.18, 1.06, 0.96 and 1.08, respectively.

83 Figure 6.5. View of the channel system in KUPB1 using natural tiling (a), a new CBU cage [32∙42∙84∙112] (b), two tiles of [32∙42] and [4∙112] (c) and (d).

It is interesting to compare the 3D open framework structure of KUPB1 with the 3D uranyl borophosphate Ag0.74(NH4)3((UO2)2(B2P5O18.74(OH)1.26)), which crystallized in the centrosymmetric space group of Pcmn (No. 62). It also has anionic borophsophate chains as in KUPB1, however, their anionic chains are comprised from different FBBs, which only contain BO4 tetrahedra and no BO3 triangle units.

The FBB of Ag0.74(NH4)3((UO2)2(B2P5O18.74(OH)1.26)) are more complex, viewing the [B2P5O20] units having a topology of (7□:□<4□>□|□), these FBBs polymerized along the b-axis forming its borophosphate chains with U-type gaps. UO7 polyhedra are filled in those U-type gaps along the [101] and [-101] directions, constructing the 3D uranyl borophosphate framework structure of

Ag0.74(NH4)3((UO2)2(B2P5O18.74(OH)1.26)). In its 3D framework, two types of 1D elliptic-ring channels can be observed, one is a 6-Rs tunnel along the c-axis and another larger one is a 12-Rs along the b-axis. The complex FBB is one of the facts that make its 3D framework less open than that for KUPB1. Neighboring anionic borophosphate chains are aligned with a more regular mode in the defining layer with ½a shift along the a-axis, producing its centrosymmetric structure. Due to the templating K+ cations in KUPB1 + having a larger ionic radii than the Ag cations in Ag0.74(NH4)3((UO2)2(B2P5O18.74(OH)1.26)), it is presumed this coerces the framework structure of KUPB1 to be exceedingly more open and contain much larger channels with diameter of ca. 7.0 Å × 8.8 Å than that in the structure of

Ag0.74(NH4)3((UO2)2B2P5O18.74(OH)1.26 with ca. 8.5 Å × 3.0 Å.

Structural relations with actinide free borophosphates. Na5[B2P3O13], Rb3[B2P3O11(OH)2] and a series 5- of organic borophosphates [195] possess the same FBB of [B2P3O13] with KUPB1, thus it is worthy to compare and contrast their structural relations (see Figure 6.6a). All of them are crystallized in monoclinic space groups, whereas Na5[B2P3O13] and KUPB1 have the same chiral P21 space group and

84 Rb3[B2P3O11(OH)2] crystallizes in the space group P21/c. Na5[B2P3O13] has a chain structure along the a- 5- axis, the [B2P3O13] anionic chains are regularly parallel arranged along the b-axis, defining a layer C on ab-plane, the layers are stacked as ...CCC… mode within its structure. The closest distance between two 5- [B2P3O13] chains is ca. 5.8 Å. Comparing to Na5[B2P3O13], the defining layers of KUPB1 and

Rb3[B2P3O11(OH)2] have the same stacking mode of …CC'CC'…, which have a ½ shift along the corresponding axis for C' with C layer. Owing to the larger ionic radii of Rb+ compared to Na+, the 2+ distance between the two neighboring anionic chains is larger with ca. 7.8 Å. The insertion of the (UO2) uranyl groups between anionic chains in the chain structure of Na5[B2P3O13] and Rb3[B2P3O11(OH)2], configures the structure towards the KUPB1 type, the distance between the adjacent anionic chains further enlarge to ca. 9.9 Å. We presume that the cations absent in Rb3[B2P3O11(OH)2] but present in KUBP1 act as scaffolding templates manipulating the spatial configuration of anionic units within the structure of these [B2P3] derived compounds. 6.3.2.2 Structure of KUPB2. KUPB2 crystallizes in the non-centrosymmetric space group of I-42m (No. 121) forming a tubular structure. The asymmetric unit of KUPB2 contains one K, two U, four B and six P atoms. It features a 3D open framework structure, composed of 1D large uranyl phosphate tube,

[(UO2)(PO4)], further bridged by a uranyl borophosphate cluster, [(UO2)4B(PO4)4] (see Figure 6.6a). The seven-fold coordinated U1O7 pentagonal bipyramids are edge (O2-O10) sharing along the c-axis, forming a 1D uranyl chain (in Figure 6.7b). P1O4 tetrahedra edge (O2-O10) and corner (O5) share with 1D uranyl chains alternatively. This structural configuration yields 1D 8-Rs [(UO2)(PO4)] tubes along the c-axis (in

Figure 6.7a). B1O4 tetrahedra vertex (O11) share with the disordered P2O4 tetrahedra forming [B(PO4)4]

FBB units. This borophosphate group can be found in the actinide free compounds of Pb6(PO4)[B(PO4)4]

[196] and Sr6[B(PO4)4](PO4) [197], in which the [B(PO4)4] units form clusters. The P(2) atoms are statistically disordered within the structure of KUBP2. They occupy a split symmetrical site within the unit cell with 50% occupation. As a result, two symmetrical O(11) positions also appear as a split atomic site. Having disordered O(11) atoms bonded to B(1) atoms, each O(11) atom has 50% spatial chance to form a B-O bond. Due to the disorder in oxygen environment, B(1) atoms are also slightly moving along the c direction and this is speculatively a reason of significant elongation of B(1) thermal ellipsoid. Four

U2O7 pentagonal bipyramids corner (O8) share with the [B(PO4)4] units, forming uranyl borophosphate clusters [(UO2)4B(PO4)4], which are parallel arranged along the c-axis (in Figure 6.7f). Uranyl borophosphate clusters [(UO2)4B(PO4)4] further bridge the [(UO2)(PO4)] tubes on the ab-plane, + constructing an unusual 3D uranyl borophosphate framework {(UO2)12[B(PO4)4](PO4)8}, the K cations are located in the center of the [(UO2)(PO4)] tubes for charging balance (see Figure 6.6a).

85 Figure 6.6. The structure KUPB2 along the c-axis (a) with its cation framework topology representation (b). K, B atoms, UO7 polyhedra and PO4 tetrahedra are shown as blue, green, yellow and pink color, respectively.

Figure 6.7. (a) An 8-R [(UO2)(PO4)] tube along the c-axis built from (b) and (c), (b) 1D uranyl chain along the c-axis with ball-and-stick presentation, (c) a P1O4 tetrahedron with ball-and-stick form, (d) a FBB [B(PO4)4] group, (e) a U2O7 pentagonal bipyramid, (f) one borophosphate cluster [(UO2)4B(PO4)4]. The colors of uranium, phosphor, boron and oxygen atoms are yellow, pink, green and red, respectively.

The KUPB2 uranyl bond lengths were found to be within the range of [1.743(14)-1.764(7) Å] which are slightly shorter than that in KUPB1, whereas the equatorial U-O bond distances for KUPB2 are comparable with that in KUPB1, in the range of [2.297(7)-2.55(2) Å]. The average P-O bond lengths for

PO4 tetrahedra are ca. 1.536 Å, whereas for B-O bond lengths are ca. 1.445 Å.

86 The tubular uranyl phosphate group, [(UO2)(PO4)], on the structural defining component of KUPB2 is composed of 1D U1O7 uranyl chains and P1O4 tetrahedra. As shown in Figure 7.6a, each 8-Rs

[(UO2)(PO4)] tube is surrounded by four [(UO2)4B(PO4)4] clusters, four 8-R [(UO2)(PO4)] tubes are also located around one [(UO2)4B(PO4)4] cluster, with a [4, 4] coordination mode. This 8-Rs [(UO2)(PO4)] tube is built from four repeating --UO7-PO4-UO7-PO4-- linkages with a diameter of ca. 7.6 Å × 7.6 Å (the distance are between two opposite O atoms), which is larger than the similar tunnels in

Rb4((UO2)6(P2O7)4(H2O)) [198]. There are 4-Rs quasi square windows (ca. 3.6 Å × 3.8 Å) on the shaft of the [(UO2)(PO4)] tubes, resulting in a 3D porous network structure as shown in Figure 6.8a and 6.8b. If the 8-Rs [(UO2)(PO4)] tube is unfolded, a hypothetical 2D uranophane topology sheet is obtained [199], which has a perimeter of ca. 26.6 Å (see Figure 6.8d and 6.8e). It is noteworthy that, in the 3D anionic framework of Rb4[(UO2)6(P2O7)4(H2O)], the existing 9-Rs tunnel, ((UO2)3(P2O7)2, is comparably smaller than that observed in KUPB2 with a size of ca. 5.2 × 6.1 Å. The tunnels have different connecting modes for Rb4((UO2)6(P2O7)4(H2O)) and KUPB2, one 9-Rs tunnel of ((UO2)3(P2O7)2 is connected with two other neighboring ones through sharing common U2-U3 dimers along the b-axis, defining a tunnel layer on the ab-plane. Those parallel tunnel layers are further linked by two P2O7 dimers (P1-P2 and P3-P8) along the c-axis, constructing its 3D framework. Noting that, the combination of BO4 tetrahedra with PO4 tetrahedra in KUPB2 has created the more complex topological structure than that in

Rb4((UO2)6(P2O7)4(H2O)).

Figure 6.8. (a), (b) the pore size of a 4R and 8-R windows of the [(UO2)(PO4)] tube, (c) side view of one 8-R tube along the [010] direction with ball-and-stick representation, (d) unfolding the 8-R [(UO2)(PO4)] tube, (e) showing a hypothetical uranophane topology sheet.

Framework Topology Studies. KUPB2 also possess a new zeolite-like topology with 1D large

[(UO2)(PO4)] tubes. In order to present the novel zeolite-like topology network of KUPB2, we simplify the anionic framework [(UO2)12B(PO4)4](PO4)8] of KUPB2 by omitting the oxygen anions. As shown in Figure 7.6b, its simplified cation network is a new 5-nodal topological type with a point symbol of 87 2 3 2 4 5 4 2 2 3 2 4 2 {3 .4 .5.6 .7.8}8{3 .4 .5 .6 }8{4.6 .8 }4{4 .6}4{4 .6 }. From its cationic network, we can observe that the 3D open framework of KUPB2 has a low framework density of ca. 14.7 M atoms per 1000 Å3, which is even lower than that in KUPB1 and comparable with the open zeolite faujasite [200-201] with framework density of ca. 13.5. The zeolite-like framework of KUPB2 can be considered as a OSI (framework type code in zeolite database) zeolite type [202], which has the similar channel topology within their network structure. From the natural tiling point of view, the anionic uranyl borophosphate framework is the same as OSI with a five coordinated net, each node occupied by a CBU. The CBU of KUPB2 is the quasi d4r cage of [38∙44∙82], whereas for OSI it is t-osi [64∙122] (in Figure 6.9b). Four neighboring quasi d4r cages of [38∙44∙82] are connected by another new cavity of [412∙68∙84]. The two adjacent CBUs are linked by two types of tiles, [34∙82] and [83], defining a 2D layer on ab-plane. Those 2D layers are further connected along the c-axis through tiles of [44∙62], forming this unusual uranyl borophosphate framework. It is apparent to see that the 3D framework of KUPB2 was built from tiling signatures [38∙44∙82] + [44∙62] + [34∙82] + [83] + [412∙68∙84], in which quasi d4r cages of [38∙44∙82] have occupied and traced the 8-Rs channel system of KUPB2 along the c-axis. For comparison, the construction tiles of OSI are less as 2 × [42∙64] + [63] + [64∙122], which indicate that the 3D framework structure topology of KUPB2 is more complex. However, the channels along the [001] direction in [Al16P16O64]-OSI [203] having a 12-Rs with a diameter of ca. 5.2 Å × 6.0 Å, are smaller than that of 8-R channels (ca. 7.6 Å × 7.6 Å) in KUPB2 along the [001] direction. The K+ cations in KUPB2 are eight-fold coordinated, existing as square antiprisms, with K-O bond lengths in the range of [2.731(9) -2.806(8) Å]. The K+ cations are face (O3-O3-O3-O3) sharing with each other, forming 1D K-chains along the c-axis, which are located in the center of 8-Rs tubes of [(UO2)(PO4)].

Figure 6.9. The channel systems in KUPB2 using natural tiling (a), a new CBU cage [412∙68∙84] (f) and four other new building tiles (b)-(e). 88 6.3.3 Thermal analyses. The thermogravimetric (TG) and Differential Scanning Calorimetry (DSC) measurements were performed from 50 °C to 1200 °C as shown in Figure 6.10. TG analysis indicates that KUPB1 shows a weight loss in the range of 300-500 °C under a flowing nitrogen atmosphere, which is attributed to the removal 4.5 mol of water molecules in one formula unit, an additional endothermic peak is exhibited at 342 oC from the DSC measurement. The mass loss observed from its TG curve is 5.68%, which matches well with the calculated one of 5.63%. The endothermic peak at 929 oC is attributed to the melting of the dehydrated product (see Figure 6.10a). We presumed that the broad DSC peak at ~1000 °C is caused by a slow decomposition of the melted sample (see Figure 6.10a). TG analysis shows that KUPB2 has a weight loss from 300 °C to 600 °C, which corresponds to the eliminating of 9.5 mol water molecules per formula unit, this is further apparent in the DSC measurement where a endothermic peak at 316 °C is observed. The mass loss observed from the TG curve of 3.65% is in agreement with the calculated one (3.73%). The endothermic peak at 907 °C corresponds to the melting of the dehydrated product (see Figure 6.10b).

