Investigations on Cesium Uranates and Related Compounds

INVESTIGATIONS ON CESIUM URANATES AND

RELATED COMPOUNDS

ACADEMISCH PROEFSCHRIFT

TER VERKRIJGING VAN DE GRAAD VAN DOCTOR Λ t' IN OE WISKUNDE EN NATUURWETENSCHAPPEN AAN DE UNIVERSITEIT VAN AMSTERDAM OP GEZAG VAN DE RECTOR MAGNIFICUS DR. G. OEN BOEF, HOOG- LERAAR IN DE FACULTEIT OER WISKUNDE EN NATUURWETENSCHAPPEN. IN HET OPENBAAR TE VERDEDIGEN IN DE AULA DER UNIVERSITEIT (TIJDELIJK IN DE LUTHERSE KERK, INGANG SINGEL 411, HOEK SPUI) OP WOENSDAG 16 JUNI 1976 OM 0 15.00 UUR PRECIES

DOOR

ANDRÊ BERNARD van EGMOND

GEBOREN TE HENGELO (O)

Ιί.'Λίά? Μ

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promotor : prof.dr. B.O. Loopstra co-promotor: prof.dr.ir. E.H.P. Cordfunke co-referent: prof.dr. J.A. Goedkoop

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V ~'\ '"·*'\ί· aan mijn ouders voor Ietje -4-

Aan allen, die in enige vorm hebben meegewerkt aan het totstandkomen van dit proefschrift, betuig ik mijn hartelijke dank. In het bijzonder dank ik Gernna van Voorst en Anneraieke Berentsen voor de syntheses van de uranaatpreparaten en Piet van Vlaanderen voor de hulp bij het Rontgenwerk. De vakgroep Chemische Fysica van de Technische Hogeschool Twente zeg ik dank voor het beschikbaarstellen van de PW1100 éénkristaldiffrac- tometer en de afdeling Fysica van het Reactor Centrum Nederland voor het gebruik van de PW1150 poederdiffTactometer. Veel dank ben ik verschuldigd aan mevr.drs. E.M.M. Rutten-Keulemans die enkele computerprogramma's voor mij toegankelijk maakte» Veel waardering heb ik voor het typewerk dat verzorgd werd door mevr. A. Schuyt-.-Fasen en voor de zorg van de reprografische dienst van het Reactor Centrum Nederland, besteed aan het drukken van dit proef- '. -Λ " " . schrift. De direktie van het Reactor Centrum Nederland ben ik erkentelijk voor de mogelijkheid de tekst van dit proefschrift ook als extern rapport (RCN-246) te laten verschijnen.

••;ii ν; "Α -5-

CONTENTS page CHAPTER I INTRODUCTION 9 1.1. Nuclear technology and cesium uranates 9 1.2. X-ray diffraction SO 1.2.1. Single-crystal X-ray diffraction 10 1.2.2. Powder X-ray diffraction 12 1.3. Crystallographic computing 13 References 15

CHAPTER II CHARACTERIZATION OF THE PHASES IN THE Cd-U-0 SYSTEM 17 2.1. Introduction 17 2.2. Experimental 17 2.3. Hexavalent cesium uranates 18 2.A. The Cs-U-0 system in air 21 2.4.1. Compositions with Cs/U > 0.5 and < 0.5 21 2.4.2. The equilibrium Cs^O^**Cs^O^ + ^ 22 2.5. The Cs-U-0 system at low oxygen pressures 24 References 24

CHAPTER III THE CRYSTAL STRUCTURES OF Cs2U4O|2 26 '-•.•'•f- 3.1. Introduction 26 im-.'-v. 3.2. Experimental 26 3.3. The crystal structure of ct-Cs U 0 26 ""- ' ί- -" '< 3.3.1. Crystal data and intensity measurement 26

3.3.2. Structure determination of β-0ε2ϋ,0 28 3.4. The crystal structures of £J- and y-Cs-U.O.- 29 3.4. ]. Crystal data 29 3.4.2., The crystal structures of β- and

Y-CS2U4OJ2 30 3.5. Discussion 31 References 33

CHAPTER IV THE CRYSTAL STRUCTURES OF Cs^O^ AND Cs^O^ 35 4.1. Introduction 35 4.2. Experimental 35 4.3. Crystal data and crystal structure of Cs.U.O.. 36 • }::.;• ν ζ^Μ 4.4. Crystal data and crystal structure of Cs.U,.O,, 38 i J ID 39 4.5. Discussion 42 Refe.. Alic Or*. Pi·:

HMM

-6-

^M CHAPTER V THE CRYSTAL STRUCTURES OF Cs2U15°46 Cs2UO4 5.1. Introduction 5.2. Experimental

ï,3. The crystal structure of Cs.U 0 ?

5.4. The crystal structure of Cs2U?022 5.5. The crystal structure of Cs U.-O,, 5.6. The crystal structure of CSjUO, 5.7. Discussion References

CHAPTER VI THE CRYSTAL STRUCTURES OF Cs^Oj 6.1. Introduction 6.2. Experimental 6.3. The X-ray analysis of a-CsXO.

6.4. The X-ray analysis of β-CsJ5J37

6.5. The X-ray analysis of γ-Cs U20_ 6.6. The neutron analysis of a-Cs.U„O_ 6.7. Discussion References

CHAPTER VII POTASSIUM AND RUBIDIUM URANATES 65 7.1. Introduction 65 7.2. Experimental 65 7.3. Hexavalent uranates 65 7.3.1. The potassium uranate system 65 7.3.2. The rubidium uranate system 67 7.3.3. The crystal structures of M-U.O-, and

M2U2O7 (M - K,Rb) 67 7.4. Pentavalent uranates 69 7.5. Discussion 69 References 71

CHAPTER VIII CRYSTAL CHEMISTRY OF THE ALKALI URANATES 73 8.1. Introduction 73 8.2. Structural characteristics of uranates(VI) 73 if 8.3. Uranium(-oxygen) motifs in alkali uranates(VI) 78 8.3.1. The mono- and diuranate region 80 8-3.2. The high M/U-ratio region 81 ' " '" ül \irn

it'!

-7- : \A t • , ''.-^

page 3&J: 8.3.3. The 3D-structure region 8!

