molecules

Review Research and Development of Zincoborates: Crystal Growth, Structural Chemistry and Physicochemical Properties

1,2,3, 1,2, 1,2,3 4 Yanna Chen † , Min Zhang †, Miriding Mutailipu , Kenneth R. Poeppelmeier and Shilie Pan 1,2,* 1 CAS Key Laboratory of Functional Materials and Devices for Special Environments, Xinjiang Technical Institute of Physics & Chemistry, CAS, 40-1 South Beijing Road, Urumqi 830011, China 2 Xinjiang Key Laboratory of Electronic Information Materials and Devices, Xinjiang Technical Institute of Physics & Chemistry, CAS, 40-1 South Beijing Road, Urumqi 830011, China 3 Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China 4 Department of Chemistry, Northwestern University, Evanston, IL 60208, USA * Correspondence: [email protected] These authors contributed equally to this work. †


Academic Editor: Michael A. Beckett
 Received: 20 June 2019; Accepted: 24 July 2019; Published: 30 July 2019 Abstract: Borates have been regarded as a rich source of functional materials due to their diverse structures and wide applications. Therein, zincobrates have aroused intensive interest owing to the effective structural and functional regulation effects of the strong-bonded zinc cations. In recent decades, numerous zincoborates with special crystal structures were obtained, such as Cs3Zn6B9O21 and AZn2BO3X2 (A = Na, K, Rb, NH4;X = Cl, Br) series with KBe2BO3F2-type layered structures were designed via substituting Be with Zn atoms, providing a feasible strategy to design promising non-linear optical materials; KZnB3O6 and Ba4Na2Zn4(B3O6)2(B12O24) with novel edge-sharing 5 [BO4] − tetrahedra were obtained under atmospheric pressure conditions, indicating that extreme 5 conditions such as high pressure are not essential to obtain edge-sharing [BO4] −-containing borates; Ba4K2Zn5(B3O6)3(B9O19) and Ba2KZn3(B3O6)(B6O13) comprise two kinds of isolated polyborate anionic groups in one borate structure, which is rarely found in borates. Besides, many zincoborates emerged with particular physicochemical properties; for instance, Bi2ZnOB2O6 and BaZnBO3F are promising non-linear optical (NLO) materials; Zn4B6O13 and KZnB3O6 possess anomalous thermal expansion properties, etc. In this review, the synthesis, crystal structure features and properties of representative zincoborates are summarized, which could provide significant guidance for the exploration and design of new zincoborates with special structures and excellent performance.

Keywords: zincoborate; structural chemistry; structure–property relationship; non-linear optical crystal

1. Introduction Over the past decades, borates have attracted burgeoning attention owning to their excellent properties and wide applications such as non-linear optical (NLO) materials, birefringent materials, electrode materials, etc [1–7]. The attractive properties of borates mainly depend on their structural diversity, as the boron atoms can be three- or four-coordinated with oxygen atoms to construct the planar 3 5 triangular [BO3] − groups or tetrahedral [BO4] − groups, respectively, which could further link together via corner- or edge-sharing to form different types of fundamental building blocks (FBBs) [8–11]. For example, the FBBs of classical KBe2BO3F2 (KBBF) [12,13] and Sr2Be2B2O7 (SBBO) [14,15] are

Molecules 2019, 24, 2763; doi:10.3390/molecules24152763 www.mdpi.com/journal/molecules Molecules 2019, 24, 2763 2 of 27

3 [BO3] − triangles, while the commercial NLO crystals β-BaB2O4 (β-BBO) [16] and LiB3O5 (LBO) [17] 3 5 are composed of [B3O6] − and [B3O7] − groups, respectively. These FBBs, in turn, may polymerize to form isolated clusters, infinite one-dimensional (1D) chains, two-dimensional (2D) layers or three-dimensional (3D) frameworks, which endow borates with rich structural chemistry and extensive applications [18–21]. From the viewpoint of structural chemistry, the anionic groups are relatively independent structural and functional modules. The expected excellent optical properties could be achieved through screening and assembling FBBs with a specific arrangement [22–28], which constitutes a tight combination between the chemistry synthesis and optical properties of materials, as well as establishes a theoretical foundation for rationally designing or synthesizing ultraviolet (UV) and deep-UV (DUV) NLO materials. In particular, the isolated planar B-O groups with coplanar or aligned arrangement could produce large microscopic second-order susceptibility and birefringence, which are suitable to design NLO and birefringent materials for UV/DUV light generation [29–31]. For instance, KBBF and 3 3 β-BBO with isolated coplanar B-O units ([BO3] − and [B3O6] −) exhibit large birefringence and second harmonic generation (SHG) response, which enables them to be world famous as optical crystals and widely applied in the field of laser technology [32–34]. However, the design and synthesis of borates with isolated coplanar B-O groups is still a research challenge. According to the statistical analysis of borate FBBs proposed by P. Becker [35], isolated borate polyhedra occur for cations/boron (A:B) > 1, which guides researchers to obtain borates with isolated B-O groups by increasing the ratio of cations and boron [36–38]. Recently, many investigations reveal that the introduction of covalent mental cations (such as, Be2+, Mg2+, Zn2+, Al3+, etc.) may effectively restrain the polymerization of B-O units and is beneficial to the formation of isolated B-O groups [39–43]. Moreover, the cooperation of highly-coordinated cations (such as, Ba2+, Sr2+, Rb+,K+, etc.) with low-coordinated covalent mental cations can synergistically affect the frameworks and facilitate the benign B-O arrangement, such as SBBO, Na2Be4B4O11 [44], Cs3Zn6B9O21 [45,46], Ba3Mg3(BO3)3F3 [47], A3B3Li2M4B6O20F (A = K, Rb; B = Ba, Sr; M = Al, Ga) series [48–54], etc. In the last few decades, borates containing zinc cations have become a research focus and zincoborates with novel structures or good properties were continuously reported [55–57]. There are several advantages motivating researchers to design new compounds in a zinc-containing borate system: (1) the zinc-containing borate has been regarded as a fertile field to search for the expected compounds with high physicochemical performance. In recent years, using Zn2+ to substitute Be2+ in the KBBF family has been proved to be a rational design strategy to synthesize desired compounds with KBBF-type layered structures, which is an effective way to explore new UV/DUV NLO materials. For example, KZn2BO3Cl2 [58,59] and Cs3Zn6B9O21 were designed with KBBF-type layered structures and exhibit enhanced SHG efficiency of about 3.0 and 3.3 KH2PO4 (KDP), respectively. Furthermore, 6 × the [ZnO4] − tetrahedron has been regarded as a NLO-active unit with good UV transparency. For instance, Ba3(ZnB5O10)PO4 [60], BaZnBO3F[61], Ba5Zn4(BO3)6 [62], Ba2Zn(BO3)2 [63,64], etc. exhibit enhanced NLO performance. In addition, KZnB3O6 and Zn4B6O13 possess anomalous thermal expansion properties [65–68], which revive the studies on new functionalities in borates that have long been overlooked, and might eventually give rise to the discovery of more exciting and exotic emerging physicochemical properties in borates. (2) Many zincoborates possess exceptional structural configurations. For instance, KZnB3O6 is the first borate possessing the novel edge-sharing (es-) 5 5 [BO4] − tetrahedra synthesized under ambient pressure [69,70]. Previously, the es-[BO4] − tetrahedra were considered only to exist in some compounds synthesized under high-pressure/high-temperature conditions [71–74]. Interestingly, Ba4Na2Zn4(B3O6)2(B12O24)[75] is the second case of borate possessing 5 the es-[BO4] − tetrahedra that obtained under ambient pressure. Besides, several zincoborates comprise of two types of isolated B-O groups, which violates the Pauling’s Rule of parsimony 3 3 (Pauling’s fifth rule) [76,77]. For instance, Cs3Zn6B9O21 ([BO3] − + [B3O6] −), Ba3Zn(BO3)(B2O5)F and 3 Ba4Zn2(BO3)2(B2O5)F2 [78] with [BO3] − triangles and another relatively low-polymerized polyborate anions have been reported; moreover, compounds with two kinds of isolated polyborate anions, such as Molecules 2019, 24, x FOR PEER REVIEW 3 of 27

([B2O5]4− + [B6O13]8−) [84] were also reported, which are rarely found in borates, implying the special role of zinc cations in regulating and controlling the B-O configuration. Herein, the aim of this review is to focus on the crystal chemistry, optical properties, thermal properties and the structure–property relationship of zincoborates due to the significant regulation Moleculeseffects2019 of ,zinc24, 2763 cations to the structures and properties. We hope that this work could open a 3way of 27 to design novel NLO crystals in zinoborate system, as well as the discovery of new zincoborates with novel structures or particular physicochemical properties. Ba2KZn3(B3O6)(B6O13)[79], Ba4K2Zn5(B3O6)3(B9O19)[80], Ba4Na2Zn4(B3O6)2(B12O24), Bi2ZnO(B2O6) 4 8 4 8 ([B2O5] − + [B2O7] −)[81–83] and α-/β-/γ-Pb2Ba4Zn4B14O31 ([B2O5] − + [B6O13] −)[84] were also reported,2. Structural which Chemistry are rarely foundof Zincoborates in borates, implying the special role of zinc cations in regulating and controlling the B-O configuration. 2.1. Statistical Analysis of Structural Configurations Herein, the aim of this review is to focus on the crystal chemistry, optical properties, thermal propertiesIn order and theto better structure–property understand the relationship structural diversity of zincoborates of zincoborates, due to the systematic significant analysis regulation of the effZnects-B-O ofzinc system cations was tocarried the structures out by taking and properties. the Inorganic We Crystal hope that Structure this work Database could open(ICSD-4.2.0, a way to the designlatestnovel release NLO of ICSD-2019/1) crystals in zinoborate as the source system, of data. as The well connection as the discovery modes of of new zinc zincoboratescations with oxygen with novelor halogen structures anions or particular as well as physicochemical the influence of properties. the zinc cations to the structures are investigated. The thresholds of bond lengths were applied for the Zn-O and B-O bonds at the maximum in the 2.continuity Structural distribution Chemistry of of Zincoboratesthe Zn-O distances (2.6 Å ) and the B-O distances (1.65 Å ), respectively. All the available anhydrous and disorder-free zinc-containing borates (64 cases) are summarized. 2.1. Statistical Analysis of Structural Configurations Structural comparisons were carried out and summarized as follows: In(1) order The toZn better-O, Zn-X understand (X = F, Cl, theBr) bond structural lengths diversity of the summarized of zincoborates, compounds systematic range analysis from 1.814 of – the2.479 Zn-B-O Å , system2.023–2.516 was Å carried , respectively, out by taking which the is Inorganicin agreement Crystal with Structure reasonable Database values. (ICSD-4.2.0, As shown in theFigure latest 1a, release most of of ICSD-2019 the Zn-O/1) bond as the lengths source are of data.in the The range connection of 1.90– modes2.16 Å of . The zinc zinc cations atoms with can oxygencoordinate or halogen with four, anions five, as or well six as O/X the atoms influence to form of the [ZnO zinc4]6 cations−, [ZnO to5]8− the, [ZnO structures6]10−, [ZnO are3X] investigated.5−, [ZnO4X]7−, The[ZnO thresholds3X2]6− polyhedra, of bond lengthsrespectively, were appliedand the forcoordination the Zn-O andof zinc B-O are bonds mostly at thefour maximum coordinated in thewith continuityoxygen (about distribution 66% of of zincoborates the Zn-O distances contain (2.6 the Å) [ZnO and the4]6− B-Otetrahedra). distances These (1.65 Å),zinc-centered respectively. polyhedra All the availablecan polymerize anhydrous into and isolated disorder-free Zn-O/X zinc-containing clusters, 1D chains, borates 2D (64 layers cases) or are 3D summarized. network through Structural vertex- comparisons/edge-sharing, were which carried can out further and summarized connect with as different follows: B-O groups to form Zn-B-O/X structures. To the(1) best The of Zn-O,our knowledge, Zn-X (X = theF,Cl, Zn-B-O Br) bond configurations lengths of in the zincoborates summarized are compounds always 2D range layers from or 3D 1.814–2.479frameworks, Å, 2.023–2.516 except that Å, respectively,the special which[Zn2(BO is in3)6 agreement]14− and [ZnB with6O reasonable18]16− isolated values. clusters As shown exist in in FigureBa2ZnSc(BO1a, most3 of)3 [8 the5] Zn-Oand Pb bond8Zn(BO lengths3)6 [8 are6], respectively. in the range of 1.90–2.16 Å. The zinc atoms can coordinate 6 8 10 5 7 with four, five, or six O/X atoms to form [ZnO4] −, [ZnO5] −, [ZnO6] −, [ZnO3X] −, [ZnO4X] −, 6 [ZnO3X2] − polyhedra, respectively, and the coordination of zinc are mostly four coordinated 6 with oxygen (about 66% of zincoborates contain the [ZnO4] − tetrahedra). These zinc-centered polyhedra can polymerize into isolated Zn-O/X clusters, 1D chains, 2D layers or 3D network through vertex-/edge-sharing, which can further connect with different B-O groups to form Zn-B-O/X structures. To the best of our knowledge, the Zn-B-O configurations in zincoborates are always 2D layers or 14 16 3D frameworks, except that the special [Zn2(BO3)6] − and [ZnB6O18] − isolated clusters exist in Ba2ZnSc(BO3)3 [85] and Pb8Zn(BO3)6 [86], respectively.

FigureFigure 1. Distribution1. Distribution of of (a )(a Zn-O) Zn- andO and B-O B- bondO bond lengths; length (sb; )(b dimensionality) dimensionality of of B-O B- configurationO configuration in in zinc-containingzinc-containing borates. borates.

(2) The B-O bond lengths of the summarized compounds range from 1.254–1.565 Å, which are in accordance with those of other reported borates. Most of the B-O bond lengths are distributed in the range of 1.30–1.52 Å (Figure1a). As shown in Figure1b, approximately 78% of zincoborate 3 4 3 structures are built up of isolated B-O groups (0D), such as isolated [BO3] −, [B2O5] −, [B3O6] −, 5 6 [B5O10] −, [B6O12] − units, etc., while no 1D B-O configuration is observed. It could be considered Molecules 2019, 24, 2763 4 of 27 that the introduction of zinc cations has profitable impact on the prevention of polymerization of B-O anionic structures. Notably, about 20 % of zincoborates with isolated B-O groups contain coplanar (or nearly coplanar) [BO ]3 /[B O ]3 units, which paves a comprehensive road map for us to design new Molecules 2019, 24, x FOR 3PEER− REVIEW3 6 − 4 of 27 compounds with benign isolated B-O groups. 2.2. Zincoborates Possessing Special Structural Features 2.2. Zincoborates Possessing Special Structural Features It is expected that the introduction of zinc cations into borates can enrich the structural diversity It is expected that the introduction of zinc cations into borates can enrich the structural diversity and be beneficial to obtain new zincoborates with special structural features. In this section, several and be beneficial to obtain new zincoborates with special structural features. In this section, several compounds with distinctive crystal structure characteristics are given. compounds with distinctive crystal structure characteristics are given.