Figure 6.10. TG-DSC curves of phases KUPB1 (a) and KUPB2 (b).

6.3.4 Raman Spectroscopy Investigation. The Raman spectra were measured for both of compounds as shown in Figure 7.11. Raman spectrum of KUPB1 was measured in a range of 100-4000 cm-1, for convenience, we have divided the spectra into two sections, a low frequency part 100-1300 cm-1 and a high-frequency region in 3000-4000 cm-1. More -1 2+ scattering peaks are in the range of 100-1000cm , which is dominated by contributions from the (UO2) -1 and BO4, PO4 tetrahedra modes of KUPB1 (see Figure 6.11a). The peaks in the range of 175-220 cm could be assigned to the v2 bending mode of the uranyl ion. The Raman spectrum show a strong and sharp -1 2+ peak at ~822 cm due to the symmetric vibration v1 mode of the uranyl (UO2) units [204]. Raman bands with week peaks around 300-500 cm−1 are attributed to the O-B-O doubly degenerated symmetric bending

89 −1 ν2 mode in BO4, the bending character v4 of BO4 are in the Raman spectrum range of 506-750 cm . The 2+ −1 characteristic vibrations of the uranyl (UO2) v3 antisymmetrical stretch mode are from 908 to 960 cm . Raman bands with a very strong peak around 1007 cm-1 and a weak peak near 1035 cm-1 has been assigned to the ν1 PO4 symmetric stretching and ν3 PO4 antisymmetric stretching modes. The Raman spectrum show a few weak bands at around the range of 1050-1250 cm−1 that have been assigned to the

O-B-O triply degenerated asymmetric stretching ν3 mode in BO4 tetrahedra. The vibrational modes that come from the coordinated water molecules are observed at around 3500 and 3600 cm-1, respectively with relatively low intensity. The Raman spectrum of KUPB2 was observed in the range of 100-1400 cm-1 and 3000-4000 cm-1 (see Figure 6.11b). Raman bands located in lower frequencies in the range of 190-300 -1 cm could be attributed to the uranyl ion with a v2 bending mode. Raman bands with a series of peaks −1 around 476 cm could be assigned to the O-B-O doubly degenerated symmetric bending ν2 mode in BO4 −1 tetrahedra, the Raman peak at ~642 cm is attributed to the bending character v4 of BO4 tetrahedra. −1 −1 Raman bands from 800 cm to 870 cm should come from the symmetric vibration v1 mode of the uranyl 2+ 2+ (UO2) units. The characteristic vibrations of uranyl (UO2) v3 antisymmetrical stretch mode are at bands −1 −1 −1 of 936 and 978 cm . The Raman bands at 1002 cm and 1018 cm can be attributed to the ν1

PO4 symmetric stretching and ν3 PO4 antisymmetric stretching modes. The Raman peaks within the range -1 of 1100-1200 cm can be attributed to the O-B-O triply degenerated asymmetric stretching ν3 mode in

BO4 groups. These assignments are according to the previously reported works [79, 205].

(a) (b)

Figure 6.11. Raman shifts of KUPB1 (a) and KUPB2 (b).

6.4 Conclusions. We have shown by use of a relatively facile hydrothermal synthetic route, two novel alkali metal uranyl borophosphates, KUBP1 and KUPB2, can be obtained with highly open framework structures. Both compounds possess unique 3D open framework structures. The 3D open framework of KUPB1 is based 5- on two types of anionic chains, 1D borophosphate chains [B2P3O13] and uranyl-phosphate chains 90 7- [(UO2)(PO4)3] . Its simplified network of KUPB1 is a new 4-nodal net topological type with a point 4 2 3 2 3 2 2 symbol of {4.8 .10}{4 .6}2{4 .6 .8 .10 }{8 .10}. KUPB2 possesses a borophosphate framework

{(UO2)12[B(PO4)4](PO4)8}, in which 1D 8-R [(UO2)(PO4)] tubes propagate along the c-axis. The parallel - stacked [(UO2)4B(PO4)4] clusters are further bridged by the isolated [(UO2)(PO4)] tubes forming its 3D open framework. Its more complex cation network is a new 5-nodal net topological type with the point 2 3 2 4 5 4 2 2 3 2 4 2 symbol {3 .4 .5.6 .7.8}8{3 .4 .5 .6 }8{4.6 .8 }4{4 .6}4{4 .6 }. Two different FBBs were isolated for the first time in actinide borophosphates family, they are [B2P3O13] in the structure of KUPB1 and [B(PO4)4] in KUPB2. The generation of the first two noncentrosymetric actinide borophosphates gives greater insight to the structural complexity of uranyl borophosphates compounds. It is noteworthy to consider the templating role of both the uranyl cations and alkali metal cations in generating these unique porous structures particularly in comparison to actinide and non-actinide borophoshphates respectively. This study further demonstrates how subtle adjustments to synthetic conditions can reveal dramatic changes to structural type and topology. Further investigation on An-B-P-O system will be continue with the similar synthetic conditions, towards obtaining structures with higher porosity whilst simultaneously examining for ion exchange properties among other associated functional properties.

91 Chapter 7. Microporous Uranyl Borophosphate with Potential Ionic Exchange Properties

7.1. Introduction The open framework materials are of high interest due to their fascinating structures and important industrial applications [206-209]. They can be used in diverse applications such as ion exchange, gas adsorption and storage, catalysis, as nonlinear optical materials etc [208, 210-213]. To the best of our knowledge, both Zeolite [180] and Metal-organic framework (MOF) [214] materials quit often possess highly porous framework structures. Zeolite and zeolite-like materials are characterized as the oxotetrahedral framework structures, such as aluminosilicates, aluminogermanates, aluminophosphates and etc. [215-217]. MOFs are defined as compounds consisting of metal ions or clusters coordinated to organic ligands to form three-dimensional (3D) framework structures. However, the thermal stabilities and expenses of MOFs have limited their vast applications in the industry 218-219]. The substitution of boron for aluminum/silicon in zeolites has long been a topic of interest, because the resulting oxo-borates own complex structural chemistry and excellent properties [220-221]. Borates possess two basic units, which are triangular BO3 and tetrahedral BO4 coordination environments [6, 10]. These two basic units via corner or edge sharing polymerized into clusters, chains, scaffolds and even condensed 3D framework structures [222]. Boralite, Zn4O(BO2)6 [223], is a rare example of complete boron substitution for both Si and Al, which is a direct topological analogue of the aluminosilicate sodalite-type framework found in Na4OH(AlSiO4)3. [224] The zeolite like network of Na2Co2B12O2 [225]. is the first infinite borate containing a discernible tunnel structure, in which, the Na+ within the tunnels are mobile, and exchangeable with Li+ preservation of the original crystal morphology. The actinide borate, [ThB5O6(OH)6][BO(OH)2]·2.5H2O (NDTB-1) [108], which has a porous super tetrahedral 3D - framework structure and possess remarkable anion exchange properties towards radioactive TcO4 [109]. 4- 5- 4- 3- The incorporation of those mixed oxo-anions, such as, [SiO4] , [AlO4] , [GeO4] , [PO4] . etc. gave rise to new families of borates derivate materials [56, 157, 226-227]. From the previously reported results, the introduction of mixed oxo-anions into borates system, the resulting structures are favorite to form higher dimensional 3D open frameworks. For example, borosilicate Cs2B4SiO9, [228] its basic B4O10 groups are connected with neighboring SiO4 tetrahedra form a 3D network. This crystal possesses deep-ultraviolet transparent nonlinear optical properties. The organic templated aluminoborate,

[CH3NH3]1.5[CH3CH2CH2NH3]0.5[H2O]5[Al(B5O10)] [156], shows an unprecedented 3D intersecting channel system. This structure is built upon the strict alternation of B5O10 clusters and AlO4 tetrahedra. Borogermanate,

SrGe2B2O8, [58] has a 3D open anionic framework structure, composed of B2O7 and Ge2O7 dimers with alternate connections. The large B-Ge 8-ring tunnels are holding the Sr2+ cations. This makes the Sr2+ cations exchangeable at higher temperatures, which can be applied for remediation of toxic Cd2+ ions from the solution. 92 Borophosphates (BPOs) have been widely studied because of their diverse structural architectures and broad applications [229]. Since the first zeolite-like BPO (C2H10N2)[CoB3P3O12(OH)12] [188] was reported in 1996, a number of open framework BPOs have been prepared under different conditions [62]. The BPO frameworks are quite complex and featured diverse structural fragments, such as oligomeric units, 1D chains and ribbons, 2D sheets and 3D open frameworks. These various BPOs frameworks were synthesized using different templating cations. These cations are alkali, alkaline earth, transition metal, lanthanides, as well as organic molecules [77, 230-231]. Among them, only a few BPO materials possess ion exchange properties, for example,

Na2[VB3P2O12(OH)]·2.92H2O [232] and Na8[Cr4B12P8O45(OH)4][P2O7]·8H2O [233].Only a limited number of open framework actinides BPOs have been reported to date [83, 84, 103]. The synthesis of novel microporous and 3D open-framework actinide BPOs is of great challenge.

Herein, the first cesium uranyl BPO, Cs3(UO2)3[B(PO4)4](H2O)0.5 (CUPB1), was prepared and characterized. It is a microporous material, features a novel 3D open framework structure. An unprecedented spherical uranyl borophosphate cage, U12P24B8, is observed within its network. The unique structural architecture has been analyzed. Results of thermogravimetric and differential scanning calorimetry characterizations, Raman spectroscopy as well as ion exchange properties are reported.

7.2 Experimental Section 7.2.1 Materials and Methods.

UO2(NO3)2∙6H2O (International BioanalyticalIndustries, Inc.), H3PO3 (Alfa-Aesar, 99.5%), CsOH∙xH2O

(x = 15-20%) (Sigma-Aldrich, 99%), H3BO3 (Alfa-Aesar, 99.5%). NaNO3 (Alfa-Aesar, 99.5%), KNO3

(Alfa-Aesar, 99.5%) RbNO3 (Alfa-Aesar, 99.5%), Mg(NO3)2∙6(H2O) (Alfa-Aesar, 99.5%),

Ca(NO3)2∙4(H2O) (Alfa-Aesar, 99.5%), SrCl2∙(H2O)6 (Alfa-Aesar, 99.5%), BaCl2∙(H2O)2 (Alfa-Aesar,

99.5%), NiCl2∙(H2O)6 (Alfa-Aesar, 99.5%), CoCl2∙(H2O)6 (Alfa-Aesar, 99.5%), Cu(NO3)2∙6(H2O) (Alfa-

Aesar, 99.5%), Zn(NO3)2∙6(H2O) (Alfa-Aesar, 99.5%), Cd(NO3)2∙6(H2O) (Alfa-Aesar, 99.5%), PbCl2

(Alfa-Aesar, 99.5%), Bi(NO3)3 (Alfa-Aesar, 99.5%), [La-Lu(NO3)3∙6(H2O)] (Alfa-Aesar, 99.5%) and

Th(NO3)4∙5(H2O)( International BioanalyticalIndustries, Inc.).

7.2.1.1 Syntheses of CUPB1: CUPB1 was synthesized via a hydrothermal method. The initial reagents are UO2(NO3)2∙6H2O (0.0515 g, 0.10 mmol), CsOH∙xH2O (0.0605 g, 0.40 mmol), H3PO3 (0.0332 g, 0.40 mmol), H3BO3 (0.0649 g, 1.05 mmol) and deionized water (0.8 ml) with a ratio of U : Cs : P : B = 1 : 4 : 4 : 10. All the chemicals were mixed thoroughly in an agate mortar and then sealed into a teflon-lined stainless steel autoclave (23 ml). The autoclave was transferred into a box furnace heating up to 220 °C for 36 hours. After that, it was cooled down to 50 °C at a cooling rate of 3 °C/h. The resulting products were washed with hot water and ethanol to get rid of an excess of boric acid. Yellow large block shaped

93 23 crystals CUPB1 together with a greenish byproduct of Cs(UO2)(PO4) were obtained. The yield of CUPB1 is as high as ca. 49% based on U content. Pure phase of CUPB1 was obtained via picking up large crystals from the mixture. The pure phase of CUPB1 was characterized and confirmed by PXRD. Energy-dispersive X-ray Spectroscopy (EDS) elemental analyses on the crystals of CUPB1 reveal a molar ratio of U : Cs : P = 3.08 : 2.96 : 3.95. This is consistent with the formula obtained from the single crystal data.

7.2.1.4 Ion-exchange experiments of CUPB1. Ion-exchange reactions were performed on polycrystalline samples of CUPB1 (50mg) in glass vials with

10 mL of ~1.0 M SrCl2∙(H2O)6 (aq), BaCl2∙(H2O)2 (aq), PbCl2 (aq), NiCl2∙(H2O)6 (aq) and CoCl2∙(H2O)6 (aq) solutions. The reactions were kept both at room temperature and at 70 °C for 12 hrs. The samples after exchanged were recovered through filtration, washed with excess of water and acetone. Then the samples were put into a drying oven for 24 hours. The compositions and elements distributions of exchanged samples were measured by EDS. The kinetic studies of Sr2+, Ba2+, Pb2+, Co2+ and Ni2+ ion-exchange in CUPB1 was carried out as follows. Nine experiments were made parallel. Solutions (10 ml) for each cation (~5.0 × 10-3 M) were mixed with the same molar ratio of A2+ : CUPB1 = 3 : 2 (A2+ : Cs+ = 1 : 2, 50 mg CUPB1 sample for each). The experiments were performed with nine different durations (1.5, 4, 21, 26, 45, 48, 52, 69 and 74 hours) at RT and 70 °C. After the experiments, the suspensions were filtered and the filtrates were analyzed by ICP-MS.