8.3.4. Cs2UI5O46 and the M^O^ (M - K,Rb,Cs) 82 8.3.5. The tetraurar.ate region 83 8.4. Uranium-uranium distances and uranium-oxygen bonding 84 8.5. The composition of the uranium layers 86

8.6. Uranium layer in Cs2U,O._ and Cs-U5O.6 90

8.7. Influence of the metal radius on the interplanar ,.•·>..<· , , distances in layer structures of uranates(VI) 91 8.8. Uranates(V) with the perovskite structure 94 References 95

APPENDICES 98

SAMENVATTING 122

CURRICULUM VITAE 124 '7 '

-9-

CHAPTER I. INTRODUCTION

1.1. Nuclear technology and cesium uranates

Uranium has been successfully applied in nuclear technology as a source Η. •'-'' of energy since the Second World W;ir. In most of the nuclear reactors 235 the isotope U - which occurs for only 0.7% in natural uranium - is used. Therefore nuclear technologists have looked for a method to harness the greater part of the available uranium, the U isotope, for the production of energy. This can be achieved in a fast-breeder reactor, in 238, which the U isotope is converted into the fissile plutonium isotope 239 Pu. The construction of a prototype reactor is a joint project of the governments of Western-Germany, Luxemburg, Belgium and The Netherlands. The work described in this thesis has been partly performed within the scope of this fast-breeder reactor project. During fission of the nuclear fuel many elements are formed such as iodine, molybdenum, rubidium, cesium [I], Some of these elements will diffuse from the hot centre of the (U,Pu)-oxide pellets (2500°-2900°C) to the cooler parts of the fuel pins (50O°-700°C) [2,3,4]. In the resulting medley of elements three types of chemical reactions can occur:

1. reactions between fission products mutually, mainly producing ir.olyb- dates [5,63; 2. corrosion of the stainless steel pin cladding by fission products, forming chromates [7,8]; 3. reactions between fission products and the nuclear fuel (U,Pu)- oxide, resulting in the formation of uranates [8,9] and piutonates. Since cesium is found in substantial quantities among the fission products [1] cesium uranates will also be formed during irradiation I 10}. The formation of cesium uranates appears to be strongly dependent on the (partial) oxygen potential in the fuel rod [2,3,10], and may rlfly a sifc~ nificant role in the swelling of the fuel and even cause fuel pin failure 19], Therefore a study of the Cs-U-0 system is clearly called for. In addition, uranates are of great interest to structural chemistry, As early as 1935 Frankuchen demonstrated I In·· existence of the linear con- 2+ figuration (O-U-0) in the crystal structm >I solium uranyl acetate [J 11. Since that time many uranium compound.·- e 'ujen shown to contain this uranyl group. In I960 Kovba investigatec o regularities in uranate structures [121, while Keller consider*d some uranates in an ex- -10-

tensive structural study on actinide compounds some years later [13]. However, both authors had no detailed structural information on cesium uranates available. In addition many investigations concerning the other alkali uranates, frequently contradicting earlier statements and even mutually conflicting, have been published since the last decade. In this thesis the crystal structures of the cesium uranates are described, whereas the rubidium and potassium uranate systems are reinvestigated. Finally the structural information on the alkali uranates is classed in a way similar as Kovba and Keller did for earth alkaline uranates mainly.

1.2. X-ray diffraction

One of the commonly adopted techniques to obtain structural information from solid compounds is X-ray diffraction. Since numerous authors have described both theory and practice of X-ray diffraction, only some sum-

ι -C - , --' marizing remarks will be made here, chiefly for a clear apprehension of the description of the computer programs used in this study.

ii^^^Single-crjrs tal_X-ray_dif fraction

When a rotating single crystal is exposed to an X-ray beam a pattern of diffracted beams is obtained. The direction of the weak diffracted beams is fixed by the orientation of the crystal, the size of the unit cell from which the crystal can be thought to be built, and the wavelength of the applied X-rays. The angle 2Θ between the diffracted beam and the beam passing through the crystal is given by

2 d sin θ = λ (1.2.1)

where λ is the wavelength of the X-rays, and d is the interpianar spacing "?/• "--• • of the reflection planes. The value d can be calculated from the unit f cell constants by

2A' + k2B' + 12C' + 2klD' 21hE' 2hkF' (1.2.2)

/..·;.ƒ,·';•• .-ir" •; - in which A', B', C', D', E' and F'.are related to the unit cell parameters a, b, c, a, 0 and γ, and h, k, 1, the so-called Laue-indices, can

•mm -•< • m -11- Η Λ forra any combination of three integers. The intensity of the diffracted beam I is fixed by the atomic arrangement in the unit cell. The atomic arrangement, i.e. the electron density ρ at a point r = (x,y,z) in the unit cell with volume V can be described by a Fourier summation

p(r) ^Γ Σ F- exp(-2iTih.r). (1.2.3)

The summation has to be carried out for all \, issible reflections with indices h = (h,k,l), whereas F^ is the so-called structure factor associated with a reflection h. The absolute value of the complex structure factor, F , • |Fe-|, is measurable by X-ray diffraction, for the intensity of a diffracted beam, I , , can be written as obs

I , ~ L rρ A F , . (1.2.4) obs obs

In expression 1.2.4 L, ρ and A denote the Lorentz, polarization and absorption correction factors, which in general depend on the diffraction angle 2Θ and the applied X-ray diffraction technique. .The phase of the complex number F^ cannot be determined directly from the X-ray intensities. The determination of the phase angles constitutes the main problem in X-ray crystallography. From a given electron-density distribution the structure factor can j: • : - , also be calculated by

= P(r) exp(2irih.r). (1.2.5)

As the electron density in the unit cell can be described by a set of atomic coordinates, the volume integral 1.2.5 can be replaced by a summation series over discrete atomic positions, yielding a calculated structure factor F^. The calculated phase of this structure factor can be assigned to the F , -value for use in equation 1.2.3. An absolute value F . » | is also associated with the calculated structure factor from equation 1.2.5. In general a set of atomic coordinates is considered to give the correct description of a crystal structure when the calculated structure factors {F . } and scaled observed structure factors {F , } agree one by one. However, discrepancies between the two sets of structure factors -12-

always will exist. Therefore a process is needed which determines atomic positions for which the F . -set fits the F . -set best. For this pur- f. pose use is often made of a so-called least-squares minimization: the atomic position parameters are varied until the expression Λ i>:

Ζ W|F 1 (1.2.6) obs - Fcalc h reaches its minimum value. The symbol w stands for an appropriate weighting factor in the least-squares process. Since the structure factor expression 1.2.5 is not a linear function of the atomic coordinates a starting set of the coordinates is needed, which is then refined by the above mentioned least-squares procedure. Finally from the obtained atomic positions conclusions can be drawn about interatomic distances and angles, the coordination of atoms, etc.

dif fraction

In the foregoing it was assumed that .single crystals of the compound to m be studied with dimensions suitable for single crystal X-ray diffraction (0 = O.I mm) can be obtained. Unfortunately many compounds are only available in powdered form. Then powder diffraction data may be used for a structural study, although some complications are met in comparison with single-crystal diffraction. The random orientation of the particles in a powder sample causes th3 diffracted radiation to be distributed over a cone from which the diffraction angles 2Θ can be obtained, either photographically (Guinier, Debeye-Sherrer) or by step-scan counter methods. With the aid of the formula 1.2.1 the 20 angles can be turned into d-values. The characteristic d-values of a sample are converted to the so-called Q-values according to