2.2.1. Zincoborates with Benign KBe2BO3F2 (KBBF)-Type Layered Structures 2.2.1. Zincoborates with Benign KBe2BO3F2 (KBBF)-Type Layered Structures The world-famousworld-famous KBBF is the unique practicalpractical NLO crystal in the DUV region that can generate the 177.3 nm coherent laser by a direct SHG method using Q-switchedQ-switched neodymium (Nd):YAG (1064 nm) laser [87]. Structurally, the perfectly coplanar [BO3]3− anionic groups in the 22[Be2BO3F2] layers of nm) laser [87]. Structurally, the perfectly coplanar [BO3] − anionic groups in the [Be2BO3F2] layers KBBF provide a relatively large SHG coefficient (d11 = 0.47 pm/V) and a moderate∞ birefringence (n = of KBBF provide a relatively large SHG coefficient (d11 = 0.47 pm/V) and a moderate birefringence (0.07∆n = @10640.07 @1064 nm), which nm), which endow endow KBBF KBBF with withunprecedented unprecedented performances performances [88,89 [88]., 89The]. Theexcellence excellence for the for theKBBF KBBF family family crystals crystals used used as asthe the DUV DUV NLO NLO crystals crystals inspirit inspiritss researchers researchers to to explore explore beryllium beryllium-free-free borates with benign KBBF-typeKBBF-type layered structuresstructures [[9090––9393].]. To inheritinherit the the favorable favorable structural structural arrangement arrangement of KBBF, of KBBF, one eoneffective effective molecular molecular engineering engineering design design strategy is to substitute the5 [BeO3F]5− tetrahedra2 in 2[Be2BO3F2] layers of KBBF with strategy is to substitute the [BeO3F] − tetrahedra in [Be2BO3F2] layers of KBBF with [MO4]/[MO3F] + 2+ 3+ 3+ ∞ (M[MO=4]/[MOLi+, Zn3F]2+ (M, Al =3 +Li, and, Zn Ga, Al3+, etc.), and tetrahedraGa , etc.) tetrahedra to achieve to structural achieve structural modification modification [94–96]. Therein,[94–96]. Therein, most of Zn− and Be-O/X polyhedra have similar coordination environment and approximate most of Zn− and Be-O/X polyhedra have similar coordination environment and approximate bond 2+ 2+ length,bond length, thus the thus substitution the substitution of Zn2+ forof Zn Be2+ forhas Be attracted has attracted the most the attention most andattention obtained and a obtained number ofa number of compounds possessing benign KBBF-type layered structures, for example, AZn2BO3X2 (A compounds possessing benign KBBF-type layered structures, for example, AZn2BO3X2 (A = Na, K, Rb, = Na, K, Rb, NH4; X = Cl, Br) series [58,59], Cs3Zn6B9O21 [45,46], BaLiZn3(BO3)3 [97,98], CdZn2KB2O6F NH4;X = Cl, Br) series [58,59], Cs3Zn6B9O21 [45,46], BaLiZn3(BO3)3 [97,98], CdZn2KB2O6F[99,100], etc. [99,100], etc. 2 AZn2BO3X2 (A = Na, K, Rb, NH4;X = Cl, Br) Series with [Zn2BO3X2] Layers ∞ AZn2BO3X2 (A = Na, K, Rb, NH4; X = Cl, Br) Series with 2[Zn2BO3X2 Layers AZn2BO3X2 (A = Na, K, Rb, NH4;X = Cl, Br) series of compounds were reported by two independentAZn2BO3 groupsX2 (A = of Na, Chen K, and Rb, Ye NH in4 2016; X = [ 58 Cl,,59 Br)], which series were of compounds developed as were the production reported by of two the 5 “transgenosis”independent groups process of onChen KBBF and structure, Ye in 2016 specifically, [58,59], which using were [ZnO developed3X] − tetrahedra as the production to substitute of the 5 2 52− [BeO“transgenosis”3F] − tetrahedra process and on yield KBBF the structure,[Zn2BO specifically,3X2] layers using similar [ZnO to the3X] tetrahedra[Be2BO3F2 to] layers substitute in KBBF the 5− 2∞ 2∞ (Figure[BeO3F]2). tetrahedra AZn 2BO3 X and2 (A yi=eldNa, the K, Rb,[Zn NH2BO4;X3X=2 Cl,layers Br) crystals similar all to crystallize the [Be2BO in R3F322 (No.layers 155) in chiralKBBF 2 5 space(Figure group 2). AZn and2BO are3X isostructural2 (A = Na, K, Rb, to KBBF. NH4; X In = the Cl, Br)[Zn crystals2BO3X all2] layers,crystallize the in [ZnO R323 X](No.− tetrahedra155) chiral 3 2 ∞ 5− inducespace group [BO3] and− groups are isostructural to arrange to into KBBF. a nearly In the coplanar [Zn2BO and3X2 aligned layers, arrangement, the [ZnO3X] which tetrahedra is beneficial induce to[BO generate3]3− groups large to SHG arrange responses into a and nearly birefringence, coplanar andindicating aligned that arrangement, they are likely which to inherit is beneficial the optical to advantagesgenerate large of KBBF.SHG responses and birefringence, indicating that they are likely to inherit the optical advantages of KBBF.

Figure 2.2. StructuralStructural evolutionevolution fromfrom KBBFKBBF toto AZnAZn22BOBO33X2.. Reprinted Reprinted with with permission permission from R Ref.ef. [ [58].58. Copyright (2016) American Chemical Society.

Cs3Zn6B9O21 with 2[Zn2BO3O2 Layers

On the basis of substituting Be2+ with Zn2+, Cs3Zn6B9O21 with 2[Zn2BO3O2 layers was synthesized and reported by two independent groups as a new UV NLO material [45,46]. Cs3Zn6B9O21 crystallizes in the orthorhombic system of space group Cmc21 (No. 36). Within the targeted 2[Zn2BO3O2 layers, the [BO3]3− triangles are in an approximately coplanar and aligned arrangement,

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2 Cs3Zn6B9O21 with [Zn2BO3O2] Layers ∞ 2+ 2+ 2 On the basis of substituting Be with Zn , Cs3Zn6B9O21 with [Zn2BO3O2] layers was ∞ synthesized and reported by two independent groups as a new UV NLO material [45,46]. Cs3Zn6B9O21 crystallizes in the orthorhombic system of space group Cmc21 (No. 36). Within the targeted 2 3 Molecules[Zn2BO 20193O2, ]24 layers,, x FOR thePEER [BO REVIEW3] − triangles are in an approximately coplanar and aligned arrangement,5 of 27 ∞ 6 2 which is arranged by the [ZnO4] − tetrahedra (Figure3b). The adjacent [Zn2BO3O2] layers are ∞ which is arranged by3 the [ZnO4]6− tetrahedra (Figure 3b). The adjacent 2[Zn2BO3O2 layers are bridged through [B3O6] − groups (Figure3a), which would reinforce the interlayer bonding compared bridged through+ [B3O6]3− groups (Figure 3a), which would reinforce the interlayer bonding compared with the weak K -F− ionic bonds in KBBF. Verified by experiments, Cs3Zn6B9O21 maintains the optical with the weak K+-F− ionic bonds in KBBF. Verified by experiments, Cs3Zn6B9O21 maintains the optical properties of KBBF, and the crystals of Cs3Zn6B9O21 are of block shape without layering tendency. properties of KBBF, and the crystals of Cs3Zn6B9O21 are of block shape without layering tendency. (a) (b)

2 FigureFigure 3. 3.(a )(a The) The crystal crystal structure structure of Csof3 CsZn36ZnB96OB219Oviewed21 viewed along along the thea-axis; a-axis; (b) ( theb) the[Zn 2[Zn2BO2BO3O32O]2 layer layer 6 3 ∞ composedcomposed of of [ZnO [ZnO4]4]−6− tetrahedratetrahedra and nearly coplanar coplanar [BO [BO3]33]− triangles.− triangles. Adapted Adapted with with permission permission from fromRef Ref.. [45,46 [45,46].. Copyright Copyright (2014) (2014) American American Chemical Chemical Society. Society. 2 2 BaLiZn3(BO3)3 with [LiZn3(BO3)3] Layers and CdZn2KB2O6F with [ZnBO3] Layers BaLiZn3(BO3)3 with ∞2[LiZn3(BO3)3 Layers and CdZn2KB2O6F with 2[ZnBO∞ 3 Layers BaLiZn3(BO3)3 [97,98] and CdZn2KB2O6F[99,100] are the other two beryllium-free borates with BaLiZn3(BO3)3 [97,98] and CdZn2KB2O6F [99,100] are the2 other two beryllium-free borates with KBBF-type structures. BaLiZn3(BO3)3 features a special zigzag [LiZn3(BO3)3] layer that is constructed KBBF-type structures. BaLiZn3(BO3)3 features a special zigzag∞ 2[LiZn3(BO3)3 layer that is constructed by 1 [LiZn O (BO )] chains and [BO ]3 units, which is evolved from 2 [Be BO F ] layer in KBBF 1 3 11/3 3 3 −3− 2 2 3 2 by∞ [LiZn3O11/3(BO3)] chains2 and [BO3] units, which is evolved from∞ [Be2BO3F2] layer in KBBF (Figure4). The adjacent [LiZn3(BO3)3] layers are tightly stacked directly via Li/Zn-O bonds in (Figure 4). The adjacent 2∞∞[LiZn3(BO3)3] layers are tightly stacked directly via Li/Zn-O bonds in the the layers, which is different from most of the KBBF derivatives that connect the adjacent layers layers, which is different from most of the KBBF derivatives that connect the adjacent layers through through cations or B-O groups between the layers. For instance, the adjacent layers in KBBF [12,13], cations or B-O groups between the layers. For instance, the adjacent layers in KBBF [12,13], Na CsBe B O [101], β/γ-KBe B O [102] and Cs Zn B O [45,46], are connected by K-F bonds, 2 6 5 15 2 3 7 3 6 9 21 3− Na23CsBe6B5O151 [101], β/-KBe2B3O7 [102] and3 Cs3Zn6B9O21 [45,46], are connected by K-F bonds, [BO3] [BO3] − groups, [BO2] chains and [B3O6] − groups, respectively. The strong Li/Zn-O covalent bonds groups, 1∞[BO2]∞ chains and [B3O6]3− groups, respectively. The strong Li/Zn-O covalent bonds in in BaLiZn3(BO3)3 can effectively reinforce the interlayer force and improve the layering tendency of BaLiZn3(BO3)3 can effectively reinforce the interlayer force and improve the layering tendency of KBBF-type structures. KBBF-type structures. CdZn2KB2O6F crystallizes in the space group of P31c (No. 163) [99,100]. In the structure, 3 4 5 the [BO3] − triangles and [ZnO3] − pyramids (from the [ZnO3F] − tetrahedra) share the vertex O 2 2 atoms to form a [ZnBO3] layer (Figure5b), which is also similar to the [Be2BO3F2] layer in KBBF. 2 ∞ ∞ The [ZnBO3] layers are connected by bridging F and Cd atoms alternately along the c-axis, and the + ∞ 2 3 K cations are filled in the interlayer to balance charge (Figure5a). In the [ZnBO3] layers, the [BO3] − 5 ∞ triangles are also in a coplanar arrangement influenced by the [ZnO3F] − tetrahedra (Figure5b).

(a) (b)

(c) 2 [Be BO F ] layer (d) 2 [LiZn (BO ) ] layer ∞ 2 3 2 ∞ 3 3 3

Figure 4. The crystal structures of (a) KBe2BO3F2 (KBBF) and (b) BaLiZn3(BO3)3 viewed along the b- axis; (c) The 2[Be2BO3F2] layer in KBBF and (d) the 2[LiZn3(BO3)3 layer in BaLiZn3(BO3)3. [97- Reproduced by permission of The Royal Society of Chemistry (RSC) on behalf of the Centre National de la Recherche Scientifique (CNRS) and the RSC.

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which is arranged by the [ZnO4]6− tetrahedra (Figure 3b). The adjacent 2[Zn2BO3O2 layers are bridged through [B3O6]3− groups (Figure 3a), which would reinforce the interlayer bonding compared with the weak K+-F− ionic bonds in KBBF. Verified by experiments, Cs3Zn6B9O21 maintains the optical properties of KBBF, and the crystals of Cs3Zn6B9O21 are of block shape without layering tendency. (a) (b)

Figure 3. (a) The crystal structure of Cs3Zn6B9O21 viewed along the a-axis; (b) the 2[Zn2BO3O2 layer composed of [ZnO4]6− tetrahedra and nearly coplanar [BO3]3− triangles. Adapted with permission from Ref. [45,46. Copyright (2014) American Chemical Society.

BaLiZn3(BO3)3 with 2[LiZn3(BO3)3 Layers and CdZn2KB2O6F with 2[ZnBO3 Layers

BaLiZn3(BO3)3 [97,98] and CdZn2KB2O6F [99,100] are the other two beryllium-free borates with KBBF-type structures. BaLiZn3(BO3)3 features a special zigzag 2[LiZn3(BO3)3 layer that is constructed by 1[LiZn3O11/3(BO3)] chains and [BO3]3− units, which is evolved from 2[Be2BO3F2] layer in KBBF (Figure 4). The adjacent 2∞[LiZn3(BO3)3] layers are tightly stacked directly via Li/Zn-O bonds in the layers, which is different from most of the KBBF derivatives that connect the adjacent layers through cations or B-O groups between the layers. For instance, the adjacent layers in KBBF [12,13], Na2CsBe6B5O15 [101], β/-KBe2B3O7 [102] and Cs3Zn6B9O21 [45,46], are connected by K-F bonds, [BO3]3− groups, 1∞[BO2] chains and [B3O6]3− groups, respectively. The strong Li/Zn-O covalent bonds in Molecules 2019, 24, 2763 6 of 27 BaLiZn3(BO3)3 can effectively reinforce the interlayer force and improve the layering tendency of KBBF-type structures.