7.2.2 Instrumental studies including Single Crystal (Table in all text of the work) and Powder XRD, SEM/EDS Analysis, Thermal Analysis and Raman Spectroscopy have been performed as described in Chapter 2. Additionally Bond-Valence Analysis was done according to the method described in Chapter 2.

94 Table 7.1. Crystal Data and Structure Refinement for compound CUPB1. a

Compound CUPB1 FW 3215.02 Space group P41212 a (Å) 12.2376(3) b (Å) 12.2376(3) c (Å) 33.9468(11) α (deg) 90 β (deg) 90 γ (deg) 90 V (Å3) 5083.8(2) Z 4 λ( Å) 0.71073 F(000) 5488 -3 Dc(g cm ) 4.201 GOF on F2 1.073 R1 0.0304 wR2 0.0740 a 2 2 2 2 2 ½ R1 = Fo -Fc/Fo, wR2 = {w[(Fo) - (Fc) ] /w[(Fo) ] }

7.3. Results and Discussion 7.3.1 Syntheses. A microporous cesium uranyl BPO, CUPB1, was obtained from mild hydrothermal conditions. It was prepared using a very simple route. For understanding the phase formation diagram, we have performed a series of experiments with the ratios of UO2(NO3)2(H2O)6 : H3BO3 : H3PO3 : CsOH∙x(H2O) = 1 : 1 : 4 : 4, 1 : 3 : 4 : 4, 1 : 5 : 4 : 4, 1 : 8 : 4 : 4, 1 : 10 : 4 : 4, 1 : 12 : 4 : 4 and 1 : 15 : 4 : 4. We only get the crystals CUPB1 in the reactions with ratios of 1 : 8 : 4 : 4, 1 : 10 : 4 : 4 and 1 : 12 : 4 : 4. The highest yield experiment is from the ratio of 1 : 10 : 4 : 4. The main byproducts of these syntheses is cesium uranyl phosphate, Cs(UO2)(PO4) [234], which is a quite stable phase in this reaction system. The first three actinide BPOs were prepared with H3BO3−NH4H2PO4 flux method at around 200 °C with very small amount of water (50 μL) [83]. Changing from H3PO3 to NH4H2PO4, the preparation of compound CUPB1 was unsuccessful. Substitution of H3PO3 to H3PO4 in the synthesis also not leads to a formation of

CUPB1. It was presumed that H3PO3 plays a simultaneous role of flux and reagent.

7.3.2 Structural description of CUPB1.

CUPB1 has a unique microporous structure, which crystallizes in the chiral space group P42212 (No. 94). Three Cs, three U, one B and four P atoms are in the asymmetric unit. The structure of CUPB1 is built from three basic building units (BBU), namely, UO7 pentagonal bipyramids, BO4 and PO4 tetrahedra (see 9- Figure 7.1). One BO4 tetrahedron shares corners with four PO4 tetrahedra, forming the FBB [B(PO4)4] of

95 9- borophosphate framework. [B(PO4)4] is also can be described as (5□:[□]□|□|□|□|) according to the borate 9- classification proposed by Burns [170]. The [B(PO4)4] FBBs are isolated in the 3D framework structure, and they also can be observed in Na3Cd3B(PO4)4 and Cs2Cr3(BP4O14)(P4O13) [79, 205]. The U(2)O7 and

U(3)O7 pentagonal bipyramids are corner or edge sharing with the FBBs, forming two mirror symmetric S-type uranyl-BPOs chains (denoted as UBP-chains) (see Figure 7.2a and 7.2b). The two S-type UBP- 9- chains are further linked by [B(PO4)4] forming 2D uranyl BPO sheets parallel to the ab-plane. In these

2D sheets, 8-MRs with four repeating linkages of -UO7-PO4- can be observed along the c-axis (see Figure

7.2c). The 2D layers are stacking in a mode of ---ABCDA---. U(1)O7 polyhedra further connected the 3- parallel 2D layers, constructed the 3D porous framework [(UO2)3B(PO4)4] (see Figure 7.3a, 7.3b). Three different multi-intersection 8-MR tunnels, which are running along the [001], [110] and [-110] directions, can be observed in the anionic network. Cs+ cations are disordered and located in the voids of the framework to balance of the charge (see Figure 7.3c and 7.5a).

Figure 7.1. The framework structure of CUPB1 with polyhedral (a) and topology (b) representations. UO7 polyhedra, BO4 and PO4 tetrahedra are shown in yellow, green and pink, respectively.

96 Figure 7.2. Zigzag uranyl borophosphate chians U(2)B(PO4)4 (a), U(3)B(PO4)4 (b) along the a-axis, a 2D uranyl borophosphate sheet on the ab-plane (c). UO7 polyhedra, BO4 and PO4 tetrahedra are shown in yellow, green and pink, respectively.

Figure 7.3. The 2D uranyl borophosphate sheets [U(2/3)B(PO4)4] are parallel arranged along the c-axis (a), a 1D [U(1)B(PO4)4] chain along the c-axis (b), view of the 3D framework structure of CUPB1 down the c-axis (c). UO7 polyhedra, BO4, PO4 tetrahedra and Cs atoms are shown in yellow, green, pink and blue, respectively.

The B-O bond lengths in the distorted BO4 tetrahedron range from 1.363(17) to 1.562(17) Å. The O-B-O bond angles are in the range of 104.0(10)° - 117.1(11)°. P-O bond distances in the PO4 tetrahedra are in the range of [1.486(9) - 1.610(9) Å]. O-P-O bond angles range from 99.5(5)° to 113.3(5)°, which are consistent with those BPOs reported previously [62]. The axial U=O bond lengths for U(1)O7 polyhedra are 1.752(8) Å and 1.765(9) Å. The equatorial U(1)-O bond distances are in the range of [2.281(9) -

2.647(7) Å]. The axial U(2)=O bond lengths for U(2)O7 polyhedra are 1.771(9) Å and 1.774(9) Å, and the equatorial U(2)-O bond distances are range from 2.280(8) Å to 2.645(8) Å. For U(3)O7 pentagonal 97 bipyramids, the axial U(3)=O bond lengths are 1.762(9) Å and 1.779(9) Å, the equatorial U(3)-O bond distances are in the range of [2.276(9) - 2.690(8) Å], which is in a good agreement with uranyl borates reported previously [10]. The calculated BVS values for B(1), P(1)-P(4), and U(1)-U(3) are ca. 3.02, 4.96, 5.08, 5.03, 4.99 and 5.95, 6.06, 5.98, respectively. These confirmed the valence states of B, P and U are 3+, 5+ and 6+ in CUPB1. The Cs+ cations are 10, 11 and 12-fold oxygen coordinated, with Cs-O bond lengths from 3.189(9) Å to 3.81(4) Å. The valence states of Cs(1), Cs(2) and Cs(3) are 1+ with the BVS values of ca. 1.12, 1.06, and 0.97, respectively.

The key feature of CUPB1 is the nanoscale size cage U12P24B8. This cage is built from twelve UO7 pentagonal bipyramids, twenty-four PO4 tetrahedra and eight BO4 tetrahedra. Its size is ~12.2 Å × 11.7 Å 3 × 11.7 Å and a volume of ~1670.1 Å . It is a first example of uranyl-based cage containing both PO4 and

BO4 tetrahedra with such a large volume. This complex cage has six 8-MRs windows (see Figure 7.5b). Six Cs+ cations are located near to the windows inside of the cages. The Cs+ cations form a square bipyramidal Cs6 fragment. Compared to the uranyl based cages reported by Burns et. al. [235, 236], the

U12P24B8 cage possesses more complex topology. This is due to the incorporation of the BO4 tetrahedra.

Uranyl groups are isolated in U12P24B8 cages and this is different to the positions of uranyl groups in U28 and U124P32 cages [237]. Four UO7 pentagonal bipyramids are linked by four [B(PO4)4] clusters, forming the U4B4P12 hemisphere. Four U(3)O7 polyhedra bridged two symmetric U4B4P12 hemispheres, creating the nanoscale sized uranyl BPO cage, U12P24B8 (see Figure 7.4).

Figure 7.4. A spherical uranyl borophosphate cage [U2P24B8] polyhedral (a) and topology (b), (c) representation. UO7 polyhedra, BO4, PO4 tetrahedra and Cs atoms are shown in yellow, green, pink and blue, respectively. The large light blue ball is a model for seeing the cage space clearly.

98 Figure 7.5. Tracing of the multi-intersection tunnels within the 3D framework of CUPB1 with stick fashion (a) and the [U2P24B8]-cages packing mode (b).

Study of Framework Topology. The 3D anionic framework of CUPB1 is complex and difficult to be described clearly. In order to give a detailed description of the zeolite-like network, anionic framework 3- [(UO2)3[B(PO4)4] was simplified through omitting of the oxygen atoms (see Figure 7.1b). The simplified cationic net of CUPB1 can be described as a new 8-nodal net topological type with a point symbol of 2 2 2 2 3 3 3 3 6 9 6 {3.4 .5 .6}3{3 .4 .8 .9 }3{3 .4 }{3 .4 .5 } (in Figure 7.1b). The 3D porous framework of CUPB1 has a low framework density of ~12.6 M atoms (M is the cations of the framework) per 1000 Å3, which is comparable with faujasite [203]. Each of the UO7 pentagonal bipyramid is surrounded by two [B(PO4)4]

FBBs, whereas there are six UO7 pentagonal bipyramids are sitting around each FBB [B(PO4)4] unit

(noted as X), existing as a XU6 square bipyramid. Thus the ratio of X and U is 1 : 3 in a XU3 network 3- structure (see Figure 7.6). If we simplified the porous anionic framework [(UO2)3B(PO4)4] of CUPB1 with FBB as a 6-connected node (X) in the 3D network, we can get a XU3 topology network with point 12 3 symbol of {8 .12 }{8}3.

99 Figure 7.6. A primary uranyl borophosphate cluster [(UO2)6B(PO4)4] with B-center (a), a simplified topology of cluster [(UO2)6B(PO4)4] (square bipyramid geometry) (b), view of the simplified topology representation of CUPB1 down the c-axis (c).

The channel system and cavities of CUPB1 can be better illustrated by natural tiling by tracing the colors of the tiles. From the natural tiling point of view, the anionic framework of CUPB1 is built from four different tiles, in which the primary build unit (PBU) is the cage [74∙82]. These PBUs are connected with four other neighboring ones parallel to the ab-plane. A large cavity is surrounded by four [74∙82] cages. These large cavities were filled with cage [38∙412∙88] bridged by the tile of [34∙82], defining a 2D layer parallel to the ab-plane. Those parallel 2D layers were further linked by [32∙82] tiles, forming its complex 3D tiling frameworks. From the tiling model of CUPB1, it is easier to trace its channels and cavities with tiling signatures of [74∙82] + [38∙412∙88] + [34∙82] + [32∙82] (see Figure 7.7).

Figure 7.7. The 3D framework structure of CUPB1 using tiling (a) and the basic tiles of its tiling structure (b).

7.3.3 Ionic Exchange Properties. Heavy metal ions in solutions are toxic to humans if the concentration is sufficiently high. For example, Pb is a highly poisonous metal (whether inhaled or swallowed), affecting almost every organ and system 100 in the human body [238]. Therefore, how to separate the toxic ions from the solutions efficiently is an actual question for the scientists. Removal of inorganic pollutants from the aqua can be achieved by electrodialysis [239], chemical precipitation [240], adsorption [241], solvent extraction [242], and ultrafiltration [243] or ion exchange [244]. CUPB1 is a porous material with a high free void volume of its 3D open framework structure. It is important that the Cs+ ions are disordered and reside in the free voids of the porous network. These have prompted us to investigate its ion-exchange properties. Ion-exchange experiments were conducted with a variety of cations, from monovalent cations A1+ to tetravalent A4+ at room and elevated temperatures. CUPB1 has shown a great ability to remove A1+ and A2+ cations from aquea solutions and for trace exchange with A3+ and A4+ cations. The ion exchanged samples were further studied by EDS elemental analyses, element distribution mapping and ICP-MS analyses.

Figure 7.8. SEM image of the Sr (a), Pb (b), Co (c), Ni (d)-exchanged samples and elemental distribution maps.

Kinetic studies of the A2+ (A = Sr, Pb, Co, Ni) cations exchange. Sorbent materials are very important since they can adsorb toxic ions, such as Pb2+, Ni2+, Co2+etc. [245, 246] and some radionuclides, such as 137Cs, 90Sr and 137mBa, etc. [247-248]. Since CUPB1 has promising ion-exchange properties, we have

101 explored more details for the toxic cations exchange experiments. The kinetic of Sr2+-exchange was investigated. It has been demonstrated that the concentrations of Sr2+ (~450 ppm at V/m ratio of 200 mL/g) decreased rapidly. CUPB1 can remove 56% of Sr2+ from aqueous solution in 24 hrs and ~70.2% in 74 hrs at room temperature (see Figure 7.10a and Table 7.2). Based on the equation (1), we can calculate that the Sr Sr-exchange capacity of CUPB1 q is ~64.1 mg/g. The distribution coefficient Kd is a measurement of Sr affinity and selectivity shown in equation (2). The Kd at RT for CUPB1 is 493.3 ml/g. It is interesting to Sr Sr note that a temperature increase to 70 °C improved both q and Kd to 74.2 mg/g and 942.8 ml/g, respectively (see Figure 7.10a and Table 7.2). The removed amount of Sr (R) increased up to 82.9% after 72 hrs, which is comparable with the Sr-exchangers reported previously by Mertz et al.[249-250] . That implies that the temperature is one of the important driving forces for these ion exchange reactions.