10 (1.2.7) :••·••*•

From 1.2.2 it follows that

2 2 2 h A + k Β 1 C + 2klD + 21hE + 2hkF (1.2.8)

in which the symbols Α,Β ... F are the tenthousandfold of the cor-

9*' -13-

responding primed quantities in 1.2.2. The obtained Q-values form a set of numbers which are well-suited for the indexing of the pattern, i.e. to trace the quantities A ... F that constitute the basis of the Q-set. •!./·? Once this regularity is known the unit cell can be calculated. Another complication in powder X-ray diffraction concerns the overlap of the diffracted beams. Not only symmetry-equivalent reflections diffract in the same cone but also reflections with little difference in 2Θ overlap in the X-ray pattern. The first fact causes the introduction of a multi- plicity factor η in the intensity formula 1.2.4, whereas the second fact raises difficulties in assigning the proper intensity to one particular reflection. Often it is only possible to assign an intensity I , to a group of j reflections. Therefore expression 1.2.4 has to be changed to

I , = Σ η. L. p. A. F , . (1.2.9) obs . j j Hj j obs.j

which yields, after reduction with an average correction factor, the 2 quantities 2. n. F , ..In addition the least-squares criterion 1.2.6 J J obs.j has to be changed to

Ζ w \l n. F2, . - I n. F2 , .I2 (1.2.10) 'j J obs.j j j calcj1

where the first summation is taken over the reflection groups.

1.3. Crystallographic computing

During the work described in this thesis the following programs have been used on the CDC-660Ö computer of Reactor Centrum Nederland.

F15 calculates corrections for non-proper alignment of equipment, film- shrinkage and zeropoint from standard lines (commonly α-quartz) in I-·?. .*'• powder patterns and applies these corrections to sample lines to yield Q-values. This program has been written by Rietveld [14].

VISSER tries to find the unit cell from the corrected Q-values of a com- ·:/?* pound. The method was given by Runge Γ15], rediscovered by Ito [16], refined by De Wolff [17] and programmed by Visser [18]. In short the program tries to recognize regularities in the Q , -set and to obs combine two-dimensional zones of reflections to three-dimensional lattices. -14-

TAUPIN, a similar program for indexing of powder patterns, has been coded by Taupin [193. In a trial-and-error method a system of six equations of type 1.2.8 is solved for the lower Q-values and cer- tain low h, k and 1-values. Other Q's are tried to fit the computed cell. The program was written in IBM-Fortran, which gives some problems upon conversion to other systems. The computing time is high compared to the Visser program.

TI23 refines the unit cell by a least-squares procedure on 40 Q-values at most, based on the linear expression 1.2.8. The program has been encoded by Rietveld [14].

PWII00. Intensities of a single crystal were measured on the PW1100 computer-controlled single-crystal X-ray diffractometer of the Chemical Physical Laboratory of Twente University of Technology. A program, also called PW1I00, was used to correct the intensities according to expression 1.2.4 [203.

SFLS, a structure-factor least-squares program, performs all refinements of structural parameters. The program has been coded by Mrs. Rutten- Keulemans [21]. The program is suited for single-crystal computations (expressions !.2.5 and 1.2.6) as well as powder computations, (expression 1.2.10). In the latter case SFLS has been adapted to the input requirements of the FOUR-program: the quantities 2 Σ. n. F , . (expression 1.2.9) are split into F , -values propor- tional to the calculated structure factors F , . calc FOUR performs Fourier summations in X-ray crystallography, of the type as given in (1.2.3). The program was written by De Graaff and Mrs. Rutten-Keulemans [21]. T418 performs similar calculations as SFLS but for neutron-diffraction data. It has been coded by Rietveld [22], The least-squares refinement is applied to the separate step-scan intensity values, per- forming a so-called profile least-squares analysis. This procedure is only possible when the intensity peak shape is accurately known: neutron diffraction peaks have a Gaussian shape. X-RAY SYSTEM. This system of programs contains a compatible set of many crystallographic computing features [23], but is not suited for computations on overlapping powder intensities. The system has been used for the single-crystal studies and for computation of interatomic distances and angles. Stv;

, ^* —/ f,*fl -15-

• j,

STEREO calculates stereo-plots of crystal structures. The program is an 1 Λ ·! enlarged local version of a program written by Van de Waal [24],

References

•. •' \'

••;.·• «·•·.- [1] F.L. Lisman, R.M. Abernathey, W.J. Maeck and J.E. Rein, Nucl. Sci. Eng., 42, 191 (1970).

[2] E.A, Aitken, S.K, Evans and B.F. Rubin, in "Behaviour and Chemical State of Irradiated Ceramic Fuels", proceedings of a panel (1972), I.A.E.A. Vienna, p.269 (1974).

L3] M.G. Adamson and E.A. Aitken, Trans. Amer. Nucl. Soc, l]\ 195 (1973).

[4] L.A. Neimark, J.D.B. Lambert, W.F. Murphy and C.W. Frenco, Nucl. Technol., J£, 75 (1972).

[5] C.E. Johnson, N.R. Stalica, C.A. Seils and K.E. Anderson, U.S. Atom. Energy Conm. Rep. No. ANL-7675, p.102 (1969).

[61 C.E. Johnson, I. Johnson, C.A. Seils, K.E. Anderson, G. Staahl and C. Wach, U.S Atom. Energy Conm. Rep. No. ANL-7877, p.29 (1972).

[7] J.W. Weber and E.D. Jensen, Trans. Amer. Nucl. Soc, JU_, 175 (1975).

[8] P.S. Maiya, U.S. Atom. Energy Coma. Rep. No. ANL-7833, p.5,11 (1971).

[9] L.A. Neimark and J.D.B. Lambert, U.S. Atom. Energy Conm. Rep. No. ANL-RDP-11, p.6,18 (1972).

[10] I. Johnson and C.E. Johnson, Trans. Amer. Nucl. Sci., JJ7, 194 (1973). -'. ν Κ'. [II] I. Frankuchen, Z. Kristallogr. 9^, 473 (1935).

[12] L.M. Kovba, Izvest. Vysshikh Ucheb. ZavedeniT, Khira. i Khim. Tekhnol. _3» 219 (I960); see Chem. Abstr. 54, No. 21900 (I960). ,

[13] C. Keller, Habilitationsschrift, Technische Hochschule Fredericiana, Karlsruhe (1964); also KFK-225, Karlsruhe (1964).

[14] H.M. Rietveld, crystallographic computer programs RCN, private communication (1972).

[15] C. Runge, Z. Physik, Jji, 509 (1917).

[16] T. Ito, Nature, J6j4, 755 (1949). [17] P.M. de Wolff, Acta Cryst., 10, 590 (1957). ϊ-¥--- JL

«*·>'.

-16- ...*

[18] J.W. Visser, J. Appl. Cryst., 2, 89 (1969).

Π9] D. Taupin, J. Appl. Cryst., b, 380 (1973).

[20] A.B. van Egmond, Direkte methoden in de kristallografie, Twente University of Technology (1973).

[21] E.W.M. Rutten-Keulemans and R.A.G. de Graaff, Handleidingen bij SFLS en FOUR, University of Leiden.