(a) (b)

Molecules 2019, 24, x FOR PEER REVIEW 6 of 27

̅ CdZn2KB2O6F crystallize(c)s 2in[Be theBO Fspace] layer group of(d) P23[LiZn1c (No.(BO ) ]163) layer [99,100]. In the structure, the ∞ 2 3 2 ∞ 3 3 3 [BO3]3− triangles and [ZnO3]4− pyramids (from the [ZnO3F]5− tetrahedra) share the vertex O atoms to form FigureFigurea 2[ZnBO 4.4. TheThe3 crystalclayerrystal structures(Figstructuresure 5b ofof), ((a whicha)) KBeKBe2 2BO is 3 also3F2 ((KBBF)KBBF similar) and and to ( (bb the)) BaLiZn BaLiZn 2[Be33(BO2BO(BO33)3F3) 23viewed] viewed layer alongin along KBBF the the b. -The 2 2 2 2 2 baxis-axis;; (c ()c )T Thehe [Be[Be2BO2BO3F23]F layer2] layer in in KBBF KBBF and and (d) ( dt)he the [LiZn[LiZn3(BO3(BO3)3 3layer)3] layer in BaLiZn in BaLiZn3(BO33(BO)3. [973)3.- + [ZnBO3 layers are∞ connected by bridging F and Cd atoms∞ alternately along the c-axis, and the K [97]-ReproducedReproduced by permission by permission of The of Royal The RoyalSociety Society of Chemistry of Chemistry (RSC) on (RSC) behalf on of behalf the Centre of the National Centre cations are filled in the interlayer to balance charge (Figure 5a). In the 2[ZnBO3 layers, the [BO3]3− Nationalde la Recherche de la Recherche Scientifique Scientifique (CNRS) and (CNRS) the RSC. and the RSC. triangles are also in a coplanar arrangement influenced by the [ZnO3F]5− tetrahedra (Figure 5b).

(a) (b) 2 Figure 5. (a) The crystal structure of CdZn2KB2O6F viewed along the b-axis; (b) the single [ZnBO3] Figure 5. (a) The crystal structure of CdZn2KB2O6F viewed along the b-axis; (b) the single 2∞[ZnBO3 layer. Reprinted from Ref. [99], Copyright (2009) with permission from Elsevier. layer. Reprinted from Ref. [99], Copyright (2009) with permission from Elsevier. 5 2.2.2. Zincoborates with Novel Edge-Sharing [BO4] − Tetrahedra 4 5− 2.2.2. Zincoborates with Novel Edge-Sharing [BO ] Tetrahedra5 On the basis of the borate structures discovered, the [BO4] − units usually connect to each other via corner-sharingOn the basis (cs-) of ratherthe borate than structures edge-sharing discovered, (es-) or face-sharing the [BO4]5− unit [103s– 105usually]. In termsconnect of to Pauling’s each other 3rd viaand corner 4th rules-sharing and the (cs orbital-) rather interpretation than edge-sharing rules, the (es connection-) or face-sharing mode of[103 es-polyhedra–105]. In terms for high-valenceof Pauling’s 3rdand and low 4th coordinated rules and the small orbital cations interpr is scarcelyetation rules, seen exceptthe connection under extreme mode of conditions es-polyhedra such for as high high- valencepressure and (HP), low for coordinated the reason thatsmall the cations repulsion is scarcely interactions seen except between under the adjacentextreme conditions cations and such anions as highmay bepressure increased (HP), when for twothe anion-basedreason that the polyhedra repulsion adopt interactions edge-sharing between connection the adjacent mode [ 76cations,77]. Thus, and 5 5 anionsthe formation may be of increased es-[BO4] − whentetrahedra two anion is extremely-based polyhedra unfavored, adopt and, theedge es-[BO-sharing4] − connectionunits can only mode be [observed76,77]. Thus, in very the fewformation borates. of es-[BO4]5− tetrahedra is extremely unfavored, and, the es-[BO4]5− units 5 can onlyIn 2002, be observed borate in with very es-[BO few borates.4] − tetrahedra, Dy4B6O15 [106], was firstly synthesized under highIn pressure 2002, borate conditions with es (8- GPa,[BO4]5 1000− tetrahedra, K) by Huppertz Dy4B6O15 and [106 van], was der firstly Eltz, whichsynthesized indicates under that high the 5 5− pressure[BO4] − tetrahedraconditions can(8 GPa, link 1000 together K) by notHuppertz only via and common van der cornersEltz, which but indicates also via commonthat the [BO edges.4] 5 tetrahedraSince then, can several link together new es-[BO not 4only] − tetrahedra-containingvia common corners but borates also via have common been synthesizededges. Since underthen, 5− severalhigh pressure new es- and[BO4 high] tetrahedra temperature-containing conditions, borates for have instance, been synthesized RE4B6O15 (RE under= Dyhigh and pressure Ho) [ 107and], 4 6 15  2 4 9 highα-(RE) temperature2B4O9 (RE =conditions,Eu, Gd, Tb, for Dy, instance, Sm, Ho) RE [108B O,109 (RE], HP-AB = Dy 3andO5 (AHo)= [K,107 NH], 4-,(RE) Rb, Tl)B O [71 (RE–73 =] andEu, 5 Gd,HP-MB Tb, 2DyO4, Sm,(M = HoFe,) [ Ni,108 Co),109] [,110 HP,111-AB].3O Recently,5 (A = K, αNH-Ba43, [BRb,10 Tl)O17 [71(OH)–732] and with HP es-[BO-MB24O] 4− (Mtetrahedra = Fe, Ni, wasCo)  3 10 17 2 4 5− 5 [synthesized110,111]. Recently, through hydrothermal-Ba [B O (OH) reactions] with at 500 es-[BO◦C and] 1000tetrahedra bar [112 was]. Although synthesized the es-[BO through4] − hydrothermal reactions at 500 °C and 1000 bar [112]. Although the es-[BO4]5− tetrahedra appear in these compounds, the high pressure condition is indispensable prerequisite. Extraordinarily, two zincoborates, KZnB3O6 [69,70] and Ba4Na2Zn4(B3O6)2(B12O24) [75], possessing es-[BO4]5− configuration were obtained under ambient pressure in 2010 and 2013, respectively, demonstrating that high pressure is not essential for the formation of es-[BO4]5− polyhedra. In addition, the synthesis of Li4Na2CsB7O14 and BaAlBO4 with es-[BO4]5− tetrahedra further enriches the es-[BO4]5− containing borate system synthesized under atmospheric environment [113,114]. Very recently, β-CsB9O14, the first triple-layered borate with es-[BO4]5− tetrahedra, was obtained under the vacuum sealed condition [115].

KZnB3O6

KZnB3O6 crystallizes in the space group of P1̅ (No. 2) [69,70]. As shown in Figure 6, its structure contains a remarkable [B6O12]6− group consisting of two es-[BO4]5− tetrahedra and four cs-[BO3]3− triangles, the [B6O12]6− groups are further connected by distorted [ZnO4]6− tetrahedra in edge-shared pairs to form a 3D framework, then the K+ cations fills in the cavities to construct the whole structure.

Molecules 2019, 24, 2763 7 of 27 tetrahedra appear in these compounds, the high pressure condition is indispensable prerequisite. Extraordinarily, two zincoborates, KZnB3O6 [69,70] and Ba4Na2Zn4(B3O6)2(B12O24)[75], possessing 5 es-[BO4] − configuration were obtained under ambient pressure in 2010 and 2013, respectively, 5 demonstrating that high pressure is not essential for the formation of es-[BO4] − polyhedra. In addition, 5 5 the synthesis of Li4Na2CsB7O14 and BaAlBO4 with es-[BO4] − tetrahedra further enriches the es-[BO4] − containing borate system synthesized under atmospheric environment [113,114]. Very recently, 5 β-CsB9O14, the first triple-layered borate with es-[BO4] − tetrahedra, was obtained under the vacuum sealed condition [115].

KZnB3O6

KZnB3O6 crystallizes in the space group of P1 (No. 2) [69,70]. As shown in Figure6, its structure 6 5 3 contains a remarkable [B6O12] − group consisting of two es-[BO4] − tetrahedra and four cs-[BO3] − 6 6 triangles, the [B6O12] − groups are further connected by distorted [ZnO4] − tetrahedra in edge-shared pairsMolecules to form2019, 24 a, 3D x FOR framework, PEER REVIEW then the K+ cations fills in the cavities to construct the whole structure.7 of 27

(a) (b) (c)

b(c) 6- [B6O12]

Figure 6. (a) The crystal structure of KZnB3O6 viewed along the [011] direction; (b) Connection Figure 6. (a) The crystal structure of KZnB3O6 viewed along the [011̅] direction; (b) Connection details 6 8 6 details of [B6O12] − and [Zn2O6] − blocks in the (1 11) plane; (c) The [B6O12] − group. Adapted from of [B6O12]6− and [Zn2O6]8− blocks in the (1̅11) plane; (c) The [B6O12]6− group. Adapted from Ref. [66,69], Ref. [66,69], with permission of John Wiley and Sons. with permission of John Wiley and Sons.

KZnB3O6 was synthesized using a conventional solid-state reaction under ambient pressure. KZnB3O6 was synthesized using a conventional solid-state reaction under ambient pressure. In In detail, a single-phase white powder of KZnB3O6 was prepared by grinding a stoichiometric mixture detail, a single-phase white powder of KZnB3O6 was prepared by grinding a stoichiometric mixture of K2CO3, ZnO, and H3BO3, which was heated to 500 ◦C to decompose the salt and annealed at 750 ◦C of K2CO3, ZnO, and H3BO3, which was heated to 500 oC to decompose the salt and annealed at 750 °C for 24 h. Single crystals of KZnB3O6 were obtained by spontaneous nucleation by melting the obtained for 24 h. Single crystals of KZnB3O6 were obtained by spontaneous nucleation by melting 1the obtained pure phase powder at 820 ◦C, then slowly cooling the melt to 600 ◦C at a rate of 1 ◦C h− . Although pure phase powder at 820 °C, then slowly cooling the melt to 600 °C at a rate of 1 °C h−1. Although the synthesis condition of KZnB3O6 is different from that of previously reported HP borates, further examinationthe synthesis of condition the edge-sharing of KZnB3O geometry6 is different reveals from that that the of B-Opreviously bond lengths reported and HP O-B-O borates, angles further in KZnBexamination3O6 are consistentof the edge with-sharing those geometry of HP borates. reveals As that a common the B-O feature, bond lengths the O-B-O and angles O-B-O within angles the in KZnB3O6 are consistent5 with those of HP borates. As a common feature, the O-B-O angles within the [B2O2] ring of es-[BO4] − are remarkably reduced and the B-O bonds within the ring are elongated − due[B2O to2] thering like-charges of es-[BO4]5 repulsion, are remarkably which willreduced push and higher the valenceB-O bonds ions within apart inthe the ring es-polyhedra are elongated to minimizedue to the the like electrostatic-charges repulsion potential., which will push higher valence ions apart in the es-polyhedra to minimizeTheoretical the electrostatic insight into potentia the structurall. stability of KZnB3O6 was carried out by Yang and coworkersTheoretical [116]. They insight investigated into the the structural molecular stability dynamics, of KZnB lattice3O dynamics6 was carried and electronic out by properties Yang and ofcoworkers es-KZnB 3O [1616and]. They cs-KZnB investigated3O6 (hypothetical the molecular one, constructed dynamics, based lattice on isostructural dynamics and KCdB electronic3O6) via densityproperties functional of es-KZnB theory.3O6 Molecularand cs-KZnB dynamics3O6 (hypothetical simulations one, show constructed that, es-KZnB based3O on6 is isostructural stable from 100KCdB to 10003O6) via K while density cs-KZnB functional3O6 deforms theory. Molecular with bond dynamics stretching. simulations Analysis of show lattice that, dynamics es-KZnB shows3O6 is that,stable a soft-mode from 100 reflecting to 1000 K the while dynamic cs-KZnB instability3O6 deforms exists in with the cs-KZnBbond stretching.3O6, which Analysis probably of comes lattice 8 fromdynamics an overlong shows that, Zn-O a bondsoft-mode in the reflecting [ZnO5] −thepolyhedra. dynamic instability Electronic exists property in the calculation cs-KZnB3O indicates6, which 5 thatprobably the longest comes B-O fromσ bonds an overlong connecting Zn- theO bond es-[BO in4 ] the− polyhedra [ZnO5]8- polyhedra are stable. enough Electronic to provide property a solidcalculation framework indicates for es-KZnB that the 3 longestO6. The B- stabilityO σ bonds of cs-KZnB connecting3O6 theis reduced es-[BO4]5 by− po thelyhedra overlong are Zn-O stable bondenough that to possesses provide a thesolid smallest framework covalent for es nature-KZnB and3O6. theThe least stability orbital of cs overlap-KZnB3 amongO6 is reduced the bonds by the in 8 aoverlong [ZnO5] −Znpolyhedron,-O bond that which possesses further the confirmssmallest thecovalent results nature that areand obtained the least fromorbital lattice overlap dynamics among the bonds in a [ZnO5]8− polyhedron, which further confirms the results that are obtained from lattice dynamics analysis. The results strongly support explanation of the structural stability origination of es-KZnB3O6, and provide a fundamental understanding on the origin of the unique es-[BO4]5− connection mode.

Ba4Na2Zn4(B3O6)2(B12O24)

Ba4Na2Zn4(B3O6)2(B12O24) is the second reported borate possessing the special es-[BO4]5− configuration obtained under ambient pressure [75]. Single crystals of Ba4Na2Zn4(B3O6)2(B12O24) were synthesized by high temperature solution method using Na2CO3, BaCO3, ZnO, H3BO3, Na2B4O710H2O as raw materials (Na2CO3/BaCO3/ZnO/H3BO3/Na2B4O7 molar ratio = 1:4:6:18:2). The basic structural units in Ba4Na2Zn4(B3O6)2(B12O24) are the [ZnO4]6− tetrahedra, [B3O6]3− and [B12O24]12− groups. Therein, the [B12O24]12− group is composed of [BO3]3− triangles and [BO4]5− tetrahedra via vertex- and edge-sharing. In detail, one [BO4]5− tetrahedron and two [BO3]3− triangles form a [B3O7]5− group via vertex-sharing, a [BO4]5− tetrahedron of the [B3O7]5− group links to a [BO3]3− triangle of another [B3O7]5− group to form a [B6O13]8− group, then two inversion-center-related [B6O13]8− groups are further connected by es-[BO4]5− tetrahedra to form a [B12O24]12− group (Figure 7b). As shown in Figure 7a, the [B12O24]12− groups are located between parallel [B3O6]3− rings to form a sandwich structural block, which are bridged by [ZnO4]6− tetrahedra to generate a 2D infinite

Molecules 2019, 24, 2763 8 of 27

analysis. The results strongly support explanation of the structural stability origination of es-KZnB3O6, 5 and provide a fundamental understanding on the origin of the unique es-[BO4] − connection mode.