In eq. (1), (2) and (3), C0 and (Ce) Cf represent the initial and equilibrium concentrations of the ions as measured by ICP-MS, V is the solution volume, m is the mass of CUPB1, R is the relative amount of cation removed.

CUPB1 shows a promising application for the ion-exchange with Pb2+ from Pb-contaminated solutions. The kinetics of Pb2+-exchange experiments showed that the Pb-amount (~950 ppm at V/m ratio of 200 mL/g) decreased on ~76.9 % after 72 hrs at RT. The amount of removed Pb has increased to 83.6 % at 70 °C (see Figure 7. 10b and table 7.2). The Pb-exchange capacity, qPb, is ~146.4 mg/g at RT and ~169.4 Pb Pb mg/g at 70 °C. The Kd at RT is 720 ml/g, whereas, the higher Kd value reached at 70 °C is 1640 ml/g (see Figure 7.10b and Table 7.2). The promising Pb-exchanged properties of CUPB1 are comparable with those previously reported modified zeolites [251-252] and resins [253].

102 Figure 7.9. Photos of CUPB1 crystals before and after exchange with colored ions (Co2+ and Ni2+).

Several groups of divalent transition-metal cation (Co2+, Ni2+, Zn2+, Cu2+. etc.) exchangers have been synthesized and characterized, such as resins, MOFs, zeolites or zeolite-like materials and polymers [254- 257]. Among them, MOFs, zeolites or zeolite-like materials preserve their original crystal structure after ion-exchange reactions. As shown in Figure 7.9, the CUPB1 crystals adopt the colors of the exchanged transition-metal ions (Co2+ and Ni2+) within a few days. The crystals can be cut, and the interior shows the same color as the surface. EDS elemental distribution maps and ICP-MS data were collected from the crystals and the solutions after exchange. These measurements can demonstrate the presence of these cations inside of the crystals. CUPB1 exhibits better Co2+ adsorption capability at high temperature of 70 °C (R~74 % after 72 hrs) compared to at RT (R~64%). Accordingly, the exchange capacity qCo is Co ~44.4 mg/g at 70 °C higher than that ~38.6 mg/g at RT. The distribution coefficient Kd increased to 569.2 ml/g (70 °C) from 355.6 ml/g (RT) (see Figure 7.10c and Table 7.2). The maximum Ni2+ exchange capacity, qNi, of CUPB1 was found to be ~40.6 mg/g at RT. The corresponding removed amount is Ni Ni Ni ~67.7 % and the Kd is ~ 425.1 ml/g. When the temperature is increased to 70 °C, the q and Kd are increased to ~45.4 mg/g and ~633.3 ml/g, respectively (see Figure 7.10d and Table 7.2). In comparison with all the exchanged cations lead has showed a better exchange property than Sr, Ni and Co. To explain this phenomenon, we calculated ionic potentials of the corresponding cations. Ionic potential (ф) is the ratio of electric charge (Z) to the radius (r) of an ion. Thus the proportion measures the charge density at the surface of the ion. The ionic potential (ф) values are in a trend of фCs (~0.6) < фPb (~1.6) < Sr Ni Co Pb Sr ф (~1.8) < ф /ф (~2.8). Consequently, the exchange properties have a tendency of Kd > Kd > Ni Co Kd >Kd . During the ion-exchange process, cations had to move through the pores of CUPB1 crystals and also through the channels in the structure, and replace the cesium cations. Commonly, diffusion was faster through the pores and was retarded when the cations moved through the smaller windows of the open channels. In this case, the cations uptake could mainly be attributed to the ion-exchange reactions in the microporous CUPB1 crystals.

103 Table 7.2. The ion-exchanged distribution coefficients Kd and exchanged capacities q of CUPB1.

Ions Sr-exchange Pb-exchange Co-exchange Ni-exchange

Kd q Kd q Kd q Kd q T (ml/g) (mg/g) (ml/g) (mg/g) (ml/g) (mg/g) (ml/g) (mg/g) RT 493.3 64.1 720 146.4 355.6 38.6 425.1 40.6 70 °C 942.8 74.2 1640 169.4 569.2 44.4 633.3 45.4

Figure 7.10. Kinetics of the A2+ (Sr2+/Pb2+/Co2+/Ni2+) ion-exchanged with ~50 mg CUPB1 (A : Cs = 1 : 2) plotted as the A-concentration (ppm) (black/gray line) and the relative A-removed amount (%) (blue/pink line) vs time t (hour), respectively, (a)-(d). Pink and gray represent reaction at RT, black and blue are at 70 °C.

FIB-SEM and EDS elemental analysis. FIB-SEM and EDS elemental analyses were performed for the CUPB1 raw crystals (sizes ~20 µm) and Sr-exchanged samples (10 min, 20 min and 2 h at RT). From the EDX images (see Figure 7.11), it can be seen that the ion-exchange rates are quite high at the beginning of the experiment. For the Sr-exchanged CUPB1 sample with 10 min exchange time, the measured molar ratio of Sr : Cs is around 1 : 4. This implies that the Cs+ cations are exchanged by ~33.3%. After 20 min, the measured molar ratio of Sr : Cs is around 1 : 2. This indicates that the amount of Cs+ cations is 104 exchanged by ~50.0%. After 2 h of ion-exchange, the measured Sr : Cs is around 10 : 1. This suggests that ~95.0 % of the Cs+ cations are exchanged in CUPB1. It has to be mention that EDS is a surface method and of course do not represent the exchange in crystal body (see Figure 7.11).

Figure 7.11. EDS spectra of the Sr-exchanged CUPB1 samples after 0 min, 10 min, 20 min and 2 h.

Figure 7.12. FIB-SEM images of the Sr-exchanged CUPB1 samples after 0 h, 2 h and 24 h.

As we can see from the pictures, the body of the crystal remains stable for at least 24 h. However, the surface of the crystal changed its morphology. Thus, we can conclude that ion-exchange is coupled with small amount of secondary phase precipitation (see Figure 7.12).

7.3.4 Thermal analysis. The thermogravimetric (TG) and Differential Scanning Calorimetry (DSC) measurement of CUPB1 was performed in the range from 50 °C to 1200 °C (see Figure 7.13). TG analysis indicates no obvious weight loss until 1000 °C. There is a small endothermic peak at 387 °C can assigned to the removal of 0.5 mol

105 water molecules per formula unit, which is too light (~0.5 %) to be clearly seen in the TG curve. The endothermic peak at 835 °C is matched with the decomposition of the anhydrated phase.

Figure 7.13 . TG-DSC curves of CUPB1.

7.3.5 Raman Analysis. The Raman spectrum of CUPB1 was measured in a range of 100-4000 cm-1. For convenience, we have divided the spectra into two parts, a low frequency part 100-1800 cm-1 and a high-frequency region in 3000-4000 cm-1 (see Figure 7.14). More peaks are in the range of 100-1000cm-1, which is dominated by contributions from the uranyl group, BO3 triangles, BO4 and PO4 tetrahedra of CUPB1. The Raman bands -1 located in lower frequencies in the range of 190-300 cm could be attributed to the uranyl ion with a v2 bending mode. Raman bands with a series of peaks around 476 cm−1 could be assigned to O-B-O doubly −1 degenerated symmetric bending ν2 mode in BO4 tetrahedra. The Raman peak at ~642 cm is attributed to −1 −1 the bending character v4 mode of BO4. Raman bands from 800 cm to 870 cm should come from the 2+ −1 −1 symmetric vibration v1 mode of the uranyl (UO2) units. The bands of 936 cm and 978 cm can be 2+ attributed to the antisymmetrical stretch v3 modes of uranyl (UO2) groups. The Raman bands at 1002 −1 −1 cm and 1018 cm can be attributed to the PO4 symmetric stretching ν3 and PO4 antisymmetric -1 stretching ν1 modes. The Raman bands within 1100-1200 cm can be attributed to O-B-O triply degenerated asymmetric stretching ν3 mode of BO4 tetrahedra. These assignments are consistent with the previously reported works [10, 258].

106 Figure 7.14. Raman shift of CUPB1.

7.4 Conclusions A remarkable ion-exchanger, CUPB1, was obtained from the mild hydrothermal method. Its 3D 9- microporous framework is based on [B(PO4)4] clusters and UO7 pentagonal bipyramids. The unusual

U12P24B8 cages with nanoscale size (~12.2 Å × 11.7 Å × 11.7 Å) are observed in the structure. In the 3D open framework, three multi-intersection 8-MR tunnels are observed along the [001], [110] and [-110] directions. The Cs+ cations are disordered and resided in the voids of the framework. The free void volume is as high as ~59%. CUPB1 is one of the most porous actinide compounds known today. Importantly, CUPB1 can adsorb monovalent and divalent cations from aqueous solution at room and elevated temperatures. The ion-exchange properties of CUPB1 can be attributed to the multi-intersection channels and the disordered guest cations in the 3D open framework. CUPB1 has proved to be a promising ion-exchanger for the separation or purification of the toxic cations. At RT, the ion-exchange capacity q for Sr2+, Pb2+, Co2+ and Ni2+ are ~64.1 mg/g, ~146.4 mg/g, ~38.6 mg/g and ~40.6 mg/g, respectively. The ion-exchange properties of CUPB1 are better at elevated temperature. At 70 °C, the q for the studied cations, Sr2+, Pb2+, Co2+ and Ni2+, increased to ~74.2 mg/g~169.4 mg/g, ~44.4 mg/g and ~45.4 mg/g, respectively. Speculatively, more porous materials can be obtained in the studied Cs-B-P-U system.

107 Chapter 8. Which Role do Counter Cation Play in the Formation of Actinide Borate-phosphates?

8.1. Introduction Borates and phosphates materials have been intensively studied in last decades because they posess diverse chemical compositions and rich structural chemistry [29, 32, 160, 260-262]. Moreover, they have a wide range of applications, such as adsorption, magnetic and nonlinear optical (NLO). etc [108, 263- 267] Interestingly, the combination of borates and phosphates forms a new family of materials with abundant structural architectures [62, 187]. In general, compounds with mixed oxo-anions of borates and phosphates can be categorized as borophosphates and borate-phosphates [62, 187]. Recently, new compounds of borophosphates have been extensively investigated. For example, MBPO5 (M = Sr and Ba) [69, 268], Na3Cd3B(PO4)4 [205],

AMBP2O8 (A = K, M = Sr, Ba and A = K, Rb, M = Pb) [230, 269, 270] and β-Zn3(BO3)(PO4) [271], are potential promising NLO materials. SrCo2BPO7 [272]and M2BP3O12 (M = Fe, Cr) have shown interesting 2+ magnetic properties [273, 274]. The Eu -doped Ba3BP3O12 can be used as a new scintillation material [275]. In contrast to the large number of borophosphates, new materials of borate-phosphates are quite scarcely reported. For example the mineral Seamanite, Mn3(OH)2[B(OH)4][PO4] [276]and series of synthetic phases of

M3(BO3)(PO4) (M = Ba, Co, Mg, Zn and Ln7O6(BO3)(PO4)2 (Ln = La, Pr, Nd, Sm, Gd, and Dy) have been reported. Borate-phosphates have shown more diverse structures and attractive properties [229, 277-280]. As mentioned in chapters 1, 6 and 7, in recent years actinide borophosphates or actinide borate- phosphates have received much attention because of their abundant structural architectures and potential applications in ion-exchange, catalysis and nuclear waste management area [6]. To the best of our knowledge, only six synthetic actinide borophosphates or actinide borate-phosphates have been reported to date [62, 83, 84, 103]. In the periodic table, strontium is in the same group (IIA) with barium. Sr2+ cation has an identical valence and a smaller ionic radius than Ba2+. Our aim was to substitute Ba2+ by Sr2+ in the uranyl borate-phosphates system and implement the same synthetic method. This study can help us to see which role plays the “templating” cation in the final structure. Herein, a novel strontium uranyl borate-phosphate, namely, [Sr8(PO4)2][(UO2)(PO4)2(B5O9)2] has been prepared and characterized. The synthetic route, microporous framework structure as well as thermal behavior, Raman and IR spectroscopic properties are discussed in detail.

8.2. Experimental Section

8.2.1 Materials and Methods. Uranyl nitrate UO2(NO3)2∙6H2O (International BioanalyticalIndustries,

Inc.), Strontium nitrate Sr(NO3)2 (Alfa-Aesar, 99.9%), Ammonium phosphate monobasic NH4H2PO4

(Alfa-Aesar, 99.8%), Boric acid H3BO3 (Alfa-Aesar, 99.5%).

108 8.2.1.1 Synthesis of SrUPB1: SrUPB1 was prepared via a high temperature solid state method. The initial reagents, UO2(NO3)2∙6H2O (0.0528 g, 0.10 mmol), Sr(NO3)2 (0.1085 g, 0.50 mmol), NH4H2PO4

(0.0573 g, 0.50 mmol) and H3BO3 (0.0929 g, 1.50 mmol) were taken with a molar ratio of U : Sr : P : B = 1 : 5 : 5 : 15. All the mixtures were ground thoroughly in an agate mortar and then transferred to a platinum crucible. The reactants were heated up to 970 °C for 5 hours in a box furnace. Then it was cooled down to 550 °C with a cooling rate of 5 °C/h and then the furnace was switched off. As a result of the reaction, orange block shaped crystals, SrUPB1, were obtained. EDS elemental analyses on single crystals of SrUPB1 showed an average molar ratio of U : Sr : P = 1 : 8.19 : 4.06, which is in good agreement with the chemical composition obtained from structural solution.