[22] H.M. Rietveld, J. Appl. Cryst., 2, 65 (1969).

[23] The X-ray system - version June 1972. Technical Report TR 192 of the computer science centre, University of Maryland, Dutch version- update September 1973.

[24] B.W. van de Waal, progress report Chemical Physical Department No. ^2, Twente University of Technology, p.H98-106.

Μ".

&'• -17-

CHAPTER II. CHARACTERIZATION OF THE PHASES IN THE Cs-U-0 SYSTEM **

2.1. Introduction

Extensive studies have been made on the alkali-metal uranates, especially by Ippolitova et al. [2], indicating the existence of a number of polyuranates, depending on the temperature and the M/U-ratio, where Μ is one of the alkali metals lithium, sodium, potassium, rubidium and cesium. But although since that time (c.1960) improved methods of investigations have led to some refinements [3,4,5] and also other investigators have studied alkali uranate systems [6,7,8], the literature data concerning their composition and crystal structures is still incomplete or even conflicting. Interest in these compounds as possible reaction products in nuclear fuel elements during fission has grown in the last decade and has given the impetus to reinvestigate these alkali uranate systems systematically. In particular cesium plays a role in the in-pile performance of oxide fuel pins. It is therefore important to have a detailed knowledge of the possibility of cesium-uranate formation in dependence of the partial oxygen pressure of the fuel. In this chapter a rather detailed investigation of the Cs-U-0 system is presented.

2.2. Experimental

Crystalline samples of the various cesium-uranate phases were prepared by heating carefully ground mixtures of amorphous UO, and cesium carbonate in a gold boat in air at 600 C, until reaction was complete.

The U03 had been obtained by decomposition of hydrated UO,, whereas in some experiments also UO., which was formed upon heating uranyl nitrate, was used. In a few cases cesium nitrate was used as a reagent instead of cesium carbonate. The atomic ratios Cs/U were accurately fixed in the starting materials and ranged from 4.0 down to 0.05. In table 2.1 the Cs/U-ratios of all prepared mixtures are collected. Since reaction proceeded slowly, especially at low Cs/U-ratios, the reaction mixtures were reground

*) This chapter is a revised and extended version of the paper by E.H.P. Cordfunke, A.B. van Egmond and G. van Voorst, J. inorg. nucl. Chem., 37, 1433 (1975).

'„• £ Λ, ' . »"ΐλ ir'·· ïrJK&^£.·*.' -18-

Table ?..!.

Cs/U-ratios of the. samples prepared during thin investigation.

0.05 0.22 0.30 0.435 0.75 1.0 0.075 0.235 0.33 0.45 0.775 1.5 0.10 0.25 0.36 0.475 0.78 2.0 0.125 0.265 0.375 0.50 0.80 2.5 0.15 0.27 0.40 0.60 0.83 3.0 0.20 0.286 0.42 0.667 0.857 4.0

several times between the heating periods. Progress of the reaction was checked by taking X-ray diffraction exposures on a Nonius Guinier focus- ing camera, using monochromated CuKa radiation. In general equilibrium was obtained after a heating period of one week. Thermal stabilities were investigated by static experiments - ignition of the sample at a fixed temperature during various periods of time - and by differential thermal, analysis (DTA) using a BDL-apparatus with a heating rate 9 /roin* Exposures taken on a Nonius high-temperature X-ray Guinier-Ill camera and thermogravimetric analysis (TGA), using a Stanton thermo-balance, completed the DTA-results. The composition of the phases formed was analyzed for U(VI), U(V) and cesium content. The uranium content was analyzed potentioinetrically after dissolution of the sample in concentrated phosphoric acid, according to the procedure described by Eberle et al. [9]. The cesium content was determined by titration of a buffered solution (pH = 5) with sodium tetraphenyl borate. Densities of powder specimens were measured pycnometrically in diethyl phtalate, n-dodecane or dioctyl phtalate at 20 C. The density measurements were carried out at least in threefold.

2.3. Hexavalent cesium uranates

From the Guinier X-ray patterns distinct cesium uranates have been found to exist in air upto 600°C at atomic ratios Cs/U - 0.133, 0.286, 0.40, 0.50, 0.80, 1.0 and 2.0. Since uranium is entirely in the hexavalent state, as followed from chemical analysis, the following formulae must be assigned to these atomic ratios: CS2U|5°A6' ε82υ7°22' Cs2US°16'

Cs2U4OJ3, Cs4U5O]7, Cs2U2O, and Cs^.

**t'^ï**t: -19-

As described in section 2.2 the formation of the cesium-uranate phases is very slow, especially at low Cs/U-ratios. Notwithstanding long heating periods (several weeks) and repeated grinding of the reac-

:t - tion mixtures, it appeared to be impossible to obtain the brown-orange coloured Cs„U_O„„ phase in a pure form: it was always contaminated with both neighbouring phases. Previously Efremova et al. [10] have found the CsΛ5-0 phase but it was assigned the formula Cs-lLO.g. Since a difference in chemical composition of the reaction mixtures with Cs/U = 0.286 and Cs/U * 0.33 can readily be detected on X-ray exposures,

the Cs„U,O1£. formula cannot be correct. More important is that the K U of wn cn a Cs υ?0 . phase appeared to be isostructural with 2 7°22 * detailed structural study has been published by Kovba [II]. This also supports the assignment of the formula Cs.l^O.o· The brown-grey cesium-uranate phase with the lowest Cs/U-ratio has not yet been described in the literature. It should be noted that its

X-ray pattern always contained lines of V-0o and Cs„ü_O__ even after long heating periods. At first only a rough composition ratio between 0.10 and 0.15 could be assigned, and merely from observations on the crystallinety of the sample mixtures the formula Cs„U.,0,„ (Cs/U=0.125) was decided [1], However, as will be described in chapter V, the formula Cs„U..O,, (Cs/U=0.133) is probably the correct one for this phase. The compounds CsJ 0 , and Cs„U,O._, both yellow-brown coloured, behave very similarly. After formation of these phases at 600°C their X-ray patterns are rather diffuse, which indicates a poor crystallinity. On heating these phases crystallization occurs above 700 C. The pattern of Cs-U_O,, is contaminated with lines of Cs„U,O„„ and Cs-U.O._. The density of Cs„U,0 _ was measured to be 6.8(2). Cs.U 0 is a bright-yellow phase, which has not.been described previously. Instead a phase with formula Cs Ü 0.» has been suggested by Efremova et al. [10]. However, the latter phase could not be detected in this investigation. In experiments to synthesize single crystals of cesium uranates single crystals of CsASJi - were often obtained, as will be described in chapter V. The measured density of Cs,U_0._ is 6.6(2).