Ba4Na2Zn4(B3O6)2(B12O24) 5 Ba4Na2Zn4(B3O6)2(B12O24) is the second reported borate possessing the special es-[BO4] − configuration obtained under ambient pressure [75]. Single crystals of Ba4Na2Zn4(B3O6)2(B12O24) were synthesized by high temperature solution method using Na2CO3, BaCO3, ZnO, H3BO3, Na2B4O7 10H2O as raw materials (Na2CO3/BaCO3/ZnO/H3BO3/Na2B4O7 molar ratio = 1:4:6:18:2). · 6 3 The basic structural units in Ba4Na2Zn4(B3O6)2(B12O24) are the [ZnO4] − tetrahedra, [B3O6] − and 12 12 3 5 [B12O24] − groups. Therein, the [B12O24] − group is composed of [BO3] − triangles and [BO4] − 5 3 tetrahedra via vertex- and edge-sharing. In detail, one [BO4] − tetrahedron and two [BO3] − triangles 5 5 5 form a [B3O7] − group via vertex-sharing, a [BO4] − tetrahedron of the [B3O7] − group links to a 3 5 8 [BO3] − triangle of another [B3O7] − group to form a [B6O13] − group, then two inversion-center-related 8 5 12 [B6O13] − groups are further connected by es-[BO4] − tetrahedra to form a [B12O24] − group (Figure7b). 12 3 As shown in Figure7a, the [B 12O24] − groups are located between parallel [B3O6] − rings to form 6 Moleculesa sandwich 2019, 24 structural, x FOR PEER block, REVIEW which are bridged by [ZnO4] − tetrahedra to generate a 2D infinite8 of 27 2 [Zn4(B3O6)2(B12O24)] layer. The intralayer open channels and interlayer void spaces are filled with ∞ 2Ba[Zn2+ 4and(B3O Na6)2(B+ 12cationsO24)] layer. to balance The intralay chargeer and open form channels a 3D framework. and interlayer void spaces are filled with Ba2+ and Na+ cations to balance charge and form a 3D framework. (a) (b)

5- es-BO4

12- the [B12O24] group

FigureFigure 7. ((aa)) The The crystal structure structure of of Ba Ba44NaNa22ZnZn44(B3OO66)2)(B2(B1212OO24)24 view) vieweded along along the the bb-axis;-axis; (b (b) )t thehe 12 5 [B[B12OO24]12]− groupgroup containing containing es es−[BO[BO4]5− ]tetrahedra.tetrahedra. Adapted Adapted from from Ref Ref.. [75 [75], ],Copyright Copyright (201 (2013)3) with with 12 24 − − 4 − permissionpermission from from Elsevier. Elsevier.

5 TThehe structural structural features features of of es es-[BO-[BO44]]5−− tetrahedratetrahedra in in afore aforementionedmentioned anhydrous anhydrous borates borates are are highly highly 6 consistent and the FBBs share common features. Taking the basic [B2O−6] − unit as the prototype, consistent and the FBBs share common features. Taking the basic [B2O6]6 unit as the prototype, all 5 all the available FBBs of these es-[BO4] − tetrahedra-containing borates can be evolved by replacing the available FBBs of these es-[BO4]5− tetrahedra-containing borates can be evolved by replacing the the four nodes with different types of B-O blocks (Figure8). For the type A model, the replaced four nodes with different types of B-O blocks (Figure6 8). For the type A model, 6the replaced nodes nodes are the same B-O blocks, such as the [B2O6] − FBB of HP-MB2O4, [B6O12] − FBB of KZnB3O6, − − are the same18 B-O blocks, such as the [B2O326]6 FBB of HP-MB2O4, [B6O12]6 FBB of KZnB3O6, [B6O18]18 [B6O18] − FBB of RE4B6O15, and [B20O46] − FBB of a-RE2B4O9 series can be regarded as the derivatives − 32− 6 6 3 8 obtained FBB of RE by4 replacingB6O15, and the [B20 nodesO46] of FBB basic of [B a2-REO6]2B−4Ounits9 series with can four be regarded identical [Bas2 Othe6] derivatives−, [BO3] −, obtained [B2O7] − , 14 − − − − byand replacing [B4O13] the− units, nodes respectively. of basic [B2 WhileO6]6 theunits replaced with four nodes identical for type [B B2O model6]6 , [BO are3] di3 ,ff [Berent,2O7]8 such, and as 3 8 3 5 the corresponding14− replaced units are [BO3] − for [B4O10] − FBB of BaAlBO4, [BO3] − and [BO4] − [B4O13] units,10 respectively. While the replaced nodes for type B model3 are different,7 such as the for [B6O14] − FBB of HP-AB3O5 (A = K, NH4, Rb, Tl), as well as [BO3] − and [B5O11] − blocks for − − − − corresponding14 replaced units are [BO3]3 for [B4O10]8 FBB of BaAlBO4, [BO3]3 and [BO4]5 for [B14O28] − FBB of Li4Na2CsB7O14. − − − [B6O14The]10 aboveFBB of findingsHP-AB3O prove5 (A = thatK, NH the4, borateRb, Tl),structure as well as is [BO very3]3 flexible and [B5 andO11]7 confirm blocks the for feasibility[B14O28]14 5 − of FBB incorporating of Li4Na2CsB the7O14 es-[BO. 4] − configuration into traditional borate chemistry to enrich the borate structure. The above findings prove that the borate structure is very flexible and confirm the feasibility of incorporating the es-[BO4]5− configuration into traditional borate chemistry to enrich the borate structure.

A

6- 6- 18- [B2O6] FBB [B6O12] FBB [B6O18] FBB 32- [B20O46] FBB 6- Basic [B2O6] unit with four nodes B 5- Edge-sharing [BO4] tetrahedra 3- [BO3] triangle 5- [B O ]8- FBB [B O ]10- FBB [B O ]14- FBB [BO4] tetrahedron 4 10 6 14 14 28 Figure 8. Structural features of all the available anhydrous borates with fundamental building blocks − (FBB) containing edge-sharing [BO4]5 tetrahedra. Adapted from Ref. [113] with permission from The Royal Society of Chemistry.

2.2.3. Zincoborates with Two Kinds of Isolated Anion Groups

Molecules 2019, 24, x FOR PEER REVIEW 8 of 27

2[Zn4(B3O6)2(B12O24)] layer. The intralayer open channels and interlayer void spaces are filled with Ba2+ and Na+ cations to balance charge and form a 3D framework. (a) (b)

5- es-BO4

12- the [B12O24] group

Figure 7. (a) The crystal structure of Ba4Na2Zn4(B3O6)2(B12O24) viewed along the b-axis; (b) the [B12O24]12− group containing es−[BO4]5− tetrahedra. Adapted from Ref. [75], Copyright (2013) with permission from Elsevier.

The structural features of es-[BO4]5− tetrahedra in aforementioned anhydrous borates are highly − consistent and the FBBs share common features. Taking the basic [B2O6]6 unit as the prototype, all

the available FBBs of these es-[BO4]5− tetrahedra-containing borates can be evolved by replacing the four nodes with different types of B-O blocks (Figure 8). For the type A model, the replaced nodes − − are the same B-O blocks, such as the [B2O6]6 FBB of HP-MB2O4, [B6O12]6 FBB of KZnB3O6, [B6O18]18

− − FBB of RE4B6O15, and [B20O46]32 FBB of a-RE2B4O9 series can be regarded as the derivatives obtained

− − − − by replacing the nodes of basic [B2O6]6 units with four identical [B2O6]6 , [BO3]3 , [B2O7]8 , and

− [B4O13]14 units, respectively. While the replaced nodes for type B model are different, such as the

− − − − corresponding replaced units are [BO3]3 for [B4O10]8 FBB of BaAlBO4, [BO3]3 and [BO4]5 for

− − − [B6O14]10 FBB of HP-AB3O5 (A = K, NH4, Rb, Tl), as well as [BO3]3 and [B5O11]7 blocks for [B14O28]14

− FBB of Li4Na2CsB7O14.

MoleculesThe2019 above, 24, 2763 findings prove that the borate structure is very flexible and confirm the feasibility9 of 27of incorporating the es-[BO4]5− configuration into traditional borate chemistry to enrich the borate structure.

A

6- 6- 18- [B2O6] FBB [B6O12] FBB [B6O18] FBB 32- [B20O46] FBB 6- Basic [B2O6] unit with four nodes B 5- Edge-sharing [BO4] tetrahedra 3- [BO3] triangle 5- [B O ]8- FBB [B O ]10- FBB [B O ]14- FBB [BO4] tetrahedron 4 10 6 14 14 28 Figure 8. Structural features of all the available anhydrous borates with fundamental building blocks 5− (FBB)(FBB) containingcontaining edge-sharingedge-sharing [BO[BO44]]5− tetrahedra.tetrahedra. Adapted fromfrom Ref.Ref. [[113113]] with permission from The Royal SocietySociety ofof Chemistry.Chemistry. 2.2.3. Zincoborates with Two Kinds of Isolated Anion Groups 2.2.3. Zincoborates with Two Kinds of Isolated Anion Groups According to the Pauling’s fifth rule [76,77], the number of essentially different kinds of constituents in a crystal tends to be small, which means that the number of components of various types in a crystal tends to be small. For most borates, there is only one kind of isolated B-O group in the structure [117,118]. Although violating Pauling’s fifth rule, several zincoborates with two kinds of isolated B-O groups 3 3 have been discovered, for instance, Cs3Zn6B9O21 ([BO3] − + [B3O6] −)[45,46], Ba3Zn(BO3)(B2O5)F and 3 Ba4Zn2(BO3)2(B2O5)F2 [78], etc., in which two kinds of isolated B-O groups are [BO3] − triangle and another relatively low-polymerized polyborate anion. Specifically, there are few examples of two kinds of isolated polyborate (polymerization is no less than 2) anions coexisting in one zincoborate structure, such as, Ba2KZn3(B3O6)(B6O13)[79], Ba4K2Zn5(B3O6)3(B9O19)[80], Ba4Na2Zn4(B3O6)2(B12O24)[75], 4 8 4 8 Bi2ZnO(B2O6) ([B2O5] − + [B2O7] −)[81–83], α-/β-/γ-Pb2Ba4Zn4B14O31 ([B2O5] − + [B6O13] −)[84], etc. The uncommon coexistence of different B-O polyanions in these zincoborates further implies the prevention effect of strong-bonded zinc cations on the polymerization of B-O configuration.

3. Zincoborates with Excellent Properties Based on the previous reports, it should be emphasized that Zn-O/F polyhedra, especially 6 5 the [ZnO4] − and [ZnO3F] − tetrahedra, have impacts on both crystal structures and properties. In this section, a series of zincoborates with UV/DUV cutoff edges, second-order NLO properties and anomalous thermal expansion properties are briefly reviewed.

3.1. Zincoborates with Short Ultraviolet (UV) Cutoff Edges With the rapid development of UV technology, NLO and birefringent materials with high transparency in the UV regions are generally required from both an academic and technological standpoint [119–121]. Since the d-d or f -f electronic transitions will have a negative influence on the large energy band gap, thus, in consideration of the absorption edge, it is a common strategy to introduce cations without d-d or f -f transitions (such as alkali and alkaline-earth metals) to blue shift the cutoff edge to the UV regions [122,123]. Besides, cations with fully occupied d or half-occupied f electronic shells, such as Zn2+, Gd3+, and Y3+, can also be used in UV materials since their electronic shells can effectively inhibit unfavorable electronic transitions [124–127]. Insofar as we know, there are a number of zinc-containing borates reported in the UV/DUV regions. For instance, Ba3(ZnB5O10)PO4 (~180 nm) [60], Cs3Zn6B9O21 (~200 nm) [45,46], AZn2BO3X2 (A = Na, K, Rb, NH4;X = Cl, Br) series (~190-209nm) [58,59], K7ZnSc2B15O30 (~200 nm) [128], K3ZnB5O10 (~190 nm) [129], Cs12Zn4(B5O10)4 (below 185 nm) [130], etc. Hence, the Zn-containing borate system is also a candidate for exploring promising UV even DUV materials. Molecules 2019, 24, 2763 10 of 27

3.2. Zincoborates with Large Second-Order Non-Linear Optical (NLO) Response The increasing need for high-power all-solid-state UV light sources promotes the development of NLO borate crystals, especially those with large second-order NLO responses and short UV-transmission cutoff edges [131,132]. After continuous efforts in the past few decades, a series of borate-based NLO materials were developed and have been widely used in many optoelectronic devices, such as KBBF [12,13], β-BBO [16], LBO [17], CsLiB6O10 (CLBO) [133,134], etc. Up to now, many NLO zincoborates with good performance in the UV/DUV regions have been discovered. In this section, we focus on recent studies of zincoborate crystals with good second-order NLO properties and the representative ones are included in Table1.

Table 1. The representative non-linear optical (NLO) zincoborates.

Second Harmonic Generation (SHG) Compounds Space Group Structural Features Absorption Edge Refs. Intensity (@ 1064nm) 2 Cs3Zn6B9O21 Cmc21 [Zn2BO3O2] layer 3.3 KH2PO4 (KDP) ~200 nm [45] ∞ × Isolated [BO ]3 KZn BO Cl R32 3 − 3.01 KDP ~194 nm [58] 2 3 2 (coplanar) × Isolated [BO ]3 RbZn BO Cl R32 3 − 2.85 KDP ~198 nm [58] 2 3 2 (coplanar) × Isolated [BO ]3 NH Zn BO Cl R32 3 − 2.82 KDP ~190 nm [58] 4 2 3 2 (coplanar) × Isolated [BO ]3 KZn BO Br R32 3 − 2.68 KDP ~209 nm [58] 2 3 2 (coplanar) × Isolated [BO ]3 RbZn BO Br R32 3 − 2.53 KDP <214 nm [58] 2 3 2 (coplanar) × Ba3(ZnB5O10)PO4 Pmn21 / 4 KDP (@ 532nm) ~180 nm [60] 2 × Ba5Zn4(BO3)6 Pc [Zn4(BO3)4O6] layer 2.6 KDP ~223 nm [62] ∞ 3 × Ba2Zn(BO3)2 Pca21 Isolated [BO3] − 1.5 KDP ~230 nm [64] 4 × Isolated [B2O5] − + Bi2ZnOB2O6 Pba2 8 3–4 KDP ~330 nm [83] [B2O7] − × 4 Isolated [B2O5] − + α Pb2Ba4Zn4B14O31 P1 8 0.6 KDP <289 nm [84] − [B6O13] − × 4 Isolated [B2O5] − + β Pb2Ba4Zn4B14O31 Cc 8 1.1 KDP <304 nm [84] − [B6O13] − × 2 Cs12Zn4(B5O10)4 [Zn(B5O10)] layer 0.5 KDP <185 nm [130] ∞ × Isolated [BO ]3 BaZnBO F P6 3 − 2.8 KDP ~223 nm [135] 3 (coplanar) × β-Zn BPO P6 / 1.8 KDP ~250 nm [136] 3 7 × Mg Na ZnB O / / 2.78 KDP ~210 nm [137] 2 2 4 10 ×

3.2.1. NLO Properties of Zincoborates Containing Alkali/Alkaline-Earth Metals

Cs3Zn6B9O21

Single crystals of Cs3Zn6B9O21 were grown by the high temperature solution method using Cs2O-B2O3-PbO (Cs2CO3:ZnO:H3BO3:PbO = 1:1:5:1) or Cs2O-ZnF2-B2O3 (Cs2CO3:ZnO:ZnF2:H3BO3 = 1.5:1:2:8) as the flux system [45,46]. The absorption edge of Cs3Zn6B9O21 is below 200 nm in the UV region and its powder SHG efficiency is approximately 3.3 times that of KDP, which implies that Cs3Zn6B9O21 has potential application prospects as an UV NLO material. 3 Remarkably, Cs3Zn6B9O21 has a small density of the [BO3] − triangles but exhibits a large SHG response in the KBBF family. Based on the calculation of the dipole moments, the inversion symmetry lifting atomic distortions (Figure9), electronic structure and atom-cutting analysis, the enhanced SHG 3 6 response originates from the cooperative effect of coparallel [BO3] − triangles and distorted [ZnO4] − 2 6 tetrahedra in the [Zn2BO3O2] layers. In particular, the contribution of the [ZnO4] − groups to the ∞ 3 6 SHG effect is significantly larger than that from the aligned [BO3] − groups, i.e., the [ZnO4] − tedrahedra 6 dominate the SHG enhancement, which distinguishes [ZnO4] − tedrahedra as an UV-transparent NLO-active units. The results imply that facile synthesis of useful NLO crystals can be achieved by 6 combining [ZnO4] − tetrahedra and π-orbital systems in borates. Molecules 2019, 24, x FOR PEER REVIEW 11 of 27 Molecules 2019, 24, 2763 11 of 27

FigureFigure 9. 9. AtomicAtomic distortion distortion patterns patterns obtained obtained from from the the symmetry-mode symmetry-mode analysis: analysis: Oxygen Oxygen atom atom 6 6− 33− displacementsdisplacements belongingbelonging to to ( a()a) the the [ZnO [ZnO4]4]− tetrahedra,tetrahedra, ((bb)) thethe [BO [BO33]] − network, and (cc)) thethe completecomplete distortiondistortion projectedprojected alongalong thethea a-axis.-axis. ReprintedReprinted (adapted)(adapted) withwith permissionpermission fromfrom Ref.Ref. [45],[45, CopyrightCopyright (2014)(2014) AmericanAmerican ChemicalChemical Society.Society.