8.2.2 Instrumental studies including Single Crystal (Table in all text of the work) and Powder XRD, SEM/EDS Analysis, Thermal Analysis and Raman Spectroscopy have been performed as described in Chapter 2. Additionally Bond-Valence Analysis was done according to the method described in Chapter 2.

Table 8.1. Crystal Data and Structure Refinements for SrUPB1.a

Compound SrUPB1 FW 1746.97 Space group P21/n a (Å) 6.5014(2) b (Å) 22.4302(9) c (Å) 9.7964(4) α (deg) 90 β (deg) 90.241(4) γ (deg) 90 V (Å3) 1428.57(9) Z 2 λ( Å) 0.71073 F(000) 1588 -3 Dc(g cm ) 4.061 GOF on F2 0.937 R1 0.0379 wR2 0.0786 a 2 2 2 2 2 ½ R1 = Fo -Fc/Fo, wR2 = {w[(Fo) - (Fc) ] /w[(Fo) ] }

8.3 Results and Discussion 8.3.1 Structure of SrUPB1. SrUPB1 is the first unhydrate alkaline earth metal uranyl borate-phosphate.

It crystallizes in a centrosymmetric space group P21/c. Five boron, one uranium, two phosphor, four strontium and twenty-three oxygen atoms are in the asymmetric unit of the unit cell (see Figure 8.1). The structure of SrUPB1 is based upon a novel 3D uranyl borate framework with two different sorts of 1D 109 channel. One phosphate tetrahedra is linked with uranyl group inside of the tunnels, whereas the second 3- one is isolated in the tunnels. The [B5O9] FBB pentamer is constructed from two BO4 tetrahedra and three BO3 triangles (see Figure 8.2c). Topology representation of the FBB can be described as 3Δ2□:<Δ2□>−<2Δ□> according to Burn’s designation. These FBBs are further sharing corners with four neighboring ones and are polymerized into the 2D polyborate layers. Oxo-borate 9-MRs are observed within these 2D layers (Figure 8.2b). The B-O bond lengths in planar BO3 triangles are in the range of

[1.345(12) Å-1.387(12) Å]. The B-O bond distances in BO4 tetrahedra are longer and range from

1.405(12) Å to 1.517(13) Å. The bond angles of O-B-O are in the range of [111.4(9)°-126.6(9)°] for BO3 triangles and [104.7(7)°-116.2(8)°] for BO4 tetrahedra. The BVS values for boron cations confirm that the valences of B cations are 3+.

Figure 8.1. The asymmetric unit of SrUPB1, showing the atom-labeling scheme and 50% thermal ellipsoids.

110 Figure 8.2. (a) The framework structure of SrUPB1 along the a-axis, (b) a 2D borate sheet on ac-plane, (c) a [B5O11] pentamer FBB with (d) its topology representation. B, P, U, Sr and O atoms are shown as green, pink, yellow, blue and red, respectively.

The uranium atoms are six-fold oxygen coordinated, and existing as UO6 square bipyramids in the structure of SrUPB1. UO6 polyhedra are playing a role of a linker between the 2D polyborate layers. As a 4- result a unique 3D uranyl polyborate framework [UO2(B5O9)2] forms in SrUPB1. In the 3D framework, 1D 18-MR channels are along the a-axis and 14-MR channels are along the c-axis. The sizes of the channels are ~6.3 Å × 16.3 Å and ~7.7 Å × 12.6 Å, respectively (see Figure 8.3). The isolated P(2)O4 tetrahedra are located in the 18-MR tunnels and are sharing corner with the UO6 polyhedra. The 4- interconnection between PO4 and UO6 forms a [UO2(PO4)2] fragment within the 3D framework 10- [(UO2)(PO4)2(B5O9)2] ∞. The isolated P(1)O4 tetrahedra are residing in the tunnels of this 3D framework (see Figure 8.2a). The uranyl axial bond distances of U=O are 1.820(6) Å. The equatorial U-O bond lengths range from 2.199(6) Å to 2.242(7) Å. The uranyl group is connected with two BO3 triangles and two PO4 tetrahedra within its equatorial plain. This coordination environment has never been found in previous actinide bearing compounds [6, 83, 84, 103] (see Figure 8.4a, 8.4b). The P-O bonds in the PO4 tetrahedra have a distance range of 1.526(7) Å-1.600(7) Å. The bond angles of O-P-O range from 103.8(4)° to 115.3(4)°. The valences of P cations are suggested to be 5+ from their BVS values of 4.98 and 5.06, whereas for U cation it is 6+ with the BVS of U(1) is 6.03.

111 Figure 8.3. (a) The uranyl borate framework of SrUPB1 along the a-axis with its simplified cationic framework topology, (b) the uranyl borate framework of SrUPB1 along the c-axis with its simplified cationic framework topology, (c) a 18-ring along the a-axis and (d) a 14-ring along the c-axis.

112 Figure 8.4. The coordination environment of the uranyl polyhedra in BaUPB1 (a) and SrUPB1 (c), and their topology representations (b), (d).

From the natural tiling illustration (see Figure 8.5), the borate framework of SrUPB1 is built from a novel (composite building unit) CBU of [34∙92∙12∙182]. This CBU is based on the intersecting 18- and 14-MR tunnels. Therefore, we can see clearly that the phosphates have no contribution to the building of the 3D skeleton for SrUPB1 framework. Each CBU [34∙92∙12∙182] is linked to five other adjacent ones via their 9, 12 or 18-MRs and is constructing the 3D uranyl polyborate tiling framework.

Figure 8.5. (a) View of the uranyl borate framework along the a-axis using natural tiling natural tiling, (b) the new [34∙92∙12∙182] CBU.

113 Framework Topology Study. As described above, each pentamer FBB is connected to four other ones via corner sharing of u2-O(10) atoms. Each uranyl group is connected with two FBBs, and is existing as a two-connected node. However, each FBB is only linked with one uranyl group. Thus, the FBBs are five- connected nodes. The simplified uranyl borate framework of SrUPB1 can be described as a new 2-nodal 4 4 2 net topological type with the point symbol of {4 .6 .8 }2{6} (see Figure 8.3a, 8.3b). Strontium cations are nine or ten-fold oxygen coordinated with Sr-O bond lengths range from 2.505(6) Å to 3.256(7) Å. The BVS of Sr(1)-Sr(4) are 2.06, 1.98, 1.96 and 2.03, respectively. This indicates that the valences of strontium cations are +2.

Figure 8.6. (a) The structure of BaUPB1 along the c-axis, (b) a nano-tubular borate tunnel of BaUPB1 along the b-axis, (c) a 2D borate sheet in SrUPB1 along the b-axis.

It is noteworthy to compare the structures of SrUPB1 and BaUPB1 [103] because both of them possess 3- identical FBB, [B5O9] , in their polyborate framework. BaUPB1 has a quasi-3D framework structure, crystallized in a centrosymmetric space group P42/n (No. 85). The structure of BaUPB1 is featured with a 10- complex nanotubular fragment of [(UO2)(PO4)3(B5O9)] . In fact, its structure is not a real 3D framework structure because these cross-like fragments are isolated. The Ba2+ cations are located in the pores among those uranyl borate-phosphate cross sections. It is interesting to note that, if we unfold the nanotubular borate fragment of BaUPB1, the resultant 2D pentaborate layer has the same topology as the 2D borate sheet in SrUPB1 (in Figure 8.6c and Figure 8.2b). The 2D borate layers are bridged by uranyl polyhedra, constructing the 3D uranyl borate framework of SrUPB1 (see Figure 8.2b). Uranium in both phases are 6- fold oxygen coordinated, presented as UO6 square bipyramids. However, the U-centers possess different borate and phosphate coordination environments. The U-center in BaUPB1 has connected to two BO3 triangles and two PO4 tetrahedra in a centrosymmetric mode of <□[ -U-□] >. Whereas the U-center in

SrUPB1 is coordinated by one BO3 triangle and three PO4 tetrahedr△a as <□△[□-U-□] > (see Figure 8.4).

114 △ Compared to Sr2+ cation, Ba2+ cation has the same charge but larger radius. It is presumed that the cations charge is predominating the topology of the polyborate unit. However, the radius of templating cation is the driving force for the final dimensionality of the polyborate structure. 8.3.2 Thermal Analysis. Thermogravimetric (TG) and Differential scanning calorimetry (DSC) were measured on SrUPB1 sample in the range from RT to 1200 ºC. The TG-DSC patterns are plotted in Figure 8.7. There are no obvious weight changes in the TG curve of SrUPB1. The DSC curve shows a relative strong endothermic peak at 972 ºC, which corresponds to the decomposition of SrUPB1. A much stronger endothermic peak at 1061 ºC is corresponded to the melting of the rest phases.

Figure 8.7. TG-DSC curves of SrUPB1.

8.3.3 Raman and IR Analyses. The Raman spectrum of SrUPB1 was measured in the range of 100 -1400 cm-1 (Figure 8.8a). Raman −1 bands around 341–500 cm could be assigned to O-B-O doubly degenerated symmetric bending ν2 mode −1 −1 in BO4 tetrahedra. The Raman bands from 502 cm to 750 cm are attributed to the bending character v4 −1 mode of BO4. A weak peak around 813 cm comes from the symmetric vibration v1 mode of the uranyl 2+ −1 (UO2) groups. The Raman bands around 1076 cm can be attributed to the PO4 symmetric stretching ν1 -1 and PO4 antisymmetric stretching ν3 modes. The Raman bands within 1178-1200 cm can be attributed to asymmetric and symmetric stretching v1, v3 modes of the B–O–B bonds in B2O5 dimers. The peaks in the −1 range of 1230–1300 cm are assigned to doubly degenerate asymmetrical stretching ν3 mode in the BO3 triangles. The IR spectrum of SrUPB1 is shown in Figure 8.8b. The IR spectrum shows strong absorption –1 bands at 1315-1454 cm that come from the asymmetric stretching of BO3 units. The absorption peaks in –1 the range of 964-1048 cm are attributed to asymmetric stretching and symmetric bending of BO4 and 115 -1 PO4 tetrahedra. The IR absorption bands of 761-950 cm are correspond to the antisymmetric stretching 2+ vibrations and symmetric stretching vibrations of uranyl (UO2) groups. The absorption peaks between –1 750 and 500 cm are caused from the synergistic effect of the in-plane bending of the B–O bonds in BO3 triangles and vibrations of the P–O bonds in PO4 tetrahedra. These assignments are according to the previously reported works [10, 204, 258].

Figure 8.8. (a) Raman and (b) IR spectra of SrUPB1.

8.4. Conclusions. An unusual actinide borate-phosphate, SrUPB1, was prepared through high temperature solid state method by substitution of counter cation Ba to Sr. Its structure was determined by X-ray crystallography and further confirmed by the Raman and IR spectroscopies. All bands observed in the Raman and IR spectra could be assigned to the corresponding groups in the structure. The complex structure of SrUPB1 is based upon a novel 3D uranyl borate framework. Two large 14- and 18-MRs channels are observed in 116 the 3D framework. The 3D uranyl borate framework is composed of corrugated polyborate layers and distorted UO6 polyhedra. The polyborate layers are interconnected by UO6 tetragonal bipyramids within the 3D network. A single PO4 tetrahedron is linked with uranyl group in the tunnels, whereas the second one is isolated resided in the 18-MR tunnels. The 2D polyborate layers are built from the pentamer FBB 3- [B5O9] . This pentamer FBB can be simplified as one node for the cationic framework topology. The new 4 4 2 simplified topological type can be described as a 2-nodal net with the point symbol of {4 .6 .8 }2{6}. Notably, by substitution of Ba2+ cations by Sr2+, the topology and FBB of the final polyborates does not show any change. This shows that the cation radius played a key role for the dimensionality of the final structure. Ba2+ cation has a larger radius than Sr2+, and this leads to the crystallization of polyborate unit into a 3D skeleton in BaUPB1 and a 2D layered structure in SrUPB1. It is presumed that, the radius of a counter cation plays a crucial role for the final structural dimensionality.

117 Chapter 9. Flux Induced Polymorphism in ThB2O5 under ambient pressure

9.1. Introduction In nature, thorium is estimated to be about three to four times more abundant element than uranium. Th has one of the longest half-life among the radioactive elements, which is 14.05 billion years [281-285]. It has a very stable oxidation state of +4, which is similar to that of some lanthanides and the group four elements, such as Ti, Zr, and Hf [286]. Boron is the only electron-deficient nonmetallic element in the group thirteen of the periodic table, therefore, it has a high affinity for oxygen, forming strong covalent boron-oxygen bonds in borates [287]. However, based on the literature analysis only three thorium oxo-borates have been reported up to now

[92, 106, 108] β-ThB2O5, the first compound in thorium borate family and was obtained by Gasperin in 1991 through high-temperature flux synthesis [92]. It is featured with a 3D thorium borate framework, in which thorium-thorium exist with a diamond packing mode forming a 3D thorium-network. B2O5 dimers are edge sharing with thorium polyhedral and are filling in the voids of the Th-cation framework along the [-101] direction. In 2010 the second thorium borate compound [ThB5O6(OH)6][BO(OH)2]·2.5H2O [108] was reported by Wang et al. This phase was synthesized with the synthetic method of low- temperature boric acid flux. This exceedingly unusual thorium borate is constructed from crown-like

B10O24 groups and twelve-fold coordinated thorium ions. These groups are forming a porous supertetrahedral 3D thorium-borate framework. The unprecedented supertetrahedra cationic framework in

[ThB5O6(OH)6][BO(OH)2]·2.5H2O makes this compound a superb anion exchanger. Hinteregger et al. has prepared the third member in the thorium borates system, ThB4O8, which was synthesized under extreme conditions (5.5 GPa/1100 °C) through high-temperature/high-pressure (HT/HP) method [106].