Cs„U.O7 has a pink-yellow colour. When heated above 300°C it transforms slowly into a slightly different phase that can easily be frozen in at room temperature upon rapid cooling. The compound, stable below the

transition temperature, is called a-Cs2U„0_, while the salmon phase,

stable at higher temperatures is referred to as 6-Cs?U_O7. Γ...... ,

ν • ft.· '- '^ y'·"'^' '• ΙΛ — ipl#?;

-20- .', 11 f. . τ

1200 υ,ο. 1100 C*,U40„

1000 υ,ο, C«,K,O„ 900 a" 3/ S u,o. S ΙΛ,Ο,Ο, a

700

1 600 ί

500

400

,300

c»«uio17

200 uCt,U2O7 υ +

Cs,UO4 100

0.5 1.0 15 2.0 C»/U. atomic ratio

Figure 2.1.

The tentative phase diagram of the pseudo-binary Ce-U-0 system in air. The dotted linea represent metastable equilibria in open atmosphere with p(CsJD) = 0. The diuranate •γ-CsJJJ)- is metastable and has therefore not been indicated in the diagram. -21- f-.'V:

A third form of Cs„U.O7 could also be synthesized at 600 C, when the synthesis is carried out with U0. prepared by decomposition of uranyl nitrate in vacuum. This phase, γ-Cs U„0_, however, was always contaminated with the β-diuranate. In general the reaction starting with U0„ from uranyl nitrate is faster than the reaction starting with Ü0. produced by decomposition of hydrated UO^. At 800°C y-Cs^u^ rapidly transforms into g-Cs-lLO., indicating that y-CSpU,^ *-s tne metastable compound. In succeeding experiments the reaction time, the temperature and the concentration of nitrate impurities were varied, but the γ-diuranate could not be synthesized in a pure form. Although

y-Cs2U2O7, previously described by Kovba et al. [12], and B-Cs2U2O7> recently studied by O'Hare and Hoekstra [13], were reported to be hygroscopic, a reaction with water vapour could not be established at room temperature in case of any of the three uranate phases. The density of B-Cs.U-O. has been measured yielding a value 6.5 (I). The orange-coloured Cs_UO, is very hygroscopic in agreement with the observations of Efremova et al. [10] and Kovba et al. [12]. There- fore it has been handled in a dry box. A uranate with a Cs/U-ratio 4.0 has been reported by Efremova et al. [14]. In this investigation no indication for the existence of the "mesouranate" Cs.UO,. has been found, in agreement with the results of Kepert [15] and Hoekstra and Siegel [16].

2.4. The Cs-U-0 sysizem in air

In figure 2.1 the phase relations in the Cs-U-0 system are represented. In this figure some lines have been dotted because they represent metastable equilibria at p(Cs.O) * 0. In the next sections a description of this phase diagram is given.

Cesium uranates with Cs/U-ratio > 0.5 decompose into Cs„0 and the neighbouring uranate with lower Cs/U-ratio at a rate dependent on the temperature and the Cs/U-ratio. When heated in air Cs-UO, already decomposes

n ••-•te ifc at 650 C into ft-C&JUJ^j» which *- turn decomposes into Cs,U.O-7 at 900°C. At 1000°C Cs.U-O.y decomposes very slowly into CSjU-O.».

sir

' V 'Λ1 a· \ -22-

i r " When cesium uranates with Cs/U-ratio <0.5 are heated in air a new compound is formed above 730 C. In this compound which has the Cs/U- ίΤΛ. ratio = 0.22, as follows from the X-ray analysis, uranium is not entirely in the hexavalent state. The U(IV)/U(VI)-ratio is 0.126 as determined by chemical analysis. Together with the cesium content this leads to the formula Cs U 0 . When Cs.U 0 is heated above 875°C a new compound with Cs/U • 0.33 can be formed. Together with the U(IV)/U(VI)-ratio (0.20) the formula Cs.U O . can be deduced for this uranate. It is not I 6 18 stable in open atmosphere: upon heating it decomposes into U-Og under evaporation of Cs.O. Cs-U^O., starts to crystallize when heated above 700 C. The phase, i which is formed then, appears to have a homogeneity range. The phase X width, as determined from the X-ray exposures of compounds with different Cs/U-ratios, which were heated at various temperatures between 700 and 1000°C, ranges from 0.400-0.435 (750°C), 0.375-0.450 (800°C) to 0.375-0.475 at 95O°C. Above !000°C the "Cs^O^-phase" reaches the limiting composition Cs/U = 0.5, i.e. Cs„U,O.~.

2.4.2. The equilibrium Cs^O^ ±5. CsΛΟ^ + j02

When Cs?U,0 _ is heated a reversible dissociation according to

CS2U4°,3 CS2Ü4°12

has been found. The oxygen pressure of this equilibrium is strongly dependent on the temperature, as observed by Cordfunke [17]. In air Cs DO . is stable up to 1040°C, CsJ 0 then formed up to at least 1250 C. This phase in turn then decomposes into UO and gaseous mix- -13 tures of cesium and oxygen [18], At low oxygen pressures (p(0„)< 10 atm) dissociation of CSjU 0.„ already occurs at 600°C. The formation of Cs-U.O., as a result of the calcination of "cesium mesouranate Cs.UO." (see section 2.3) has been observed by Efremova et al. [143. They also found a reversible transition into Cs_U,O._ upon cooling Cs.U.O down to 500°C in air. However the X-ray pattern, reported by these authors, does not agree very well with the data resulting from this investigation. i .·.- ι.

-23-

1200

1100

1000 -

I 900 C^lbOe a C,U4O,2 I 800 -

700

600 i 500 °» sc^o, 4-

o" ο ê C^,U04 400 a" ί Μ υ δ C^U4Ol3 • * + i 300 ο* 'i 3 200 υ υΙ υ ê 4

CS2UO4 too

0.5 1.0 1.5 2.0 C*/U . atomic ratio

Figure 2.2.

The tentative phase diagram of the pseudo-hinary Cs-U-0 system at low oxygen pressure (p(0 ) < / ' aim).

The phase transitions of CsJ}.0no at β'. ' and 695 C have not been indicated in the diagram (take:· fvom L171).

i t -•, "5. if' - fo- Ί , t

-24-

t! •)>. '-.;••$ 2.5. The Cs-U-0 system at low oxygen pressure

As stated above Cs.U O dissociates into Cs.U.O „ and 0„ at 600°C when the oxygen pressure is sufficiently low. Moreover, all cesium uranates(VI)

decompose slowly into UO-, Cs2U,O.2 and Cs?O(g) above 600 C. Therefore the upper part of the phase diagram of the Cs-U-O system at low oxygen pressure is rather simple, as can be seen in figure 2.2.

The compound Cs2U,O)2 exhibits two phase changes before decomposition into U0„ and Cs_O(g) [18]. The different phases will be called a-, β- and y-CsJü.O y. The α -*• 3 transition occurs at 625°C and the β •*• γ \ transition at 695°C as has been found from high-temperature X-ray work (see chapter III) and DTA experiments [17]. The density of the α-phase has been measured to be 7.2 (1).

As Cs2U,O._ is the only stable cesium uranate at low oxygen pressures and elevated temperatures, there is clear evidence that it is the cesium uranate which is formed in reactor fuel pins during fission (see section 1.1).