AZn BO X (A = Na, K, Rb, NH ;X = Cl, Br) Series AZn22BO33X2 (A = Na, K, Rb, NH44; X = Cl, Br) Series Crystals of KZn BO Cl , RbZn BO Cl , KZn BO Br , and RbZn BO Br can be obtained by high Crystals of KZn2 2BO33Cl22, RbZn22BO33Cl22, KZn2BO3Br22, and RbZn2BO33Br22 can be obtained by high temperature solution method as well as solvothermal techniques, while crystals of NH Zn BO Cl temperature solution method as well as solvothermal techniques, while crystals of NH4 4Zn2 2BO3 3Cl22 werewere growngrown only only by by solvothermal solvothermal techniques techniques due due to to the the decomposition decomposition of of ammonium ammonium compounds compounds at highat high temperature. temperature. The The series series of borates of borate are isostructurals are isostructural with KBBF with and KBBF preserve and preserve the NLO-favorable the NLO- structuralfavorable features structural [58 features,59]. Remarkably, [58,59]. Remarkably, this series ofthis materials series of exhibits materials strong exhibit SHGs responsesstrong SHG of approximatelyresponses of approximately more than 2 times more that than of benchmark 2 times that KBBF, of benchmark and the compounds KBBF, and are the phase-matchable compounds are inphase the- visiblematchable and in UV the regions visible and and possessUV regions UV-transmission and possess UV cuto-transmissionff edges (~200 cutoff nm), edge indicatings (~200 nm), that thisindicating series ofthat crystals this series may of have crystals potential may have application potential in application the short-wave in the NLO short field.-wave TheoreticalNLO field. 5 calculationsTheoretical calculations reveal that the reveal SHG that enhancement the SHG enhancement mainly originates mainly from originates the distorted from the [ZnO distorted3X] − tetrahedra. The cooperative effect of [ZnO X]5 tetrahedra and the coparallel [BO ]3 triangles results [ZnO3X]5− tetrahedra. The cooperative effect3 − of [ZnO3X]5− tetrahedra and the3 − coparallel [BO3]3− in the large SHG responses. In particular, it is the first case where [ZnO X]5 groups can be used as the triangles results in the large SHG responses. In particular, it is the first3 case− where [ZnO3X]5− groups NLO-activecan be used structuralas the NLO units-active in NLOstructural materials. units in NLO materials.

BaZnBO3F BaZnBO3F Initially, structure of BaZnBO3F was determined by powder X-ray diffraction data in 2010. Single Initially, structure of BaZnBO3F was determined by powder X-ray diffraction data in 2010. Single crystal growing trials of BaZnBO3F with different fluxes have not been successful until 2016 [61,135]. crystal growing trials of BaZnBO3F with different6 fluxes have not been successful until 2016 [61,135]. In the structure of BaZnBO3F, the [ZnO3F2] − bipyramid shares its three equatorial oxygen atoms with In the structure of BaZnBO3F, the [ZnO3F2]6− bipyramid shares its three equatorial oxygen atoms with three [BO ]3 groups to form a flat 2 [ZnBO F] layer, and the adjacent layers are further linked via 3 3− 2 3 three [BO3] groups to form a flat6 [ZnBO∞ 3F] layer, and the adjacent layers are further linked via the the apical F atoms of [ZnO3F2] − bipyramids to form a 3D framework (Figure 10). Within a single apical F atoms of [ZnO3F2]6− bipyramids to form a 3D framework (Figure 10). Within a single 2 [ZnBO F] layer, the [ZnO F ]6 bipyramid facilitates its three neighboring [BO ]3 units to arrange 2 3 3 2 6−− 3 3−− ∞[ZnBO3F] layer, the [ZnO3F2] bipyramid facilitates its three neighboring [BO3] units to arrange6 into a perfect coplanar alignment in the plane through three basal or equatorial bonds of [ZnO3F2] −6− into a perfect coplanar alignment in the plane through three3 basal or equatorial bonds of [ZnO3F132] bipyramid. Meanwhile, among different layers, the [BO3] − units are also governed by the [BaO6F3] − bipyramid. Meanwhile, among different layers, the [BO3]3− units are also governed by the [BaO6F3]13− polyhedra and arranged parallel to each other in neighboring layers. The perfectly coplanar manner polyhedra and3 arranged parallel to each other in neighboring layers. The perfectly coplanar manner of the [BO3] − groups produces a cooperative effect and gives maximum contribution to the NLO of the [BO3]3− groups produces a cooperative effect and gives maximum contribution to the NLO response. As a result, a large NLO effective coefficient, 2.8 deff (KDP), is observed. response. As a result, a large NLO effective coefficient, 2.8× × deff (KDP), is observed. (a) (b) (c)

(d) Molecules 2019, 24, x FOR PEER REVIEW 11 of 27 results imply that facile synthesis of useful NLO crystals can be achieved by combining [ZnO4]6− tetrahedra and π-orbital systems in borates.

Figure 9. Atomic distortion patterns obtained from the symmetry-mode analysis: Oxygen atom displacements belonging to (a) the [ZnO4]6− tetrahedra, (b) the [BO3]3− network, and (c) the complete distortion projected along the a-axis. Reprinted (adapted) with permission from Ref. [45, Copyright (2014) American Chemical Society.

AZn2BO3X2 (A = Na, K, Rb, NH4; X = Cl, Br) Series

Crystals of KZn2BO3Cl2, RbZn2BO3Cl2, KZn2BO3Br2, and RbZn2BO3Br2 can be obtained by high temperature solution method as well as solvothermal techniques, while crystals of NH4Zn2BO3Cl2 were grown only by solvothermal techniques due to the decomposition of ammonium compounds at high temperature. The series of borates are isostructural with KBBF and preserve the NLO- favorable structural features [58,59]. Remarkably, this series of materials exhibits strong SHG responses of approximately more than 2 times that of benchmark KBBF, and the compounds are phase-matchable in the visible and UV regions and possess UV-transmission cutoff edges (~200 nm), indicating that this series of crystals may have potential application in the short-wave NLO field. Theoretical calculations reveal that the SHG enhancement mainly originates from the distorted [ZnO3X]5− tetrahedra. The cooperative effect of [ZnO3X]5− tetrahedra and the coparallel [BO3]3− triangles results in the large SHG responses. In particular, it is the first case where [ZnO3X]5− groups can be used as the NLO-active structural units in NLO materials.

BaZnBO3F

Initially, structure of BaZnBO3F was determined by powder X-ray diffraction data in 2010. Single crystal growing trials of BaZnBO3F with different fluxes have not been successful until 2016 [61,135]. In the structure of BaZnBO3F, the [ZnO3F2]6− bipyramid shares its three equatorial oxygen atoms with three [BO3]3− groups to form a flat 2[ZnBO3F] layer, and the adjacent layers are further linked via the apical F atoms of [ZnO3F2]6− bipyramids to form a 3D framework (Figure 10). Within a single 2[ZnBO3F] layer, the [ZnO3F2]6− bipyramid facilitates its three neighboring [BO3]3− units to arrange into a perfect coplanar alignment in the plane through three basal or equatorial bonds of [ZnO3F2]6− bipyramid. Meanwhile, among different layers, the [BO3]3− units are also governed by the [BaO6F3]13− polyhedra and arranged parallel to each other in neighboring layers. The perfectly coplanar manner Molecules 2019, 24, 2763 12 of 27 of the [BO3]3− groups produces a cooperative effect and gives maximum contribution to the NLO response. As a result, a large NLO effective coefficient, 2.8 × deff (KDP), is observed. (a) (b) (c)

(d)

Molecules 2019, 24, x FOR PEER REVIEW 12 of 27

2 Figure 10. (a) Crystal structure of BaZnBO3F viewed along the b-axis; (b) Flat [ZnBO2 3F] layer; (c) Figure 10. (a) Crystal structure of BaZnBO3F viewed along the b-axis; (b) Flat [ZnBO3F] layer; 13− 3− 6− [BaO6F3] polyhedron13 connects to six [BO3] groups3 ; (d) [ZnO3F2] trigonal6 bipyramid∞ connects to (c) [BaO6F3] − polyhedron connects to six [BO3] − groups; (d) [ZnO3F2] − trigonal bipyramid connects 3− three [BO3] groups.3 Adapted from Ref. [29], Copyright (2018) with permission from Elsevier. to three [BO3] − groups. Adapted from Ref. [29], Copyright (2018) with permission from Elsevier.

Ba5ZnZn44(BO(BO3)36) 6

A new NLO crystal Ba5Zn44(BO(BO33)6) 6waswas obtained obtained b byy substituti substitutingng the the Be Be atom atom with with the the Zn Zn atom atom,, ssingleingle crystals of which were obtained from aa high-temperaturehigh-temperature solutionsolution withwith BaCOBaCO33, ZnO,ZnO, HH33BO3,, 2 2 and NaF in a a molar molar ratio ratio of of 2:2:4:1 2:2:4:1 [ 62[62].]. The The structure structure is is constructed constructed with with [Zn∞[Zn44(BO(BO33))44O6]] layers 33− ∞ 2 2 bridged by planar planar [BO [BO33]] − groups (Figure 11 11),), and and the the distance distance between between adjacent adjacent [Zn∞[Zn4(BO4(BO33))44O6] ∞ layerslayers in Ba Ba5Zn44(BO(BO33)6) 6isis much much shorter shorter than than that that of of KBBFKBBF,, thus,thus, Ba Ba55Zn4(BO(BO33))66 maymay show show a a better growth habit. Also, Ba 5ZnZn44(BO(BO3)36) 6featuresfeatures a arelatively relatively large large SHG SHG response response of of about about 2.6 2.6 times times that that of 33− 6− KDPKDP,, owing owing to to thethe incorporationincorporation of of[BO [BO3]3]− and [ZnO [ZnO44] − NLONLO active active groups. groups. Calculation Calculation results results,, based on on the the anion anion group group theory theory [138 [13,1398,1],39 show], show that thatthe theoretically the theoretical calculatedly calculat SHGed response SHG response coming 3 3− cominfrom theg from [BO3 ] the− groups [BO3] ( dgroups111 and (dd122111 coeandffi cientsd122 coefficients with values with of + values0.403 and of +0.4030.42 pm and/V, − respectively)0.42 pm/V, 6 − 6− respectively)is far smaller than is far the smaller total SHG than response, the total which SHG implies response, that thewhich [ZnO implies4] − groups that contributethe [ZnO4] a lot groups to the 6 6− contributelarge SHG a response, lot to the and large further SHG confirmsresponse, the and function further ofconfirm the [ZnOs th4e] function− groups of as the NLO [ZnO active4] groups groups. as NLO active groups.

Figure 11. ((aa)) The crystalcrystal structurestructure of of BaBa55ZnZn44(BO(BO3))66;;( (bb)) Basic building building blocks blocks in in Ba Ba55ZnZn44(BO3))66.. Reprinted with permission from R Ref.ef. [ [62],62, Copyright (2017) American Chemical Society.

3.2.2. Other Zinc-Containing Compounds with NLO Properties 3.2.2. Other Zinc-Containing Compounds with NLO Properties

Bi2ZnOB2O6 Bi2ZnOB2O6 Bi2ZnOB2O6 was first reported by J. Barbier et al. in 2005 and its structure was determined Bi2ZnOB2O6 was first reported by J. Barbier et al. in 2005 and its structure was determined by by powder X-ray diffraction and refined by the Rietveld method using powder neutron diffraction powder X-ray diffraction and refined by the Rietveld method using powder neutron diffraction3 data data [81]. Two years later, the crystal of Bi2ZnOB2O6 with a size of 0.4 0.4 0.5 mm was prepared [81]. Two years later, the crystal of Bi2ZnOB2O6 with a size of 0.4  0.4 ×× 0.5 mm× 3 was prepared by the by the conventional solid-state reaction method [82]. Until 2009, Pan group firstly obtained the high conventional solid-state reaction method [82]. Until 2009, Pan group firstly obtained the high quality quality sizable single crystal by the top-seeded growth method [83]. The structure of Bi2ZnOB2O6 sizable single crystal by the top-seeded growth method [83]. The structure of Bi2ZnOB2O6 consists of consists of 2 [ZnB O ] layers alternating with six-coordinated Bi3+ cations (Figure 12)[140]. In the 2 2 7 3+ 2 2∞[ZnB2O7] layers∞ alternating4 with six-8coordinated Bi cations (Figure6 12) [140]. In the ∞[ZnB2O7] [ZnB2O7] layer, [B2O5] − and [B2O7] − units are bridged by [ZnO4] − tetrahedra via sharing oxygen ∞ 4− 8− 6− layeratoms., [B It2O is5] apositive and [B2O biaxial7] units optical are crystalbridged with by [ largeZnO4] birefringence tetrahedra (0.1066-0.0794)via sharing oxygen and atoms. has a large It is a positive biaxial optical crystal with large birefringence (0.1066-0.0794) and has a large SHG effect of about 3–4 times that of KDP. These advantages make Bi2ZnOB2O6 a promising candidate for NLO materials and attractive for continued research.