Its structure is based on a 2D polyborate layer composed of corner-sharing BO4 tetrahedra. The ten-fold coordinated thorium cations are located in the interlayer space. With the motivation of understanding the structural chemistry and behavior of thorium borates under different conditions, our research interests are focus on Th-B-O-A (A= alkali, alkaline earth metal elements) system. Herein, we report a new polymorph α-ThB2O5 in ThB2O5 family, which has a large volume change compared to the previously reported β phase. The synthetic conditions, structural chemistry and topology, phase relationship and phase transformation, thermodynamics stability, theoretical analysis as well as Raman and IR spectra of both polymorphs are reported.

9.2. Experimental Section

9.3.1 Materials and Methods. Thorium nitrate Th(NO3)4∙5H2O (International BioanalyticalIndustries,

Inc.), Lithium carbonate Li2CO3 (Alfa-Aesar, 99.9%), Sodium carbonate Na2CO3 (Alfa-Aesar, 99.9%),

118 Potassium carbonate K2CO3 (Alfa-Aesar, 99.9%), Rubidium carbonate Rb2CO3 (Alfa-Aesar, 99.9%),

Cesium carbonate Cs2CO3 (Alfa-Aesar, 99.9%), Boric acid H3BO3 (Alfa-Aesar, 99.9%).

9.2.1.1 Synthesis of α-ThB2O5. Single crystals of monoclinic α-ThB2O5 were obtained from a high temperature flux method. They were grown from a high temperature solution using H3BO3-Li2CO3 as flux.

Th(NO3)4∙5H2O (0.105 g, 0.18 mmol), H3BO3 (0.0618 g, 1.00 mmol) and Li2CO3 (0.0182 g, 0.25 mmol) were mixed in the mortar and grinding thoroughly, then transferred into a platinum crucible. The mixture was holded in a box furnace at 450 °C for 1 hour for decomposing of thorium nitrate and boric acid. Then the temperature was increased to 970 °C for 5 hours until the homogeneous melt was formed. After this, the melt was cooled down to 700 °C at a rate of 5 °C/h and then it was cooled down to room temperature with a cooling rate of 10 °C/h. The resulting products were washed with hot water to remove the excess of boric acid. Large colorless block shaped crystals of α-ThB2O5 were obtained.

The nearly pure polycrystalline samples of α-ThB2O5 were prepared quantitatively by the reaction of a mixture Th(NO3)4∙5H2O (0.306 g, 0.54 mmol) and H3BO3 (0.090 g, 1.45 mmol) with a molar ratio of 3 : 8 at 850 °C for 36 hours. Then the samples were washed with boiled water several times for eliminating the impurities. Energy dispersive X-ray spectroscopy (EDS) elemental analysis on several single crystals gave an average molar ratio of Th : O= 1 : 5.35 for α-ThB2O5, which is in good agreement with those obtained from single crystal X-ray diffraction studies.

9.2.2 Instrumental studies including Single Crystal (Table in all text of the work) and Powder XRD, SEM/EDS Analysis, Thermal Analysis and Raman Spectroscopy have been performed as described in Chapter 2. Additionally Bond-Valence Analysis was done according to the method described in Chapter 2.

9.2.3 Ab initio calculations

The computational studies of the ThB2O5 system were performed with density functional theory (DFT) method using Quantum-ESPRESSO [288] simulation package. Because we are interested in good computations of structures of considered materials we applied PBEsol exchange/correlation functional [289], which was successfully applied in the previous studies of An-bearing systems [286, 290]. The core electrons of the constituent atoms were modeled with ultrasoft pseudopotentials [291] and the 6s26p66d27s2 electrons of thorium atom were computed explicitly. Basing on previous experience with computing Th-bearing systems, we selected 50Ryd as plane-wave energy cutoff.

119 a Table 9.1. Crystal Data and Structure Refinements for α-ThB2O5 and Crystal Data for β-ThB2O5.

Compound α-ThB2O5 β-ThB2O5 FW 333.66 333.66 Space group P21/m C2/c a (Å) 4.2535(2) 11.545(3) b (Å) 6.8733(3) 6.937(2) c (Å) 6.3242(3) 10.263(3) α (deg) 90 90 β (deg) 106.316(5) 101.5(3) γ (deg) 90 90 V (Å3) 177.445(14) 805.44 Z 2 8 λ( Å) 0.71073 F(000) 280 -3 Dc(g cm ) 6.245 5.500 GOOF on F2 0.851 R1 0.0261 wR2 0.0609 a 2 2 2 2 2 ½ R1 = Fo -Fc/Fo, wR2 = {w[(Fo) - (Fc) ] /w[(Fo) ] }

The α- and β-ThB2O5 structures were modeled with 16 and 64 atoms contained supercells and the calculations were performed on the 6 × 4 × 4 and 2 × 4 × 3 Methfessel−Paxton k-points grids [292].The calculations of phonons and vibrational entropy contributions to Gibbs free energies were performed with the density functional perturbation theory using PHONON and DYNMAT packages of the Quantum- ESPRESSO code.

9.3. Results and Discussion 9.3.1 Synthesis. Investigation of the simplest thorium borate Th-B-O system reveals a novel polymorph of α-ThB2O5 which was obtained through high temperature flux method. It is necessary to note that we used the different initial reaction chemicals Th(NO3)4∙5H2O/Li2CO3/H3BO3 for preparation of α-ThB2O5 compared to ThO2/Na2B4O7/B2O3 for the synthesis β-ThB2O5 [92]. The reaction temperature was decreased to 970 °C from 1200 °C for synthesis of α-ThB2O5. If Li2CO3 was substituted by Na2CO3,

K2CO3, Rb2CO3, Cs2CO3 then α-ThB2O5 can be successfully synthesized at the temperature 1000 °C as well. Hence, we presumed that the initial flux and temperature have a great impact on the resulting phases.

If we put a mixture of α-ThB2O5 and β-ThB2O5 phases or a pure β phase into a platinum crucible under HT/HP (4 GPa, 1000 °C) conditions, we can get pure α phase finally. This indicates that the phase transition from β to α-ThB2O5 can happen under extreme conditions (HT/HP). This indicates that the fluxes played a crucial role during the structure formation process. In performed experiments fluxes are

120 not only lowering the melting point of the mixtured chemicals, but they are also strongly influencing the formation of the phase modification.

Both polymorphs of α-ThB2O5 and β-ThB2O5 were prepared at ambient pressure. Hinteregger et al. have reported a thorium borate ThB4O8 synthesized through HT/HP method at extreme conditions (5.5 GPa, 3 -3 1000 °C) [93]. The density of ThB4O8 is 5.970 g/cm is larger than this of β-ThB2O5 (5.500 g·cm ). The -3 density of α-ThB2O5 is 6.245 g·cm and even larger than ThB4O8.

9.3.2. Description of Structure. Structure refinement reveals that α-ThB2O5 crystallizes in the monoclinic space group P21/m. It has one crystallographically independent Th, two B and four O atoms in the structure. The 3D framework of α-ThB2O5 is based on 1D boron chains and 2D thorium layers (see

Figure 9.1). In the structure of α-ThB2O5, B(1) atoms are existing as B(1)O4 tetrahedra and B(2) atoms are forming B(2)O3 planar triangles. B(1)O4 tetrahedra and B(2)O3 triangles are connected with each other through corner sharing, forming a 1D zigzag boron-oxygen chain along the a-axis (see Figure 9.1a). The oxo-borate fragments are polymerizing into 2D layers in the structure of the phases from the family of closely related compounds AB2O5 (A = Zr, Hf) [293, 294]. Boron atoms demonstrate only BO4 tetrahedra within the structures of A2B2O5 (A = Zr, Hf).

In α-ThB2O5, two BO4 tetrahedra are corner sharing with each other, forming four- and eight-MRs parallel to the bc-plain. The zigzag oxo-borate chains are uncovered for the first time. B-O bond lengths in B(1)O4 tetrahedra are ranging from 1.455(7) Å to 1.472(11) Å. The O-B(1)-O bond angles are in the range of 105.8(7)° - 113.1(5)°. Whereas the B-O bond distances in B(2)O3 range from 1.356(11) Å to 1.390(11) Å. The O-B(2)-O bond angles are in the range of 114.2(7) °-127.4(8) °, which are consistent with the borates reported previously [10] (see Table 9.2).

Figure 9.1. Polyhedral representation of α-ThB2O5 structure. A 1D boron-oxygen chain along the a-axis (a), a 2D thorium layer on ab-plane (b), the 3D thorium borate structure along the a-axis (c).Thorium polyhedra, BO3 triangles and BO4 tetrahedra are shown in yellow and green, oxygen atoms are shown in red. 121 Table 9.2. Selected Important Bond Lengths (angstroms) for α-ThB2O5.

Th(1)-O(3)#1 2.302(5) Th(1)-O(3)#2 2.302(5) Th(1)-O(2)#3 2.370(6) Th(1)-O(1) 2.583(6) Th(1)-O(4)#4 2.520(7) Th(1)-O(2) 2.514(6) Th(1)-O(3)#5 2.539(5) Th(1)-O(3)#6 2.539(5) Th(1)-O(3) 3.067(6) Th(1)-O(3) 3.067(6)

B(1)-O(1) 1.472(11) B(1)-O(3) 1.455(7) B(1)-O(4) 1.471(11) B(1)-O(3)#7 1.455(7) B(2)-O(2) 1.356(11) B(2) - O(1) 1.380(11) B(2)-O(4) #3 1.390(11) Symmetry transformations used to generate equivalent atoms: #1 -x, -y, -z+2, #2 -x, y+1/2, -z+2, #3 x-1, y, z, #4 x+1, y, z+1, #5 x, -y+1/2, z+1, #6 x, y, z+1, #7 x, - y+1/2, z.

Figure 9.2. Representation of the β-ThB2O5 structure. A one-capped distorted thorium pentagonal bipyramid (a), a four-fold Th-coordinated thorium polyhedron (b), the 3D thorium network structure along the b-axis (c), a B2O5 dimer (d), the 3D thorium borate framework structure along the b-axis (e). ThO8 polyhedra and BO3 triangles are shown in yellow and green, O atoms are shown in red.

Thorium atoms are ten-fold oxygen coordinated, existing as ThO10 four caped trigonal prisms (in Figure

9.2c). These ThO10 polyhedra are edge and face sharing with each other forming a 2D Th sheet parallel to the ab-plain (Figure 9.1b). The eight Th-O bonds are ranging from 2.302(5) Å to 2.583(6) Å. The two Th- O bonds are longer with 3.067(6) Å. Because of the large difference between Th-O bond lengths, one can

122 call the thorium coordination number as (8+2). The 2D Th-based sheets are further linked by the zigzag boron-oxygen chains, through corner, edge and face sharing, and are forming the 3D thorium borate framework (see Figure 9.1c). In the 3D thorium borate framework, a six-MR channel was observed along the c-axis. The size of this channel is ~4.5 Å × 3.5 Å (distances are between two opposite O-O atoms) (see Figure 9.1d). Coordination environment and geometry of B and Th centers. Tetravalent Th cation has complex oxygen coordination. The coordination numbers are ranging from six to fifteen, depending mainly on the nature of ligands [295-296]. The connection between Th polyhedra and borates is producing numerous thorium borates architectures. In the structure of α-ThB2O5, B(1)O4 tetrahedra are face sharing with one and corner sharing with four Th polyhedra, denoted as Th4BTh, existing in form of square pyramid geometry (see Figure 9.3a, d). B(2)O3 triangles are edge sharing with one and corner sharing with two Th polyhedra, exhibiting a planar triangle geometry as ThBTh2. ThO10 polyhedra are corner, edge and face sharing with five BO4 tetrahedra and three BO3 triangles, forming a Th-centered hexagonal bipyramidal

(ThB8) geometry (see Figure 9.4). This Th coordination environment is reported for the first time in the inorganic thorium bearing compounds.

Figure 9.3. (a) The oxygen coordination environment of thorium cation with ball and stick mode, (b) the four caped trigonal prismatic shape of thorium polyhedra. Th and O atoms are shown as yellow and red, respectively.

123 Figure 9.4. The coordination environment of a B(1)O4 tetrahedron (a) schematic topological representation of square pyramid geometry (d), coordination environment of a B(2)O3 triangle (b) schematic topological representation of a trigonal geometry (e), coordination environment of ThO10 polyhedra (c) schematic topological representation of a hexagonal bipyramidal shape of Th coordination (f).