References

[1] E.H.P. Cordfunke, A.B. van Egmond and G. van Voorst, J, inorg. nucl. Chem., y}_, 1433 (1975).

[2] Investigations in the field of Uranium Chemistry, a symposium of papers, edited by V.I. Spitsyn, Pub. House Mosc. Univ., (1961), translated and released by Argonne National Laboratory, Illinois, Rep.no. ANL.-Trans.-33, (1964).

[3] L.M. Kovba and V.I. Trunova, Radiokhimiya, _Π· 773 (1971).

[4] L.M. Kovba, Radiokhimiya, JJ2, 522 ('970).

[5] L.M. Kovba, Russ. J. Inorg. Chem., J^, 1639 (1971).

[6] J. Hauck, J. inorg. nucl. Chem., 3£, 2291 (1974).

[73 J.S. Anderson, Chimia, 23, 438 (1969).

[8] E.H.P. Cordfunke and B.O. Loopstra, J. inorg. nucl. Chem., 33_, 2427 (1971).

[9] A.R. Eberle, M.W. Lerner, C.G. Goldbeck and C.J. Rodden, NBL-report, no. 252 (1970). •<· 'φ

i .

f; -25- -a Η • C

[10] K.M. Efremova, E.Α. Ippolitova, Yu.P. Simanov and V.I. Spitsyn, Dokl. Akad. Nauk. SSSR., ^2A„ 1057 (1959).

[11] L.M. Kovba, Zh. Strukt. Khim., _Π, 256 (1972).

[12] L.M. Kovba, I.A. Murav'eva and A.S. Orlova, Radiokhimiya J_6, 648 ι,;, Λ.-,- (1974).

[13] P.A.G. O'Hare and H.R. Hoekstra, J. Chem. Thermodynamics, T_, 831 (1975).

[14] K.M. Efremova, E.A. Ippolitova and Yu.P. Simanov, Vestn. Mosk.

Univ., Khiro. Ser.t 24, 57 (1969). [15] D.G. Kepert, Thesis, University of Melbourne (1960).

[16] H.R. Hoekstra and S. Siegel, J. inorg. nucl. Chem., 2£, 693 (1964).

[17] E.H.P. Cordfunke, in "Thermodynamics of Nuclear Materials", proceedings of a symposium (1974), I.A.E.Α., Vienna, Vol. II, p.185 (1975). ΑίΤ-Ι [18] E.H.P. Cordfunke and G. Prins, Fast Reactor Programme Fourth

Quarter 1974 Progress Report, RCN-2°5, p.39 (1975). Ï'.·:•••-•-."••·

ff'T

/•Μ:: [i

-26-

CHAPTER III. THE CRYSTAL STRUCTURES OF Cs2U4O]2 '

3,1. Introduction

In chapter II of this thesis it was concluded that Cs U^O 2 is formed in reactor fuel pins during irradiation. Therefore this study of the structures of cesium uranates starts with the description of the structure of

the technically most-important compound Cs2U,0 „. In the foregoing chapter it also appeared that Cs-U.O „ has three different phases, which were called α-, β~ and γ-Cs U 0 . The transitions, α + 6 at 625°C and 0 ·+ γ at 695°C, are reversible. The γ-phase slowly de-

composes into U0_+ at a temperature depending on the oxygen pressure. In figure 3.1 a high-temperature X-ray film of a Cs ϋ,Ο., sample, heated in nitrogen, has been reproduced, showing the phase transitions and the final decomposition of Cs_U,O._.

3.2. Experimental

Single crystals of Cs„U,Oj0 were grown at about 1150 C from molten cesium sulphate on amorphous UO- in a platinum cup. Black pyramidal crystals were formed after 45 hours. The crystals, which were partly clustered, were washed from bulk cesium sulphate with ethanol and methanol. Attempts to grind the jagged crystals in a crystal spherizer yielded ellipsoidal

ν" '5·'^Ιΐ crystals. Powder samples of Cs.U.O „ were prepared by calcination of Cs-U,0 . samples in inert atmosphere, as described in chapter II,

3.3. The crystal structure of a-Cs.U.O „

3. 3Λ1 i_Cr2£ ta l_da ta_and_in t ensi t j;_mea sur emen t

Single crystals of a-Cs-U.O,», mounted on a Philips PW1100 computer- controlled single-crystal X-ray diffractometer, showed a pseudo-cubic unit cell. Due to the rapid cooling down to room temperature to avoid oxidation of the crystals to Cs_U,0._ the crystals were of a poor quality.

*) This chapter is a revised and slightly extended version of the paper by A.B. van Egmond, J. inorg. nucl. Chem., 3Τ_, 1929 (1975).

* :~jjy

c ''<· · .: * t r-'-. '' ί V* ,'-'ν' • -,- 'x\,.y.:,./.,·,_ ,••'-•'"

"SS5ίf^.^

'2Θ 5 25 ? 5ΟΟ- α:

700-

900- °c L Ü

Figure 3.1.

High-temperature X-ray film of CsJJ 0. , showing the X-ray patterns of the

three CsJi.O^ phases and U0„+: a •* β transition at 626 C,

β -> γ transition at 695°C, y •* U00 decomposition at - 900 C. (Nonius Guinier-III camera, monoehromated CuKa. radiation) -28-

The unit-cell parameters were determined from a powder specimen, mounted en a PWII50 powder diffTactometer using CuKa radiation and quartz as an internal standard. A least-squares refinement on Q-values led to a rhombohedral cell with a - 10.9623(6)8 and α « 89.402(7)°. The calculated

density 7.110, based on four Cs„U,012 units in the cell, is in good agreement with the measured density 7.2(2), determined pyenometrieally in diocthyl phtalate. Thereupon 2635 reflections were measured from a single crystal using monobromated ΜοΚα-radiation in the range 2°<θ<20°. The crystal was an irregular sphere with a diameter of 0.16 mm. As there were no systematic extinctions in the reflection set the space group of a-Cs.U.O.- can be R3, R3, R3m, R32 and R3m [1]. Because a significant difference in intensity between reflections hkl and khl could not be detected, the space group is limited to R3m, R32 or R3m. The reflection set was averaged, yielding 478 independent reflections, 398 of which were significantly greater than three times the average counting standard deviation.