Molecules 2019, 24, 2763 13 of 27

SHG effect of about 3–4 times that of KDP. These advantages make Bi2ZnOB2O6 a promising candidate forMolecules NLO materials 2019, 24, x FOR and PEER attractive REVIEW for continued research. 13 of 27 Molecules 2019, 24, x FOR PEER REVIEW 13 of 27

Figure 12. The crystal structure of Bi ZnOB O viewed along b-axis. Reprinted with permission from Figure 12. The crystal structure of Bi22ZnOB22O6 viewed along b-axis. Reprinted with permission from Ref. [140], Copyright (2009) American Chemical Society. RFigureef. [14 012.], CopyrightThe crystal (2009) structure American of Bi2ZnOB Chemical2O6 viewed Society. along b-axis. Reprinted with permission from Ref. [140], Copyright (2009) American Chemical Society. Ba3(ZnB5O10)PO4 Ba3(ZnB5O10)PO4 BaBa3(ZnB3(ZnB5O105)POO104)PO 4 was successfully synthesized as the first DUV NLO zincoborate-phosphate 3 5 10 4 Ba (ZnB O )PO was 6 successfully synthesized3 as the first DUV NLO zincoborate-phosphate crystal by combining [ZnO4] − tetrahedra, [PO4] − tetrahedra, and B-O groups into one compound [60]. crystalBa by3(ZnB combining5O10)PO 4[ZnO was4 ] successfully6− tetrahedra, synthesized [PO4]3− tetrahedra, as the and first B DUV-O groups NLO into zincoborate one compound-phosphate [60]. In the crystal structure, the basic building unit [ZnB O ]3 is composed of three [BO ]3 triangles, Incrystal the crystal by combining structure, [ZnO the 4basic]6− tetrahedra, building [POunit4 ][ZnB3− tetrahedra,5O510]103− is− andcomposed B-O groups of three into [BO one3] 3compound− triangles,3 − [two60]. 5 6 two [BO5− ] tetrahedra, and one [ZnO6− ] tetrahedron via3 sharing− the corner oxygen atoms3− (Figure 13a). [BOIn the4] 4 crystal tetrahedra,− structure, and onethe basic[ZnO building4] 4tetrahedron− unit [ZnB via5 Osharing10] is composed the corner of oxygen three [BO atoms3] triangles,(Figure 1 two3a). 3 TheThe[BO adjacent 4] adjacent5− tetrahedra, [ZnB [ZnB5 O5andO1010]] one3−− building [ZnO4] 6− units unitstetrahedron are are further further via sharing interconnected interconnected the corner through throughoxygen cornersatoms corners (Figure to to create create 13a a). a 3 3 3[ZnBThe[ZnB adjacent5O5O1010]] frameworkframework [ZnB5O10 ](Figure 3− building 1133bb). ).units The The Ba Baare atoms atoms further and and interconnected the the [PO [PO4]43]− tetrahedra− tetrahedra through are corners are embedded embedded to create in inthe a the ∞ 3 voidsvoids3[ZnB of of the5O the10] framework3[ZnB[ZnB55OO1010]] framework(Figure framework 13b ).(Figure (Figure The Ba 1 3c13 atoms).c). Ba Ba3 (ZnBand3(ZnB 5theO510 O)PO[PO10)PO44 ]exhibits3− 4tetrahedraexhibits a DUV a are DUV absorptio embedded absorptionn edge in the edgeof ∞ of180 voids nm, of largethe large 3 SHG[ZnB SHG 5responsesO responses10] framework of of approximately approximately (Figure 13c). 4 Ba 4 3KDP(ZnBKDP 5atO at101064)PO 1064 4nm, exhibits nm, and and is a DUV istype type-I- Іabsorptio phase phase-matchable.-matchable.n edge of × AllAll180 these these nm, results largeresults indicateSHG indicate responses that that Ba Ba of3(ZnB3 (ZnBapproximately55OO1010)PO4 isis 4a a promising promising KDP at 1064 NLO NLO nm, material. material. and is Basedtype Based-І onphase on the the -calculationmatchable. calculation results,resultAll these thes, the rotationresults rotation indicate of of the the B-Othat B-O groupsBa groups3(ZnB is5isO pivotalpivotal10)PO4 is for a promising enabling enabling the theNLO SHG SHG. material.. In In particular, particular, Based on the thethe valence valencecalculation band band 6−6 maximummaximumresults, the consists consists rotation of of of Zn Znthe 3 d3 Bdstates- Ostates groups of of O O is 2 2ppivotalpstates states for derivedderived enabling fromfrom the the SHG [ZnO . In 4particular,4]] −tetrahedratetrahedra, the, valencewhich which along band along 3 3− 6− withwithmaximum the the [BO [BO 3consists]3]− triangles triangles of Zn make make3d states the mostof most O 2important importantp states derived contributions contributions from the to to [ZnOthe the NLO4 NLO] tetrahedra response. response. , which along with the [BO3]3− triangles make the most important contributions to the NLO response.

(a) (a)

(b) (c) (c) (b) Figure 13. The crystal structure of Ba3(ZnB5O10)PO4. (a) The [ZnB5O10]3− units consist of [BO3]3−, [BO4]5−, 6− 3 3 3− 2+ 3 FigureandFigure [ZnO 13. 13.The4 ]The polyhedra; crystal crystal structurestructure (b) The of of Ba Ba[ZnB3(ZnB3(ZnB5O5O105]10O network)PO10)PO4. (a4).( Twithinhea) The[ZnB which [ZnB5O10]3 5−the Ounits10 (]c) −consist [POunits4] of consistanions [BO3] 3− ofand, [BO [BO Ba4]35−], −, 5 6 3 3 [BOcationsand4] −[ZnO, andoccupy4] [ZnO6− polyhedra; the4] channels.− polyhedra; (b) ReprintedThe (3b[ZnB) The from5O10 ][ZnBR networkef. [560O]10, w ]withinith network permission which within the of whichJohn(c) [PO Wiley the4]3− (anions cand) [PO Sons. 4and] − anionsBa2+ ∞ andcations Ba2+ occupycations the occupy channels. the Reprinted channels. from Reprinted Ref. [60] from, with Ref. permission [60], with of John permission Wiley and of Sons. John Wiley and Sons.

3.3. Zincoborates with Anomalous Thermal Expansion Properties 3.3. Zincoborates with Anomalous Thermal Expansion Properties

Molecules 2019, 24, 2763 14 of 27

Molecules3.3. Zincoborates 2019, 24, x FOR with PEER Anomalous REVIEW Thermal Expansion Properties 14 of 27 Most of the materials exhibit positive thermal expansion, i.e., expanding on heating and contracting Most of the materials exhibit positive thermal expansion, i.e., expanding on heating and on cooling in three dimensions. Interestingly, an increasing quantity of materials with anomalous contracting on cooling in three dimensions. Interestingly, an increasing quantity of materials with thermal expansion properties, such as negative thermal expansion (NTE) (materials contract along anomalous thermal expansion properties, such as negative thermal expansion (NTE) (materials some specific directions when heated) and zero thermal expansion (ZTE) (materials can retain a contract along some specific directions when heated) and zero thermal expansion (ZTE) (materials constant size in a specified temperature range), have attracted a great deal of attention in laboratories can retain a constant size in a specified temperature range), have attracted a great deal of attention in and industries [141,142]. laboratories and industries [141,142]. As abundant inorganic compounds resources, borates not only have promising applications as As abundant inorganic compounds resources, borates not only have promising applications as optical materials, but also are recognized with unusual thermal expansion behavior [143,144]. The bond optical materials, but also are3 recognized with unusual5 thermal expansion behavior [143,144]. The lengths and angles of [BO3] − triangles or [BO4] − tetrahedra in a borate structure remain almost bond lengths and angles of [BO3]3− triangles or [BO4]5− tetrahedra in a borate structure remain almost constant as the ambient temperature varies. When these rigid B-O groups further construct 0D clusters, constant as the ambient temperature varies. When these rigid B-O groups further construct 0D 1D chains, 2D layers, or 3D frameworks, the rotation between the rigid B-O groups combined with clusters, 1D chains, 2D layers, or 3D frameworks, the rotation between the rigid B-O groups combined expansion and/or tilting of other polyhedra in borate structures will control the thermal expansion with expansion and/or tilting of other polyhedra in borate structures will control the thermal property and may result in the anomalous thermal expansion. In recent years, many borate crystals expansion property and may result in the anomalous thermal expansion. In recent years, many borate have been reported to exhibit abnormal thermal expansion behaviors. For instance, the 1D NTE crystals have been reported to exhibit abnormal thermal expansion behaviors. For instance, the 1D behavior has been detected in LiB3O5 [145] and BiB3O6 [146], the area NTE behaviors were discovered NTE behavior has been detected in LiB3O5 [145] and BiB3O6 [146], the area NTE behaviors were in LiBeBO3 [147] and KZnB3O6 [65,66], and the isotropic area NTE effect were found in KBBF [148]. discovered in LiBeBO3 [147] and KZnB3O6 [65,66], and the isotropic area NTE effect were found in Most interestingly, the 3D ZTE effect was discovered in Zn4B6O13 [67,68], which possess the intrinsic KBBF [148]. Most interestingly, the 3D ZTE effect was discovered in Zn4B6O13 [67,68], which possess isotropic near-ZTE behavior as the first case. The discoveries of presented zincoborates add important the intrinsic isotropic near-ZTE behavior as the first case. The discoveries of presented zincoborates members to the family of materials with anomalous thermal expansion properties. add important members to the family of materials with anomalous thermal expansion properties.

3.3.1. Near-Zero Thermal Expansion Properties in Zn4B6O13 3.3.1. Near-Zero Thermal Expansion Properties in Zn4B6O13 Zn4B6O13 crystallizes in the cubic space group of I43m (No. 217) and possesses a very rare Zn4B6O13 crystallizes in the cubic space group of I4̅3m (No. 217) and24 possesses a very rare sodalite sodalite cage structure [67,68]. As shown in Figure 14, each [B24O48] − sodalite cage is constructed cage structure5 [67,68]. As shown in Figure 14, each [B24O48]24− sodalite cage is constructed by 24 [BO4]55− by 24 [BO4] − tetrahedra via sharing the corner oxygen atoms (O2 atoms). In detail, the [BO4] − tetrahedra via sharing the corner oxygen atoms (O2 atoms). In detail, the [BO4]5− tetrahedra5 are tetrahedra are interconnected to form the quadrangles and hexagons with four and six [BO4] − units, 5− interconnectedrespectively. Further, to form every the quadrangles six B4 quadrangles and hexagons and eight with B6 four hexagons and six are[BO combined4] units, respectively. to construct Further, every six 24 B4 quadrangles and eight B6 18 hexagons are combined to construct24 the closed the closed [B24O48] − sodalite cage. The [Zn4O13] − cluster, locked in the [B24O48] − sodalite cage, [B24O48]24− sodalite cage. The6 [Zn4O13]18− cluster, locked in the [B24O48]24− sodalite cage, is composed of is composed of four [ZnO4] − tetrahedra via sharing the vertex O1 atom located at the center of cage. four [ZnO4]6− tetrahedra18 via sharing the vertex O1 atom located at 24 the center of cage. The inside The inside [Zn4O13] − cluster can effectively reinforce the [B24O48] − cage through the relatively 18− 24− [Znstrong4O13 Zn-O2] cluster covalent can effectively bonds, which reinforce could decreasethe [B24O the48] thermal cage through expansion. the relatively strong Zn-O2 covalent bonds, which could decrease the thermal expansion.

Figure 14. The crystal structure of Zn4B6O13. The neighboring boron atoms are connected by thick Figure 14. The crystal structure of Zn4B6O13. The neighboring boron atoms are connected by thick black lines to explicitly display the sodalite cages. Reprinted from Ref. [67], with permission of John black lines to explicitly display the sodalite cages. Reprinted from Ref. [67], with permission of John Wiley and Sons. Wiley and Sons.

The thermal expansion behavior of Zn4B6O13 between 13 and to 270 K was investigated by the variable-temperature X-ray diffraction (XRD) and variation of refined cell parameters (refined by the Rietveld method). As results, in the measured temperature range, no new peaks appear in all the

Molecules 2019, 24, 2763 15 of 27

The thermal expansion behavior of Zn4B6O13 between 13 and to 270 K was investigated by the variable-temperature X-ray diffraction (XRD) and variation of refined cell parameters (refined by the RietveldMolecules 2019 method)., 24, x FOR As PEER results, REVIEW in the measured temperature range, no new peaks appear in15 all of the 27 XRD patterns, which indicates that the structure of Zn4B6O13 is kept in the cubic I43m space group, ̅ andXRD the patterns, thermal which expansion indicates is completelythat the structure 3D isotropic. of Zn4B The6O13 is cell kept parameter in the cubic of Zn I443Bm6O space13 increases group, byand just the 0.03%, thermal this expansion thermal-expansion is completely behavior 3D isotropic. is very T lowhe cell and parameter consistent of with Zn4 theB6O observation13 increases by of positionsjust 0.03%, of the this XRD thermal peaks,-expansion as shown behavior in the insert is very in Figure low 15 and, the consistent (004) peaks with remain the nearly observation constant of inpositions the varying of the temperature XRD peaks, as environment. shown in the Further,insert in theFigure fitted 15, average the (004) thermal peaks remain expansion nearly coe constantfficient (byin the PASCal varying software) temperature in the wholeenvironment. temperature Further, range the isfitted 1.00(14) average/MK. thermal Particularly, expansion from 13 coefficient to 110 K, the(by thermalPASCal software) expansion in coe theffi wholecient intemperature Zn4B6O13 isrange even is much1.00(14)/MK. smaller Particularly, (0.28(06)/MK), from which 13 to can110 beK, accuratelythe thermal cataloged expansion to ZTE. coefficient in Zn4B6O13 is even much smaller (0.28(06)/MK), which can be accurately cataloged to ZTE.

Figure 15. Variation ofof thethe refinedrefined cellcell parametersparametersof ofZn Zn44BB66O1313 withwith respect to temperature. Reprinted fromfrom Ref.Ref. [[6767],], withwith permissionpermission ofof JohnJohn WileyWiley andand Sons.Sons.

First principles calculations were carried out for further investigation and demonstrate that the intrinsicintrinsic isotropicisotropic near-ZTEnear-ZTE behaviorbehavior ofof ZnZn44BB66O13 mainlymainly originates originates from the invariability of the solid 24 18 [B[B24OO4848]24] − cage− cage fixed fixed by bythe the [Zn [Zn4O134]O1813− clusters,] − clusters, affirming affirming the important the important impact impact of the relatively of the relatively strong strongZn-O bonds. Zn-O bonds.The discovery The discovery of Zn4B of6O13 Zn with4B6O intrinsic13 with intrinsicisotropic isotropicnear-ZTE near-ZTE behavior behaviornot only notgain onlys an gainsimportant an important member to member the family to theof ZTE family materials, of ZTE but materials, also revives but the also studies revives on the new studies functionalities on new functionalitiesin borates, which in borates, may eventually which may lead eventually to the discovery lead to the of discovery more exciting of more and exciting exotic and emerging exotic emergingphysicochemical physicochemical properties properties in borates. in borates.