It is necessary to compare the structure of α-ThB2O5 with β-ThB2O5 to reveal a structural difference between two polymorphs. Both of them possess 3D framework structures, which are composed of two independent B and one Th atoms per unit cell. However, B atoms are only presented as BO3 triangles in the β phase. In the structure of β-ThB2O5, two BO3 triangles are corner sharing linked forming an isolated

B2O5 dimer. Each BO3 triangle is corner sharing with two and edge sharing with one Th polyhedra, forming a BTh3 fragment with linear geometry. In the structure of α-ThB2O5, as described above, B(1)O4 tetrahedra and B(2)O3 triangles are corner sharing and are forming a 1D zigzag chain. Within the 1D B-O chains, the two shortest B-B distances are 2.489(3) Å and 2.594(1) Å, respectively. For comparison, the shortest B(1)-B(2) distance in the structure of β-ThB2O5 is 2.445(1) Å (see Figure 9.4a). Thorium cations are eight-fold coordinated in the structure of β-ThB2O5 and existing as ThO8 distorted one-caped pentagonal bipyramids. These ThO8 distorted one-caped pentagonal bipyramids are edge sharing with each other, forming a 3D Th-O framework. Each Th cation is connected to four other Th cations in a diamond packing mode in the structure of β-ThB2O5. The four shortest Th-Th distances are ranging from

4.071(1) Å to 4.131(3) Å in the structure of β-ThB2O5. There are seven neighboring Th cations with Th-

Th distances ranging from 3.965(5) Å to 4.411(2) Å in the structure of α-ThB2O5 (see Figure 9.5a, b). We supposed that changing of packing mode between β- and α-ThB2O5 modifications is necessary for the large molar volume change (~12%). There is a density increase of ~15% from β to α phase.

124 Figure 9.5. The adjacent Th-Th bond distances in two polymorphs.

In order to see the thorium-thorium coordination environments and spatial interactions more clearly, Voronoi-Dirichlet polyhedra (VDP) of Th atoms in Th sublattices have been described for both polymorphs of ThB2O5 (see Figure 9.6). The Th VDP of α-ThB2O5 shows a truncated cuboctahedron with the combinatorial–topological type (CTT) of 45·52·65. This face distribution symbol means that there are three different numbers (5, 2, 5) of different (4, 5, 6) faces. Whereas, the CTT of the Th atoms for β- 4 4 8 4 ThB2O5 is totally different, shown as 3 ·4 ·5 ·10 in Figure 9.6. The large differences between CTTs for the Th atoms in the VDP indicate that, Th atoms bear a totally different spatial distribution in the two polymorphic modifications.

Figure 9.6. Schlegel projections of the Voronoi–Dirichlet polyhedral (VDP) with 45·52·65 and 34·44·58·104 combinatorial–topological types (CTTs).

125 Figure 9.7. Cationic topology representation of α-ThB2O5. (a) A zigzag boron chain ∙∙∙B1B2B1B2B1∙∙∙along the a-axis, (b) a 2D {36} Th-sheet parallel to the ab-plain, (c) Topological view of the 3D new 3-nodal cation network with a point symbol of {34, 410, 510, 64}{34, 410, 56, 6}{34, 44, 52}. The Th cations are shown as 8-connected nodes with blackballs, B1 and B2 are 7 and 5-connected nodes as hollow balls.

Structural topology analysis. In order to reveal the cationic topology of the structures, we omitted the anions whilst retaining the connectivity between the cations from the 3D thorium borate framework of α-

ThB2O5. As shown in Figure 9.7, B1 and B2 are linked along a-axis forming a zigzag boron chain ∙∙∙B1B2B1B2B1∙∙∙ (see Figure 9.7). Thorium cations are connected with six other neighboring ones forming a 2D Th layer parallel to the ab-plain. Each Th is a [36] node within the 2D Th-sheet topology (Figure 9.7b). The zigzag B-chains bridged over those parallel Th-sheets along c-axis and constructed the

3D ThB2 framework (see Figure 9.7c). The simplified ThB2 network can be depicted as a new 3-nodal net topological type with a point symbol of {34, 410, 510, 64}{34, 410, 56, 6}{34, 44, 52}. 9.3.3 Thermal and HT-PXRD Analyses. The Thermogravimetric (TG) and Differential scanning calorimetry (DSC) experiments were performed in the temperature range of 200-1200 °C. For this experiment we used two different samples. First was a pure β-ThB2O5 and second sample was a mixture of α and β-ThB2O5 .The HT-PXRD pattern and TG-DSC curves of pure β-ThB2O5 are shown in Figure 9.8. There is no obvious weight loss up to 1200 °C based on the TG curve of ThB2O5. At ~630 °C, a slow transformation from β to α-ThB2O5 is starting. Until ~870 °C the β-ThB2O5 has transformed into α-ThB2O5 based on the HT-PXRD pattern (see Figure 9.8a). This is corresponding to the broad endothermal peak in the range of (650 °C - 850 °C) shown in the DSC curve of β-ThB2O5. The DSC curve shows a strong endothermic peak at 1053 °C, which corresponds to the phase decomposition of the α-phase.

126 (a)

(b)

Figure 9.8. (a) HT-PXRD pattern and (b) TG and DSC curves of β-ThB2O5.

Figure 9.9. HT-PXRD pattern of a mixture of α-(66 %) and β-ThB2O5 (34 %).

127 The HT-PXRD experiment on the mixture of β (34 %) and α (66 %) phases reveals similar results. The pattern is presented on the Figure 9.9. As shown in Figure 9.9, the phase transformation process from β to α happened in the temperature range of ~ 630–850 °C. The α phase can be stable up to ~1060 °C based on the HT-PXRD pattern.

9.3.4 Vibrational spectroscopy. -1 Raman spectrum of α-ThB2O5 was measured in the region of [100 -1500 cm ]. As shown in Figure 9.10a, the modes in low-frequency region from 187-389 cm−1 are raised from the lattice vibrations. Raman band −1 with a very strong peak around 526 cm can be attributed to the bending character ν4 mode of BO4 −1 tetrahedra. Raman bands with weak peaks around 800 cm are due to the symmetric stretching ν1 mode −1 of BO4 units. The Raman band at around 966 cm has been assigned to the B-O-B symmetrical stretching −1 ν3 mode in BO3 triangles. The peaks around 1362 cm can be attributed to the doubly degenerated asymmetrical stretching ν4 mode of the B–O bonds in the trigonal BO3 units. For β-ThB2O5, Raman band −1 with a strong peak at 472 cm is arisen by the doubly degenerated in-plane O–B–O bending ν1 mode −1 −1 from the planar trigonal BO3 groups. The weak peaks at 700 cm and 803 cm can be assigned to the B–

O bending ν2 mode in the BO3 triangles. The symmetrical stretching ν3 modes of BO3 groups have caused the peaks at Raman bands of 918 cm−1 and 1000 cm−1. Raman band with a weak peak at 1416 cm−1 is attributed to the doubly degenerated asymmetrical stretching ν4 mode of BO3 units [297-299]. IR spectra of both phases were measured in the range of [4000 - 400 cm-1] (see Figure 9.9b). The IR spectra of both phases show very high transmittance in the range of [4000-1600 cm−1] (2.50−6.25 μm). The absorption bands between 1027 cm-1 and 1438 cm-1 in both curves can be assigned to the antisymmetric stretching vibrations of the BO3 groups in the two compounds. The peaks associated with -1 the BO4 symmetric stretch vibrations appeared in the range of [928-735 cm ] for α-ThB2O5. The −1 absorption peaks at 529-652 cm can be assigned to bending vibrations of BO3 and BO4 groups for α-

ThB2O5 and BO3 groups for β-ThB2O5. These assignments are consistent with the borates reported previously [10, 258].

128 Figure 9.10. Raman spectra of compound α-ThB2O5 (a) and β-ThB2O5 (b).

Figure 9.10. IR spectra experimental (black) and calculated (red) curves of α-ThB2O5 and β-ThB2O5.

9.3.5 DFT investigations of polymorphs.

The computed lattice parameters for the α-ThB2O5 and β-ThB2O5 phases are reported in Table 9.3. The lattice parameters of α-ThB2O5 phase are reproduced with surprisingly good accuracy with relative error much smaller than 1% and the volume being off from the experimental value by just 0.6%. On the other hand, the lattice parameters of β-ThB2O5 are off the experimental values by up to 1.5% with the volume 129 being overestimated by 2.5%, although this is still reasonably good performance of DFT. In spite of these offsets we notice that both structures are well reproduced and the DFT-based geometry relaxation did not result in significant deformation of the experimental structures, which were taken as starting configurations.

Table 9.3. The lattice parameters of α-ThB2O5 and β-ThB2O5 obtained by DFT studies. In parentheses we report the offset from the measured values.

α-ThB2O5 β-ThB2O5 a 4.239 (-0.014) 11.643 (+0.098) b 6.934 (+0.061) 7.037 (+0.100) c 6.339 (+0.015) 10.258 (-0.005) Alpaha 90 90 Betha 106.69 (+0.374) 100.68 (-0.82) Gamma 90 90 Vol 178.47 (+1.03) 825.96 (+20.52)

In Table 10.4 we present the values of relative internal energies (enthalpies) and free energies computed adding vibrational entropy and zero point energy contributions for temperatures of 300K (ambient conditions), 1000 K, 1200 K and 1500 K. The Gibbs free energies are computed as:

G=U+ZPE-ST where U is the internal energy, ZPE is the zero point energy contribution, S is the vibrational entropy and T is the temperature. We report the differences in Gibbs free energies of the two phases ∆G=G(β-

ThB2O5)-G(α-ThB2O5) at selected temperatures (see Figure 9.11). The calculations show that β-ThB2O5 phase is the more stable phase at low temperature with the internal energy difference at 0 K of 34.4 kJ/mol. This is consistent with experimental observation reported in Section Thermal analysis. Increasing the temperature, the free energy difference decreases reaching zero at temperature just above 1500 K.

130 Figure 9.11. The computed difference in Gibbs free energy between β-ThB2O5 and α-ThB2O5 phases. ∆G=G(β -ThB2O5)-G(α-ThB2O5).

Table 9.4. The difference in Gibbs free energy between β-ThB2O5 and α-ThB2O5 phases. The energies are reported in kJ/mol.

∆U ∆U+ZPE ∆G ∆G ∆G ∆G (0K) (0K) (300K) (1000K) (1200K) (1500K)

∆G=G(β-ThB2O5)- 34.4 31.8 28.1 12.6 8.1 1.5 G(α-ThB2O5)

In order to validate our calculations of phonons we also computed the IR spectra of the two considered polymorphs. The resulted estimate of the Transmittances is provided in Figure 9.10. These were computed assuming Lorentz-type broadening of the computed absorption peaks with the broadening parameter Γ of 200 cm-1. As indicated in Figure 9.10 we obtained good qualitative match to the measured spectra with the reasonable reproduction of the measured peak intensities and peak positions. In Table 9.5 we compare the measured and computed absorption frequencies. There is a good correspondence between the computed and measured absorption peak positions Section 9.3.4.

131 Table 9.5. Computed and measured IR bands positions. The frequencies are reported in cm-1.

α-ThB2O5 β -ThB2O5 computed Experimental computed experimental 1411 1438,1385 1400 1414 1294 1268 1250, 1252 1257 1078 1072 1028 1027 990,945,914 928 634,630,618 652 790 807 513,508,505 529 715 735 628 652 562, 543 580,557

9.4 Conclusions

A novel polymorph, α-ThB2O5, was obtained through high temperature flux method under ambient conditions. Difference between α and β modifications is indicating a high level of structural flexibility of

ThB2O5. Changing the flux from borax to a mixture of boric acid and alkaline metal carbonates leads to a formation of a denser α phase at lower temperature ~970 °C. It has to be mention that this novel modification was not obtained from the pure boron oxide flux. Hence, we presumed that the flux has played a crucial role during the structure formation process. The structure of α-ThB2O5 is totally different in comparison to the β phase and to the other compounds in the AB2O5 (A = Zr, Hf) family. α modification is featured with a remarkable 3D thorium borate framework, which is composed of a new [36] topological 2D thorium layers on ab-plane and 1D boron-oxygen zigzag chains along the a-axis. The four-capped trigonal prism of ThO10 polyhedra in this structure is uncovered for the first time among all of the thorium inorganic compounds. The coordination number of thorium has increased from eight to ten for β-ThB2O5 and α-ThB2O5, respectively. This is a result of a giant molar volume change appearing via phase transformation between these two polymorphs. The α-phase is more stable according to the experimental and computation studies. With this study we demonstrated that a change in synthetic conditions can lead to a drastical change in formation of robust polymorphs. The effect of flux is comparable to implementation of extreme pressure and temperature on the system.

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144 Chapter 10. List of Uranyl Phosphates Phases

The compounds in the Table 10.1 are synthesized from the H3BO3-flux methods. Their structures are not described in this dissertation. Further characterization and properties of them will be studied in the future work. Table. 10.1 Crystals that Obtained in the Uranyl Phosphpates Systems.