3.3.2. Structure determination of ct-Cs-U.O „

The space group of a vector set (Patterson function) corresponding to R3m, R32 and R3m, itself is R3m [2]. The rough positions of the uranium atoms were derived from a three-dimensional Patterson synthesis. After refinement of these positions in space group R3m and calculation of a Fourier map cesium atoms could be located. Isotropic refinement in R3m

t •- '-' Λ resulted in an R -factor of 21.55!. Refinement of the similar structure in space group R32 gave an R -factor of 17.9% and finally refinement in space group R3m resulted in an R -factor of 14.92. In the refinements scattering factors were taken from Cromer and Waber [3], corrected with &£' for anomalous dispersion, taken from Cromer [4], From Hamilton's test of the R-factor [5,6] the space group of

a-Cs2U,0.- was concluded to be R3m, with a reliability better than 99.5Z. By including a spherical absorption correction, anisotropic refinement of the metal atoms and omitting three heavily weighted reflections the R -factor dropped to 10.8Z. The final coordinates of o-Cs_U,0._ are

*) R »[£wA• F ο / Ε w F 2 , ]24 , weights from statistics, w obs :m i -29-

listed in table 3.1. As three of the nine anisotropic temperature factors were non-positive definite, these factors have not been tabu- lated. A list of observed and calculated structure factors is given in appendix A3; the powder X-ray pattern of a-Cs V 0 is listed in appendix C4.

Table 2.2.

Least-squares coordinates of u-Cs;>.0U i (RSm). f 1 Ci

ATOM x/a o(x/a) y/b o(y/b)

UI 0.000 U2 0.500 0.003 U3 0.242 0.002 U4 0.733 0.002 0.235 0.004 Csl 0.369 0.004 Cs2 0.660 0.007 Cs3 0.122 0.004 Cs4 0.869 0.003

3.4. The crystal structures of β- and Y-CS„U,0 „

The splitting of the powder reflections of B-Cs.U.O.- (see figure 3.1) could be explained by a slight distortion of the rhombohedral cell. Thia distorted cell turned out to have two equal axes and angles, a = b = 11.235(2)8, c « 10.793(2)8, a = 0 = 88.17(2)°, γ - 90.83(2)°, and to be C-centered. Therefore the cell symmetry is monoclinic; taking half of the diagonals of the triclinic ab-plane, the monoclinic unit cell can be deduced to have the following parameters: a « 7.886(1)8, b « 8.002(1)8, c - 10.793(2)8, β • 92.62(1)°. Systematic extinctions, (0k0 absent for k * 2n+l) correspond to the space groups P2./m and P2.. The X-ray pattern of Y-CS-U.O.- is much simpler than that of «- #·

and B-Cs-U.O.» as can be seen from figure 3.1. The pattern of Y-CS.U,0.2 can be indexed on a cubic cell with a - 11.2295(6)8. The space group of this cubic unit cell is Fd3m, while from the systematic extinctions also -30-

follows that the atoms in the Y-CS.U.O.. structure occupy special positions [1].

3.4.2. The crystal structures of β- and y-Cs^U.O

From the strong reflections of the X-ray pattern of γ-Cs-U.O „ it follows that the sixteen uranium atoms occupy position c(16) and the eight cesium atoms position b(8) (space group Fd3m, origin at centre 3m). J Since the Cs-Cs distance is rather short (4.86 8) it is impossible that an oxygen atom occupies position (i,i,i). More likely the oxygen atoms occupy position f(48). From five high-temperature Guinier films taken at 800° ± 20 C with different exposure times, 22 intensities were measured with a Nonius micro-densitometer. According to [73 the reflections were corrected with a combined factor for absorption, oblique incidence and geometry corrections Ν - 1.0+ l.O(l-cosX), in addition to the normal Lp-correction. Testing the above described model for the y-Cs-U.Oj, structure, oxygen atoms were located at position f(48) with χ • -0.051. The isotropic temperature factors of uranium, cesium and oxygen were 2.5, 9.4 and 3.9 A respectively. As the (sin θ/λ) -range was rather small, how- 2 ever, these values are not very reliable. The R -factor based on F was 7Z. Weights were estimated on basis of a comparison of the five exposures. The atomic positions in the Y-CS„U,0.„ structure are summarized in table 3.2.

Table 3.2. Least-squarescoordinates of y-Cs U 0 .·

ATOM x/a σ (x/a) y/b o(y/b) z/c σίζ/c)

U 0.000 0.000 0.000 Cs 0.375 - 0.375 - 0.375 0 0.125 0.125 -0.051 .005

As B-Cs-U.O., is strongly related to the other two phases, as can be seen from figure 3.1, its crystal structure will not greatly differ from the a- and γ-structures. From films taken at 660° ± 20°C 77 intensities covering 213 partly overlapping reflections with 2Θ < 60° were measured with a micro-densitometer and corrected as described for the γ-phase in- -31-

tensities. Initial atomic positions were obtained from the γ-structure, but adapted to theraonoclinic uni t cell. Refinement of the uranium and 2 cesium coordinates in space group P2./m led to an R-factor based on F of 26.1%. Refinement of the structure in P2. led to an R-factor of 20.4Z. According to Hamilton [5,6] the refinement in P2 gives the better description of the fi-Cs-U.O.- structure. Oxygen atoms could not be located in a computed Fourier map. Due to the small range of (sin θ/λ) , an overall temperature factor only has been refined: Β · 1.7 A • The final position parameters of uranium and cesium are given in table 3.3.

Table 3.3. Least-squares coordinates of ^~Cs2U4°i2 ^pzi^'

ATOM x/a a(x/a) y/b a(y/b) z/c o(z/c)

in 0.523 0.013 0.000 - 0.531 0.009 U2 -0.018 0.012 0.052 0.010 0.020 0.007

.·•;ƒ>••• U3 0.264 0.014 0.234 0.013 0.265 0.009 U4 0.738 0.012 0.264 0.020 0.222 0.009 Csl -0.040 0.015 0.811 0.026 0.386 0.010 Cs2 0.560 0.014 0.274 0.022 0.880 0.008

Listings of the observed and calculated intensities of the X-ray powder patterns of β- and γ-Cs-U.O _ are presented in appendices B7 and B8.

3.5. Discussion

Most of the cesium uranates have crystal structures containing (pseudo) hexagonal uranium layers like in U_0_ [8], as will be shown in the next chapters. The three CSjU.O., crystal structures, however, are related to the UO„ crystal structure. In figure 3.2 the y-Cs.U.O _ structure is drawn, together with the unit cell of U0_ t9]: the position of the uranium atoms in Cs-U/O „ is similar to that of half of the uranium atoms in U0_, while cesium atoms are substituted for the other half. The introduction of cesium in the UO_ structure causes a decrease

Cs U O in specific density from 10.952 for U0_ [10] to 6.613 for Y~ 2 4 i2· As this compound was shown to be present in reactor fuel pins during -32-

••••'·' 'ί.·';·.Ί

5.6iA

(O) Figure 3.2.

Unit oells of y-CsJJ. (a) and UO» (b) and an isolated structural

motif in Ί-Cs^l^^ (β). The unit oell of i- '0 ? has been built from 8 motifs such as given in figure a. From the figures b and a

the structural relation between U00 and y-Cs0U.O _ is evident (oxygen atoms are not drawn in these figures; small spheres designate uranium, large spheres designate cesium).