3.3.2. Unidirectional Thermal Expansion in KZnB O 3.3.2. Unidirectional Thermal Expansion in KZnB3O6 5 As described before, KZnB3O6 is the first borate that contains the es-[BO4] 5−− tetrahedra under As described before, KZnB3O6 is the first borate that contains the es-[BO4] tetrahedra under ambient pressure [69,70]. Lou et al. investigated the thermal expansions of KZnB O from room ambient pressure [69,70]. Lou et al. investigated the thermal expansions of KZnB33O6 from room temperature to 1013 K (Figure 16a) [65,66]. Interestingly, KZnB O shows an unusual unidirectional temperature to 1013 K (Figure 16a) [65,66]. Interestingly, KZnB33O66 shows an unusual unidirectional thermal expansion along the approximate [302] direction, i.e., the X axis direction, over the entire thermal expansion along the approximate [3̅02] direction, i.e., the X33 axis direction, over the entire measured temperature (from 298 K to 1013 K).K). TheThe expansionsexpansions along other directions on the plane perpendicularperpendicular to [3̅02]02] areare negligiblynegligibly small,small, i.e.,i.e., thethe areaarea showsshows zerozero expansionexpansion (Figure(Figure 1616b)b).. Further investigationsinvestigations reveal reveal that that the the abnormal abnormal thermal thermal behavior behavior originates originates from the from cooperative the cooperative hinge rotations hinge 6 5 8 of [B O ] − (contain6− es-[BO ] − tetrahedra)5− and [Zn O ] − rigid8− groups, which are probably driven rotations6 12 of [B6O12] (contain4 es-[BO4] tetrahedra) and2 6 [Zn2O6] rigid groups, which are probably bydriven asymmetrical by asymmetrical elongations elongations of K-O of bonds K-O andbonds only and leads only toleads a quasi-unidirectional to a quasi-unidirectional expansion expansion upon heating.upon heating. These These findings findings will help will us help better us understand better understand the relationship the relationship between between structure structure and property and andproperty might and broaden might thebroaden applications the applicat of borates.ions of borates.

Molecules 2019, 24, 2763 16 of 27 Molecules 2019, 24, x FOR PEER REVIEW 16 of 27

Figure 16.16. Thermal expansionexpansion behavior ofof thethe KZnBKZnB33O6..( (aa)) The temperature dependence of lattice constants a, b, c and and cell cell volume volume;; ( (bb)) n normalizedormalized components components of of the the principal principal axes axes versus versus temperature temperature.. Reprinted with permission permission from from Ref.Ref. [ [66],66, Copyright (201(2015)5) Creative Commons Attribution 4.0 International License.License. 4. Single Crystal Growth of Zincoborates 4. Single Crystal Growth of Zincoborates High-quality and sizable single crystals are essential to measure fundamental properties and High-quality and sizable single crystals are essential to measure fundamental properties and to to accurately evaluate practical applications. Although a great deal of effort has been put into the accurately evaluate practical applications. Although a great deal of effort has been put into the exploration of growing sizable single crystals with high optical quality, it is still a great challenge exploration of growing sizable single crystals with high optical quality, it is still a great challenge to to obtain the large-scale crystals for practical devices [149–151]. As for the zinc-containing system, obtain the large-scale crystals for practical devices [149–151]. As for the zinc-containing system, the the growth of large crystal seems more difficult since compounds containing zinc element usually growth of large crystal seems more difficult since compounds containing zinc element usually have have a high melting point (ZnO, 1975 ◦C at 5.2 MPa). Usually, the effective fluxes, such as PbO, PbF2, a high melting point (ZnO, 1975 °C at 5.2 MPa). Usually, the effective fluxes, such as PbO, PbF2, H3BO3, etc., are introduced to decrease the melting point and the viscosity during the growth of single H3BO3, etc., are introduced to decrease the melting point and the viscosity during the growth of single crystals. Fortunately, some of them melt congruently and sizable crystals have been grown from a crystals. Fortunately, some of them melt congruently and sizable crystals have been grown from a stoichiometric melt by the top-seeded solution growth (TSSG) and Czochralski method. stoichiometric melt by the top-seeded solution growth (TSSG) and Czochralski method.

4.1. Bi2ZnOB2O6 4.1. Bi2ZnOB2O6 In 2009, Li et al. successfully grew the single crystal of Bi2ZnOB2O6 with high quality and In 2009, Li et al. successfully grew the single crystal of Bi2ZnOB2O6 with high quality and dimensions of 18 mm 13 mm 6 mm through the TSSG method (Figure 17a) [83]. The low melt dimensions of 18 mm × 13 mm × 6 mm through the TSSG method (Figure 17a) 83. The low melt point (no more than 700 ◦C), non-viscous properties, and the congruent melting performance make point (no more than 700 °C), non-viscous properties, and the congruent melting performance make Bi2ZnOB2O6 capable to grow sizable single crystals. Followed by these, a sizable single crystal with Bi2ZnOB2O6 capable to grow sizable single crystals. Followed by these, a sizable single crystal with sizes up to Φ 30 mm 55 mm has been obtained along the c-axis direction using the Czochralski sizes up to Φ 30 mm × 55 mm has been obtained along the c-axis direction using the Czochralski method (Figure 17b) [152]. The NLO coefficients have been determined through the Maker fringes method (Figure 17b) 152. The NLO coefficients have been determined through the Maker fringes method at 1064 nm [140]. Results show that the coefficients of Bi2ZnOB2O6 relative to d36 for KDP are method at 1064 nm 140. Results show that the coefficients of Bi2ZnOB2O6 relative to d36 for KDP are d31 = (2.34 0.05) d36 (KDP), d32 = (7.90 0.16) d36 (KDP) and d33 = (2.60 0.06) d36 (KDP). The large d31 = (2.34 ±± 0.05) d36 (KDP), d32 = (7.90 ± 0.16)± d36 (KDP) and d33 = (2.60 ± 0.06)± d36 (KDP). The large NLO NLO coefficients and the easy crystal growth behavior suggest that Bi2ZnOB2O6 is a promising coefficients and the easy crystal growth behavior suggest that Bi2ZnOB2O6 is a promising candidate candidate for NLO materials. for NLO materials.

Molecules 2019, 24, 2763 17 of 27 Molecules 2019, 24, x FOR PEER REVIEW 17 of 27 Molecules 2019, 24, x FOR PEER REVIEW 17 of 27 (a) (b) (a) (b)

FigureFigure 17. 17. ((aa)) Photograph Photograph of of Bi Bi22ZnOBZnOB22OO6 6crystalcrystal grown grown by by the the top top-seeded-seeded solution solution growth growth ( (TSSG)TSSG) methodmethodFigure 17.and and ( a () (b bPhotograph)) Czochralski Czochralski of method method,Bi2ZnOB, respectively.2 respectively.O6 crystal grown Reprinted Reprinted by the with withtop -seeded permission permission solution from from growth R Ref.ef. [ [83, 83(TSSG15,1522]],,) CopyrightCopyrightmethod and ( (2009)2009 (b)) American AmericanCzochralski Chemical Chemical method Society Society,, respectively., Copyright Reprinted (201 (2010)0) Elsevier,Elsevierwith permission, respectively.respectively from. Ref. [83,152], Copyright (2009) American Chemical Society, Copyright (2010) Elsevier, respectively. 4.2. Ba3(ZnB5O10)PO4 4.2. Ba3(ZnB5O10)PO4 4.2. Ba3(ZnB5O10)PO4 Ba (ZnB O )PO melts congruently, but its relatively high viscosity and melting temperature Ba33(ZnB5O5 1010)PO4 melts4 congruently, but its relatively high viscosity and melting temperature are unfavorableare unfavorableBa3(ZnB to5O obtain10 to)PO obtain 4high melts- high-qualityquality congruently, crystals crystals but from its arelatively from stoichiometric a stoichiometric high viscosity melt. Therefore, melt. and melting Therefore, large temperature single large crystals single are crystalsunfavorable of Ba to(ZnB obtainO high)PO-qualitywere crystals grown throughfrom a stoichiometric a TSSG method melt. by Therefore, using the ZnO-Blarge singleO self-flux crystals of Ba3(ZnB5O103)PO4 5were10 grown4 through a TSSG method by using the ZnO-B2O3 self-2flux3 system (Figuresystemof Ba3(ZnB 1 (Figure8) 5O15103 )PO18. The)4 [153]. were seed The grown orientations seed through orientations have a TSSG a havegreat method aimpact great by impact on using the ongrowth the the ZnO growth rate,-B2O morphology, rate,3 self morphology,-flux system and and(Figure quality 18)  of15 the3. The crystals. seed Withorientations the [010]- have and a [101]-orientedgreat impact on seed, the Bagrowth(ZnB rate,O )PO morphology,crystals withand quality of the crystals. With the [010]- and [101]-oriented seed, Ba3(ZnB3 5O10)PO5 104 crys4tals with size sizequality dimensions of the crystals. of 35 mm With20 the mm [010]5- mmand and[101] 34-oriented mm 15 seed, mm Ba83 mm(ZnB were5O10)PO obtained,4 crystals respectively, with size dimensions of 35 mm  20 mm×  5 mm× and 34 mm  15 mm×  8 mm× were obtained, respectively, both     ofbothdimensions which of which have of have high35 mm highoptical 20 optical mm quality. quality. 5 mm Compared and Compared 34 mm with with the15 mm the crystal crystal 8 mmgrown grown were with withobtained, the the [010] [010]-oriented respectively,-oriented seed, seed,both thetheof whichcrystal crystal have grown grown high with with optical the the [101] [101]-orientedquality.-oriented Compared seed seed ha haswiths a a thickerthe thicker crystal dimension dimension grown withand and exhibitsthe exhibits [010] a a- orientedmore more regular regular seed, shapethe crystal (Figure grown 18a). with Refractive the [101] index-oriented measurements seed has showa thicker that dimension Ba (ZnB O and)PO exhibitsis a negative a more regular biaxial shape (Figure 18a). Refractive index measurements show that Ba33(ZnB5O1010)PO44 is a negative biaxial crystalcrystalshape (Figurewith with birefringence birefringence 18a). Refractive ranging ranging index from from measurements 0.0418 0.0418 to to 0.0306 0.0306 show in in that a a wavelength wavelength Ba3(ZnB5O range10 range)PO4 ofis of 253.6a 253.6–2325.4 negative–2325.4 biaxial nm. nm. BasedBasedcrystal on on with the the measuredbirefringence measured refractive refractive ranging index index from and and 0.0418 fitted fitted to Sellmeier Sellmeier0.0306 in equations, equations,a wavelength the the calculated calculatedrange of 253.6 phase phase–2325.4 match matching ingnm. (PM)(PM)Based regions on the formeasuredfor SHGSHG basedbased refractive on on the the index fundamental fundamental and fitted light lightSellmeier are are 730–3386 730 equations,–3386 nm nm for the for type calculated type I SHG I SHG PMphase PM (Figure match (Figure 18ingb). The(PM) measurement regions for SHG results based indicate on the that fundamental Ba (ZnB O light)PO arecrystal 730–3386 is a promisingnm for type NLO I SHG material PM (Figure in the 18b). The measurement results indicate that3 Ba3(ZnB5 105O10)PO4 4 crystal is a promising NLO material in theUV18b). UV region. The region measurement. results indicate that Ba3(ZnB5O10)PO4 crystal is a promising NLO material in the UV region(a). (b) (a) (b)

Figure 18. (a) The as-grown Ba3(ZnB5O10)PO4 crystal with [101]-oriented seed; (b) Phase-matching Figure 18.18. ((aa)) TheTheas-grown as-grown Ba Ba(ZnB3(ZnB5OO10)PO4 crystalcrystal with [101]-oriented[101]-oriented seed; ((bb)) Phase-matchingPhase-matching calculation for Ba3(ZnB5O10)PO4 3crystal:5 PM10 angle4 curves for type-I (black) and type-II (red) SHG as a functioncalculationcalculation of for the Ba fundamental33(ZnB(ZnB55OO1010)PO)PO wavelength.4 crystal:4 crystal: PM PM Reprintedangle angle curves curves (adapted) for fortype type-I - withI (black) (black) permission and and type type-II- fromII (red) R (red)ef SHG. [15 SHG as3] ,a Copyrightasfunction a function of(2016) the of the American fundamental fundamental Chemical wavelength. wavelength. Society. Reprinted Reprinted (adapted) (adapted) with with permission permission from from R Ref.ef. [151533],], Copyright (2016)(2016) AmericanAmerican ChemicalChemical Society.Society.

4.3. -Zn3BPO7 4.3. β-Zn BPO 4.3. -Zn3BPO7 IInn 1982 1982,, Liebertz Liebertz and and Stahr Stahr repo reportedrted the the existence existence of of Zn 3BPOBPO7 thatthat occur occur in in two two phase phasess with with a a In 1982, Liebertz and Stahr reported the existence of Zn33BPO7 that occur in two phases with a phase transition at 602 °C 154. -Zn3BPO7 (high-temperature phase) has been characterized as a phase transition at 602 C[154]. β-Zn BPO (high-temperature phase) has been characterized as a phase transition at 602 ◦°C 154. -Zn33BPO7 (high-temperature phase) has been characterized as a NLO crystal owning to its significant properties. However, the growth of large crystals of -Zn3BPO7 NLO crystal owning to its significant properties. However, the growth of large crystals of β-Zn33BPO77 isNLO difficult crystal since own theing crystal to its significant will transfer properties. from - toH owever,-phase. the Unremitting growth of effortslarge crystals have been of -madeZn BPO to isis didifficultfficult sincesince thethe crystalcrystal willwill transfertransfer fromfrom β-- toto α-phase.-phase. UnremittingUnremitting eeffortsfforts havehave beenbeen mademade to obtain sizable and high-quality -Zn3BPO7 crystals. Consequently, the phase transition of -Zn3BPO7 obtain sizable and high-quality β-Zn BPO crystals. Consequently, the phase transition of β-Zn BPO obtain sizable and high-quality -Zn33BPO77 crystals. Consequently, the phase transition of -Zn33BPO77 to -Zn3BPO7 is effectively suppressed through adopting appropriate heat treatment. In 2000 to 2002 to α-Zn BPO is effectively suppressed through adopting appropriate heat treatment. In 2000 to to -Zn3BPO77 is effectively suppressed through adopting appropriate heat treatment. In 2000 to 2002 136,155,156, the transparent and crack free single crystals of -Zn3BPO7 with size dimensions of 35 2002 [136,155,156], the transparent and crack free single crystals of β3 -Zn37BPO7 with size dimensions mm136 ,15 205 ,15mm6 , the 10 transparentmm and 43 andmm crack 43 mm free single 12 mm crystal (Figures of 19a)-Zn wBPOere grown with sizeby Wu dimensions and Wang of et35     almm. using 20 the mm Czochralski 10 mm and and 43 TSSG mm method 43 mms, respectively 12 mm (Figure. 19a) were grown by Wu and Wang et al. using the Czochralski and TSSG methods, respectively.