Compounds SG Parameters Str. 13.1464, 7.0632, 10.7660, 90.000, (UO2)3(PO4)2(H2O)5 P21/m 3D 127.629, 90.000, V = 791.73 10.2814, 13.0198, 16.9129, 111.114, (UO2)15(HPO4)12(OH)6(H2O) P-1 3D 102.002, 91.151, V= 2054.60 7.0145, 7.5271, 9.4584, 108.372, Na[(UO2)(HPO4)NO3] P-1 1D 101.354, 102.338, V = 443.79 9.3503, 15.3825, 9.6629, 90.0000, K2(UO2)5(PO4)4 C2/m 3D 93.4300, 90.0000, V = 1387.35 10.9368, 26.8452, 13.5777, 90.000, K5(UO2)12(HPO4)13(OH)3(H2O)13 Pbcm 3D 90.000, 90.000, V = 3986.42 9.1708, 7.9484, 17.1115, 90, 90.529, 90, Cs(UO2)(PO4) Pc 3D V = 1247.28 8.0382, 9.5197, 13.3734, 92.4397, Cs(UO2)2(H2PO4)3(OH)2(H2O)2 P-1 2D 95.0886, 107.7919, V = 967.95 11.9240, 12.6419, 13.1631, 90.0000, Ba3(UO2)4(PO4)2O4 P21/n 2D 115.6817, 90.0000, V = 1788.24 19.046862 9.173266 9.474164 90.0000 Ba3((UO2)2(PO4)2(P2O7)) C2/c 3D 96.8889 90.0000, V = 1643.39 6.9649, 17.2983, 24.3325, 90.000, Ca2(UO2)3(PO4)2O2 C2/m 2D 94.622, 90.000, V = 2922.07 9.524584 8.733452 10.552507 90.0000 Sr3(UO2)2(HPO4)2(PO4)2 P21/c 1D 97.4928 90.0000, V = 870.29 5.3488, 7.9286, 9.7090, 90.0000 SrLi2(UO2)(PO4)2 P21 3D 93.1178 90.0000, V = 411.14 6.9767, 9.3209, 12.6803, 69.307, SrNa2(UO2)6(PO4)4O2 P-1 2D 89.341, 69.186, V = 714.89

PS. SG = space group, Str. = Structural dimension.

145 Conclusions and Outlook

A systematic investigation of An-B-P-O (An = U and Th) system using different synthetic methods result in obtaining and characterization of a number of novel inorganic actinide borates, borophospahtes, borate- phosphate and phosphates. All of the new phases possess novel structural arrangements and properties. Moreover, the synthetic conditions have been proved to be a key factor for the final structural architectures of actinide compounds in studied systems. The topologies of crystal structures of obtained phases were studied in detail and compared to the previously reported phases. A high level of diversity of structural topologies demonstrates the structural flexibility of actinide oxo-compounds. In this work the actinide elements were presented by U and Th. The choice of these elements is based on their long half- life and their relatively weak α-emition. Th was used as a crystal chemical surrogate for tetravalent actinides. Uranium is perfectly simulating crystal chemistry of high valent actinides. However, this not includes very complex redox chemical behavior of transuranium elements, especially for Np and Pu. For the better understanding of the transuranium chemistry in oxo-borate systems, we need to perform real experiments to confirm our assumptions. The main results of this work are presented below.

Uranyl borates with monovalent cations. The system of A+-U-B-O has been systematically investigated using three different synthetic methods. All of the new phases that form in different reaction conditions are 2D layered structures. (H3O)(UO2)(BO3), Li(UO2)(BO3)∙(H2O), K2(UO2)5(BO3)2O3∙(H2O)4 and

Rb3[(UO2)3(BO3)2O(OH)]∙(H2O) were obtained from mild hydrothermal syntheses with different mineralizers. This study has demonstrated that the mineralizer is a very important factor in the formation of different structures of actinide borates. Additionally, we found that water content in reaction media and the pH values are playing a crucial role in the hydrothermal reactions. We demonstrated that an excess of water in the reactions lead to formation of 2D layered structures which are very similar to the phases from

HT solid state synthesis. α-K4[(UO2)5(BO3)2O4], K4Sr4[(UO2)13(B2O5)2(BO3)2O12] and

A6[(UO2)12(BO3)8O3]∙(H2O)6 (A = Rb and Cs) were synthesized using the high temperature solid state reaction method. Their structures are more complex than the structures of the phases obtained from mild hydrothermal and flux syntheses. We demonstrated that the nature of counter cations is a key factor for the complexity of the uranyl borate structures. β-K4[(UO2)5(BO3)2O4] was obtained from a HT/HP solid state reaction (~1000 ºC and ~4 GPa). The 2D structural topology is identical for both HT and HT/HP phases which have identical composition α-K4[(UO2)5(BO3)2O4]. However, the HT/HP-

K4[(UO2)5(BO3)2O4] crystallizes in other space group with higher symmetry. That implies that pressure has played an important role in the formation of the final structures of uranyl borates.

146 Uranyl borates with divalent cations. A series of novel uranyl borates were synthesized in the A2+-U-B-O system under different reaction conditions. Four novel alkaline earth metal uranyl borates were synthesized, namely A[(UO2)5(BO3)2O2(OH)2]∙5H2O and A[(UO2)2(B2O5)O] (A = Sr, Ba). We demonstrated that change of A1+ cations to A2+ leads to the formation of rare CCI (Cation-cation interaction) in the 3D framework structures of A[(UO2)2(B2O5)O]. A unique zeolite-like actinide polyborate (H2O)Pb3(UO2)3B14O27 (LUBO) was obtained in a mild hydrothermal synthesis. The key 2+ 2+ feature of LUBO is a 3D oxo-borate open framework with zeolite-like topology. Pb and (UO2) are 12- playing a role of charge compensaters and templates for 3D oxo-borate [B14O27] framework. In this study we demonstrated that stereo active lone electron pairs of Pb2+ are important for stabilizing of oxo- brate framework with uranyl cations.

Uranyl borophsopahtes/borate-phosphate. Three novel alkali metal uranyl borophosphates have been prepared through a mild hydrothermal synthetic route. These phases possess novel 3D open framework structures. The synthesis of these phases leads to a better understanding of the structural complexity of 9- actinide phases. CUPB1 possesses a microporous framework structure, which is based on the [B(PO4)4] clusters and UO7 pentagonal bipyramids. Importantly, CUPB1 can remove aliovalent cations from aqueous solution at room or elevated temperatures. We demonstrated that the most of the removement goes through absorbtion and minor part because of precipitation of secondary phase. The first unhydrated uranyl borate-phosphate, [Sr8(PO4)2][(UO2)(PO4)2(B5O9)2] (SrUPB1), was synthesized via a high- temperature flux method. The structure of SrUPB1 is composed of a 3D open uranyl borate-phosphate framework. Two large 14R- and 18R-channels are observed in this structure. The framework is based on 2D corrugated polyborate layers. These polyborate layers possess the same FBB with that observed in the polyborate units of BaUPB1. It is presumed that the cations charge determine the composition of the polyborate unit. However, the radius of templating cation is the driving force for the final dimensionality of the polyborate structure.

Thorium borates. A new polymorph of thorium borate, ThB2O5, was obtained using the high temperature flux method. We designated this as α-ThB2O5 due to its higher density compared to the previously known polymorph. This phase was obtained using carbonate-boron oxide flux while the beta phase was made from pure boron oxide flux. It is featured with a remarkable 3D thorium borate framework structure. The thorium-coordination number in α phase is ten while in less dense β phase it is eight. This is a result of a giant molar volume change (~12%) which happened in the phase transformation process. The α-phase is more stable at high temperatures based on the experimental and the computational studies. With this study we demonstrated that a change in synthetic conditions can lead to a drastical change in formation of

147 polymorphs. The effect of flux modification is comparable to implementation of extreme pressure and temperature on the system.

Generally, our work demonstrates that actinide borates can form in a wide range of conditions and can include additional oxo-anions. The chemical composition of the flux, the amount of water in reaction media and the nature of counter cations are the key factors for the phase formation of actinide borates. 3- The introduction of PO4 oxo-anion into actinide borates system leads to a formation of more complex materials. The complexity of these phases allows advanced materials properties such as aliovalent ion- exchange. The phases were obtaind in this work are very stable in the air. The thermal stabilities of the phases are in the range of 600-1000 °C. The results of this work are giving us better understanding of the chemical behavior of the actinides in complex oxo-borate systems.

148 Acknowledgment

Firstly, I would like to express my special appreciation and thanks to my supervisor Prof. Dr. Evgeny V. Alekseev. It’s you, who led me to the beautiful actinide inorganic chemistry world. Before I came here for my PhD study, I just have little knowledge about actinide chemistry. You have taught me a lot in this wonderful area. Your patience, motivation, enthusiasm, selfless and immense knowledge give me great support of my PhD study and research work. I would say I am very lucky that having a great adviser, mentor and friend for my PhD study. Thanks for everything you have made for my research and career. I am also very very thankful to my office-mate, colleague and best friend Dr. Kegler. It’s you, who always give me great help whenever I have met questions and problems on my work. You are so professional, kind hearted and patient scientist, who has taught me a lot on many aspects. My sincere gratitude goes to the director of our institute IEK-6, Prof. Dr. Bosbach, thanks for your support that I have got this precious chance to do my PhD research work here. I want to say thanks to Prof. Jianggao Mao, who has firstly led me to the science world during my master study, I have learned a lot from him of the basic knowledge as a crystal chemist. I also want to say many thanks to my group colleagues, Dr. Philip Kegler, Eike Langer, Haijian Li, Gabriel Murphy and former colleagues Dr. Bin Xiao and Dr. Na Yu. They have given me great helps both in my research work and life. We have studied and lived abroad here. There are lots of problems that we have met, both Dr. Philip Kegler and Eike Langer have helped me as much as they can, which made my study and life easier than I have expected. I would like to say thank you Dr. Vladislav Klepov, though we have not met each other before mainly contact by email, we have collaborate together did nice data analyses and impressive discussion, we have produced many exciting results and I really appreciate the opportunity of working with him. For this dissertation, I would also say thanks to our colleagues Dr. Hartmut Schlenz, Dr. Philip Kegler, Simone Weigelt and Jakob Dellen for the PXRD or Raman measurements, Dr. Martina Klinkenberg and Murat Güngör for the EDS measurements, Prof. Dr. Giuseppe Modolo for the TG-DSC measurements, Fabian Sadowski and Dimitri Schneider for the ICP-MS measurements, Dr. Hildegard Curtius and Zaina Paparigas for the IR measurements. I would say without their kind help I can’t finish my dissertation so smoothly. Meanwhile I also want to say thanks all the colleagues in IEK-6, thanks all your help during my PhD study here. I also need to say thanks to Dr. Piotr Kowalski and Dr. Yan Li, who have helped me to do the theoretical analysis for my research work. I really appreciate the Chinese Scholarship Council has supported me to stay in the Juelich Research Center for my PhD study here.

149 Most importantly, I want to acknowledge my family, my parents who gave birth to me and brought me up with endless of love. They always gave me the great encouragement whatever I have met in my work and life. All of the sacrifices that they’ve made on me are countless. I am also grateful for my sisters and brother, they have helped me to take care of our parents. I really want to say thanks to my love and my wife, she gave up her job in her hometown and came to me. She gave me so much help in my life and my work. She is a great soulmate to share joy, sad and everything in my life.

150 Appendix: Papers and Conference

8. Yucheng Hao, Philip Kegler, Martina Klinkenberg, Dirk Bosbach, Thomas E. Albrecht-Schmitt, Shuao Wang and Evgeny V. Alekseev,* “Microporous Uranyl Borophosphate with Potential Ionic Exchange Properties ”(In preparation)

7. Yucheng Hao, Vladislav V. Klepov, Philip Kegler, Giuseppe Modolo, Dirk Bosbach, Thomas E. Albrecht-Schmitt, Shuao Wang and Evgeny V. Alekseev* “Synthesis and Study of the First Zeolitic Uranium Borate.” Crystal Growth & Design, 2018, 18, 498-505.

6. Yucheng Hao, Philip Kegler, Dirk Bosbach, Thomas E. Albrecht-Schmitt, Shuao Wang and Evgeny V. Alekseev,* “Divergent Structural Chemistry of Uranyl Borates Obtained from Solid State and Hydrothermal Conditions” Crystal Growth & Design, 2017, 17, 5898-5907.

5. Yucheng Hao, Gabriel L. Murphy, Dirk Bosbach, Thomas E. Albrecht-Schmitt, and Evgeny V. Alekseev* “Highly Porous Alkali-metal Uranyl Borophosphates with Unique three dimensional Open Framework Structure.”Inorganic Chemistry, 2017, 56, 9311-9320.

4. Yucheng Hao, Vladislav V. Klepov, Gabriel L. Murphy, Giuseppe Modolo, Dirk Bosbach, Thomas E. Albrecht-Schmitt, Brendan J. Kennedy, Shuao Wang, and Evgeny V. Alekseev*. "Influence of Synthetic Conditions on Chemistry and Structural Properties of Alkaline Earth Uranyl Borates." Crystal Growth & Design, 2016, 16, 5923-5931.

3. Yucheng Hao, Xiang Xu, Fang Kong, Jun-Ling Song, Jiang-Gao Mao*. PbCd2B6O12 and EuZnB5O10: Syntheses, Crystal Structures and Characterizations of two New Mixed Metal Borates. CrystEngComm, 2014, 16, 7689-7695.

2. Yucheng Hao, Chun-Li Hu, Xiang Xu, Fang Kong and Jiang-Gao Mao*. SrGe2B2O8 and Sr3Ge2B6O16: Novel Strontium Borogermanates with Three-Dimensional and Layered Anionic Architectures. Inorganic Chemistry, 2013, 52, 13644 -13650.

1. Liping Wang., Jingcheng Hao,...,Yucheng Hao and Fei Ma. Fabrication of block copolymer brushes on hollow sphere surface via reverse iodine transfer polymerization. Journal of colloid and interface science, 2011, 361, 400-406.

Conference 1. 25th Annual Meeting of the German Crystallographic Society, 27–30 March 2017, Karlsruhe/Germany (DGK): A poster was presented: “Exploration of Structural Chemistry and Ion- exchange Properties of Uranyl Borophosphates” Yucheng Hao and Evgeny V. Alekseev. (2017.03)

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