Molecules 2019, 24, 2763 18 of 27 of 35 mm 20 mm 10 mm and 43 mm 43 mm 12 mm (Figure 19a) were grown by Wu and × × × × MoleculesWang et 2019 al., using24, x FOR the PEER Czochralski REVIEW and TSSG methods, respectively. 18 of 27 Molecules 2019, 24, x FOR PEER REVIEW 18 of 27

(a) (b) (a) (b)

FigureFigure 19 19.. (a(a) )B Bottomottom of of the the as as-grown-grown β-βZn-Zn3BPO3BPO7 crystal7 crystal with with size size up upto 43 to mm 43 mm  43 mm43 mm  12 mm12; mm; (b) Figure 19. (a) Bottom of the as-grown β-Zn3BPO7 crystal with size up to 43 mm  43× mm  12× mm; (b) t(ransmittanceb) transmittance spectrum spectrum of β of-Znβ-Zn3BPO3BPO7. Reprinted7. Reprinted with with permission permission from from Ref Ref.. [13 [136],6, Copyright Copyright (20 (2002)02) transmittance spectrum of β-Zn3BPO7. Reprinted with permission from Ref. [136, Copyright (2002) AmericanAmerican Chemical Chemical Society. Society. American Chemical Society.

TThehe linear linear and and non non-linear-linear optical propertiesproperties areare investigated.investigated. ResultsResults show show that thatβ -Zn-Zn3BPO3BPO7 7has has a The linear and non-linear optical properties are investigated. Results show that d-Zn3BPO7 has aUV UV absorption absorption edge edge atatabout about 250250 nmnm (Figure(Figure 1919b)b) andand thethe none-zeronone-zero NLO coecoefficientfficient d11 measuredmeasured a UV absorption edge at about 250 nm (Figure 19b) and the none-zero NLOd coefficient d11 measured byby the the Maker Maker fringes fringes method method is is 0.69 0.69 pm/V pm/V (1.8 (1.8 times times as as large large as as that that of of d36 (KDP)).(KDP)). The The Sellmeier Sellmeier by the Maker fringes method is 0.69 pm/V (1.8 times as large as that of d36 (KDP)). The Sellmeier equationsequations suggest suggest that that the the shortest shortest SHG SHG wavelengths wavelengths for for the the crystal crystal are are 39 3999 and and 605 605 nm nm for for types types I I equations suggest that the shortest SHG wavelengths for the crystal are 399 and 605 nm for types I andand II, II, respectively. respectively. The The eas easyy growth growth habit habit and and good good NLO NLO properties properties make make β--ZnZn3BPOBPO77 attractiveattractive for for and II, respectively. The easy growth habit and good NLO properties make -Zn3BPO7 attractive for continuedcontinued research research as as NLO NLO materials. materials. continued research as NLO materials. 4.4. Zn4B6O13 4.4. Zn4B6O13 4.4. Zn4B6O13 The large-sized Zn4B6O13 single crystal with dimensions of about 40 mm 40 mm 18 mm The large-sized Zn4B6O13 single crystal with dimensions of about 40 mm  40× mm  18 ×mm and and exhibitingThe large-sized good opticalZn4B6O quality13 single was crystal grown with using dimensions the conventional of about 40 TSSG mm method  40 mm (Figure  18 mm 20) [67].and exhibiting good optical quality was grown using the conventional TSSG method (Figure 20) 67. Opticalexhibiting transmittance good optical measurements quality was show grown that using Zn4B the6O13 conventionalpossesses a wide TSSG transmission method (Figure range covering20) 67. Optical transmittance measurements show that Zn4B6O13 possesses a wide transmission range aOptical wide spectral transmittance region from measurements the UV to the show near-infrared that Zn4B6 (wavelengthO13 possesses from a wide 217 to transmission 3100 nm). The range UV covering a wide spectral region from the UV to the near-infrared (wavelength from 217 to 3100 nm). cutocoverffingedge a wide of Zn spectral4B6O13 isregion the shortest from the among UV to thethe ZTEnear crystals,-infrared implying (wavelength the potentialfrom 217 applicationsto 3100 nm). The UV cutoff edge of Zn4B6O13 is the shortest among the ZTE crystals, implying the potential ofThe Zn UV4B6 O cutoff13 in ultra edgeprecise of Zn4 opticalB6O13 is instruments. the shortest Notably, among the short ZTE UVcrystals, cuto ff implyingedge of Zn the4B 6 potentialO13 also applications of Zn4B6O13 in ultra precise optical instruments. Notably, the short UV cutoff edge of stemsapplications from the of relativelyZn4B6O13 in strong ultra Zn-Oprecise bond optical based instruments. on the analysis Notably, of the the ab short initio UV partial cutoff density edge of Zn4B6O13 also stems from the relatively strong Zn-O bond based on the analysis of the ab initio partial states.Zn4B6O Moreover,13 also stems Zn fro4B6mO the13 exhibits relatively high strong thermal Zn-O stability, bond based thermal on conductivitythe analysis of and the high ab initio mechanical partial density of states. Moreover, Zn4B6O13 exhibits high thermal stability, thermal conductivity and high hardness,density of whichstates. areMoreover, also important Zn4B6O13 for exhibits practical high applications. thermal stability, Combined thermal with conductivity the intrinsic and isotropic high mechanical hardness, which are also important for practical applications. Combined with the near-ZTEmechanical behavior, hardness, the environmentallywhich are also importantfriendly feature for practical and easy applications.growth habit facilitateCombined the with practical the intrinsic isotropic near-ZTE behavior, the environmentally friendly feature and easy growth habit applicationsintrinsic isotropic of Zn 4nearB6O-13ZTE. behavior, the environmentally friendly feature and easy growth habit facilitate the practical applications of Zn4B6O13. facilitate the practical applications of Zn4B6O13.

Figure 20. (a) The as-grown Zn4B6O13 single crystal with a dimension of 40 mm  40 mm  18 mm; (b) Figure 20.20. ((aa)) The The as as-grown-grown Zn Zn44BB6O6O1313 singlesingle crystal crystal with with a dimension a dimension of of40 40mm mm  40 mm40 mm  18 mm18 mm;; (b) Fabricated single crystal with a dimension of 20 mm  20 mm × 10mm. Reprinted from× Ref. [67× ], with (Fbabricated) Fabricated single single crystal crystal with with a dimension a dimension of 20 of mm 20 mm 20 mm20 mm× 10mm.10mm. Reprinted Reprinted from fromRef. [67 Ref.], with [67], permission of John Wiley and Sons. × × withpermission permission of John of JohnWiley Wiley and Sons. and Sons.

4.5. BaZnBO3F 4.5. BaZnBO3F BaZnBO3F exhibits typical layer habit due to structural characteristics, which is familiar with the BaZnBO3F exhibits typical layer habit due to structural characteristics, which is familiar with the KBBF family crystals. Crystal of BaZnBO3F with the dimensions of about 20 mm  20 mm  0.5 mm KBBF family crystals. Crystal of BaZnBO3F with the dimensions of about 20 mm  20 mm  0.5 mm has been grown by high temperature solution method from BaF2-NaF flux (Figure 21) 135. The has been grown by high temperature solution method from BaF2-NaF flux (Figure 21) 135. The perfect coplanar and alignment [BO3]3- groups in the structure result in an observed large effective perfect coplanar and alignment [BO3]3- groups in the structure result in an observed large effective

Molecules 2019, 24, 2763 19 of 27

4.5. BaZnBO3F

BaZnBO3F exhibits typical layer habit due to structural characteristics, which is familiar with the KBBF family crystals. Crystal of BaZnBO F with the dimensions of about 20 mm 20 mm 0.5 mm 3 × × has been grown by high temperature solution method from BaF2-NaF flux (Figure 21) [135]. The perfect Molecules 2019, 24, x FOR PEER REVIEW3 19 of 27 coplanar and alignment [BO3] − groups in the structure result in an observed large effective NLO coefficient (2.8 deff KDP). BaZnBO3F crystal possesses chemical stability and high transmittance in NLO coefficient× (2.8  deff KDP). BaZnBO3F crystal possesses chemical stability and high the range of 300–3000 nm wavelength with the UV cut-off edge of 223 nm. In the consideration of transmittance in the range of 300–3000 nm wavelength with the UV cut-off edge of 223 nm. In the its superior optical properties in the visible to UV range, larger-size crystals should be developed consideration of its superior optical properties in the visible to UV range, larger-size crystals should for practical applications by exploiting several methods or flux to overcome the strong anisotropic be developed for practical applications by exploiting several methods or flux to overcome the strong growth habit. anisotropic growth habit.

Figure 21. (a) The as-grown BaZnBO3F crystal; (b) Interference pattern of BaZnBO3F along the c-axis. Figure 21. (a) The as-grown BaZnBO3F crystal; (b) Interference pattern of BaZnBO3F along the c-axis. Reprinted from Ref. [135], Copyright (2016) with permission from Elsevier. Reprinted from Ref. [135], Copyright (2016) with permission from Elsevier. 5. Conclusions 5. Conclusions In this review, we have examined the recent development of zincoborates, focusing on the crystalIn this structure review, we chemistry have examined as well the as theirrecent physicochemical development of properties. zincoborates, The focusing introduction on the crystalof the structurestrong-bonded chemistry zinc as cations well as into their borates physicochemical effectively enriches properties. the The structural introduction diversity of the of strongborates-bonded and further zinc cations results in into extensive borates effectively applications. enriches Several the examplesstructural werediversity given. of borates (1) A series and furtherof zincoborates results in display extensive unique applications. structural Several features examples in crystal were chemistry given. (1) ofA borates,series of forzincoborates example, display unique structural features in crystal chemistry of5 borates, for example, KZnB3O6 and KZnB3O6 and Ba4Na2Zn4(B3O6)2(B12O24) with novel es-[BO4] − tetrahedra; Ba4K2Zn5(B3O6)3(B9O19), Ba4Na2Zn4(B3O6)2(B12O24) with novel es-BO45− tetrahedra; Ba4K2Zn5(B3O6)3(B9O19), Ba2KZn3(B3O6)(O(B3O6)2) with two kinds of isolated polyborate anionic groups coexisting in one Ba2KZn3(B3O6)(O(B3O6)2) with two kinds of isolated polyborate anionic groups coexisting in one borate structure; AZn2BO3X2 (A = Na, K, Rb, NH4;X = Cl, Br) series, Cs3Zn6B9O21, BaLiZn3(BO3)3 with boratebenign structure; KBBF-type AZn layered2BO3X structures,2 (A = Na, K, etc. Rb, (2) NH Numerous4; X = Cl, zincoboratesBr) series, Cs with3Zn6B brilliant9O21, BaLiZn physicochemical3(BO3)3 with benign KBBF-type layered structures, etc. (2) Numerous zincoborates with brilliant physicochemical performance have emerged. For instance, Cs3Zn6B9O21, BaZnBO3F, Bi2ZnOB2O6, Ba5Zn4(BO3)6, performance have emerged. For instance, Cs3Zn6B9O21, BaZnBO3F, Bi2ZnOB2O6, Ba5Zn4(BO3)6, Ba3(ZnB5O10)PO4, etc. have been suggested to be suitable for UV NLO applications; Zn4B6O13 and Ba3(ZnB5O10)PO4, etc. have been suggested to be suitable for UV NLO applications; Zn4B6O13 and KZnB3O6 with anomalous thermal expansion properties have inspired the discovery of different KZnBapplications3O6 with for anomalous borates. thermal expansion properties have inspired the discovery of different applicationsBased on for the borates. aforementioned findings, the regulation effect of introducing Zn-O/F polyhedra into boratesBased can on be the emphasized. aforementioned Firstly, findings, the introduction the regulation of Zn-O effect/F polyhedra of introducing can eff ectivelyZn-O/F inhibitpolyhedra the intopolymerization borates can of be B-O emphasized. anionic structures, Firstly, the which introd is beneficialuction of to Zn obtain-O/F isolatedpolyhedra B-O can groups. effectively In particular, inhibit theit is polymerization propitious to obtain of B good-O anionic NLO or structures birefringent, which properties is beneficial when the to isolatedobtain isolated B-O groups B-O aregroups. induced In particularby Zn-O/F, it polyhedra is propitious and exhibitto obtain acoplanar good NLO arrangement. or birefringent The terminalproperties oxygen when atomsthe isolated of the BB-O-O groups are induced linked with by Zn zinc-O/F atoms, polyhedra eliminating and exhibit the dangling a coplanar bonds arrangement of the B-O. groups,The terminal which oxygen would atoms of the B-O groups are linked with zinc atoms, eliminating the dangling bonds 6of the B-O groups,5 further widen the transparence in the UV region. Moreover, the distorted [ZnO4] − and [ZnO3F] − whichtetrahedra would are further NLO-active widen structural the transparence units, which in the shouldUV region. provide Moreover, an enhanced the distorted contribution ZnO46 to− and the SHGZnO3 response,F5− tetrahedr indicatinga are that NLO the-active zinc-containing structural borate units, system which is optimalshould for provide exploring an new enhanced NLO contributionmaterials. However, to the SHG it isnot response yet clearly, indicating understood that whichthe zinc factors-containing determine borate the specialsystem e ffisect optimal of Zn-O for/F exploringpolyhedra new in zincoborates. NLO materials The. However, intrinsic mechanism it is not yet understandingclearly understood of the which special factors contribution determine of the specialcovalent effect zinc of cations Zn-O/F on polyhedra structural andin zincoborates. functional regulation The intrinsic should mechanism be theoretically understanding elucidated of andthe special contribution of the covalent zinc cations on structural and functional regulation should be theoretically elucidated and exploited in the future, which will present an useful guide for the exploration of undiscovered NLO crystals in zincoborate system that can be practically applied for UV/DUV NLO materials. In addition, although several zincoborates have been grown with sizable single crystals, there are still great hurdles to develop new zincoborates with excellent properties that are feasible for growing large single crystals. Looking into the future, continuous exploration and considerable effort should be made in growing large-size single crystals for more detailed physical measurements and practical applications.

Molecules 2019, 24, 2763 20 of 27 exploited in the future, which will present an useful guide for the exploration of undiscovered NLO crystals in zincoborate system that can be practically applied for UV/DUV NLO materials. In addition, although several zincoborates have been grown with sizable single crystals, there are still great hurdles to develop new zincoborates with excellent properties that are feasible for growing large single crystals. Looking into the future, continuous exploration and considerable effort should be made in growing large-size single crystals for more detailed physical measurements and practical applications.

Author Contributions: Every author contributed to this overview. Funding: This research was funded by the National Natural Science Foundation of China (Grant Nos. 21501194, 51872323, 91622107) and NSF-DMR- Solid State and Materials Chemistry (Grant No. 1904701). Conflicts of Interest: The authors declare no conflict of interest.

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