•Wwiw^MBeim^s^-^na^Wer1 =Γ' -33-

irradiation [chapter II], the swelling of the U0„ pellets during fission can be readily understood. The coordination of uranium by oxygen in γ-Cs-U.O „ is not the same as in U0„. For ÜO„ an eightfold coordination with V-0 » 2.37 X has been reported [9], whereas in Y-CS.U.O.J a deformed octahedral coordination with U-0 * 2.07(2) has been found. Each cesium atom in y-CsJ}.0.„ is coordinated by six oxygen atoms at 3.64(6) A and twelve oxygen atoms at 4.06(1) A. Since these cesium-oxygen distances are rather large Cl] the large temperature factor of cesium probably indicates static disorder or oscillation of the cesium atoms in the structure. The differences between the uranium and cesium positions in the Cs„U,O.„ crystal structures are rather small. The average Ü-U distances for a-, 0- and y-Cs.U.O . are all in the range 3.8-4.0 X, while the average Cs-Cs distances are in the range 4.8-4.9 8. Larger differences may be found in the oxygen positions in the three structures, but neutron investigations are necessary to affirm this hypothesis. Finally there is the question whether CsAJ.O.* should be written as VI IV VI V Cs„U„ U O.» or as Cs„U„ U„O.„. Measurement of the magnetic suscepti- bility cf a-Cs.U.O.^j using a Gouy balance, showed a magnetic moment which was not found in Cs_U_07 and Cs.U.O.,. Kemmler-Sack et al. [II] ill ZHIJ ^ yj have indicated that o-U_0o should be written as U_0,.U 0_. Similarly JO i D i the uranium in a-Cs_U,0._ is assumed to be partly in the pentavalent and VI V hexavalent state respectively, which implies that Cs„U_ U?0 „ is the correct notation for

References

[1] Internationa). Tables for X-ray Crystallography, Birmingham, (1962).

[2] M.J. Buerger, Vector Space, New York, (1959).

[3] D.T. Cromer and J.T. Waber, Acta Cryst., 2§_, 104 (1965).

[4] D.T. Cromer, Acta Cryst., _1£, 502 (1965).

C5] W.C. Hamilton, Acta Cryst., JJJ, 502 (1965).

[6] G.C. Ford and J.S. Rollett, Acta Cryst., A26, 162 (1970).

[7] W.H. Sas and P.M. de Wolff, Acta Cryst., ^8, 104 (1965),

[8] 3.0. Loopstra, Acta Cryst., \7, 651 (1964). <

-34-

t9] Thermodynamic and Transport Properties of Uranium Dioxide and Related Phases, I.A.E.A. Technical reports series no. 39, Vienna, (1965).

[10] E.H.P. Cordfunke, The Chemistry of Uranium, Amsterdam, (1969).

[II] S. Keranler-Sack, E. Stumpp, W. Riidorff and H. Erfurth, Z. anorg. allg. Chem., 354, 287 (1967).

..I -35-

*) CHAPTER IV. THE CRYSTAL STRUCTURES OF j3 AND CSjU Ojfi

4.1. Introduction

In sectipn 2.4.2 the reversible dissociation reaction

Cs Cs U + 2Vs3^ 2 4°12 *°2 has been discussed. Since in the foregoing chapter the crystal structures of Cs.U.O.» have been described, now a study of the Cs_U,0 „ structure is called for. In chapter II it was found that the crystallinity of Cs„U,0 „ and CsJ 0 , powder samples increases upon heating above 600 - 700 C. Furthermore Cs.U.O , appeared to have a homogeneity range when heated above 650°C, whereas at 1000°C a solid solution is obtained with 0.375 < Cs/U < 0.5, enclosing the original compounds Cs.U 0 and Cs„U O ,. In this chapter the crystal structures of Cs^U^O,., and Cs-U.O., will be described. ••-Ui

4.2. Experimental

Powder samples of Cs-U.O., were prepared as described in the second chapter. For the structure determination of Cs.U-O., a sample with Cs/U = 0.42 was used. After heating the sample in air at 760 C for 50 hours it was cooled down to room temperature very rapidly to avoid

decomposition of the one-phase component into Cs„U,0 and Cs?U.O., (Cs/U= 0.40),. as could be expected from earlier experiments (see figure 2.1). Single crystals of Cs.U.O., were grown from molten cesium chloride, on amorphous UO. in a gold boat. The gold boat had been placed in a quartz tube, which was filled with pure oxygen. After one day crystals had formed along a temperature gradient, the temperature ranging from 950° to 900°C in the gold boat. The crystals had grown at • -.-•4

the outside of the boat, while the gold boat had been partly corroded by ,&?:;

. ί-'J.Y some of the reagents. The plate-like, even scaly crystals were of a very poor quality. Attempts to grow better crystals from different reaction ?&3

*) This chapter is a revised version of the paper by A.B. van Egmond, J. inorg. nucl. Chenu, in the press. &! tftt*

-36-

circumstances and from other molten-salt techniques failed. In a number of cases Cs,U,O._ crystals could be isolated from the reaction products. A description of the crystal structure of this compound will be given in the next chapter.

•ν,Λ.Α» A.3. Crystal data and crystal structure of CsJJ.O.-

Guinier exposures of powder samples of Cs„U,O., could be indexed with a C-face centered orthorhombic unit cell with a = 13.494(2) A, b • 15.476(2) 8 and c « 7.911(2) 8, as follows from a least-squares

refinement on Q-values. The density of Cs„U,0.3 had been measured to be 6.8 (chapter II). Assuming 4 units Cs.U.O in the unit cell a density 5.73 can be calculated, which, however, is much lower than the experimental value. With 5 molecules in the cell the density is calculated to be 7.17, which in turn is too high. Weissenberg exposures of some Cs^U.O., crystals showed that the unit cell of Cs.U.C· has a c-axis five times the above mentioned c-axis. Thus the true unit cell of Cs„ü,0 „ is a C-centered one with a = 13.494(2) 8, b » 15.476(2) 8 and c » 39.56(1) 8. The systematic extinctions, determined from the Weissenberg exposures, correspond to the space groups Cmcm, Cmc2 or C2cm [1]. From this cell a density 6.879 is calculated, assuming 24 units CsJJ,O.~ in the cell, which is close to the experimental value 6.8. Therefore the subcell of CsJJ.O „ contains 24/5 4.8 units

An irregular crystal of Cs-U.O.-, with a diameter < 0.05 mm, was mounted on a PW1100 computer-controlled single-crystal X-ray diffractometer. Using monobromated ΜοΚα-radiation a set of 360 subcell reflections with 2Θ < 50° was measured. Since the unit cell consists of five subcells the symmetry of the subcell is the same as the symmetry of the large unit cell. Therefore the crystal structure of Cs-U 0 has been solved in the subcell with spacegroup Cmcm. From a Patterson synthesis two pseudo hexagonal uranium layers, consisting of 8 atoms each, could be constructed in the Cs.U.O.- subcell, perpendicular to the y-axis. In a calculated Fourier synthesis the remaining uranium atoms could also be located in position f(8). As the subcell contains only 4x4.8 » 19.2 uranium-atom equivalents this eightfold position is occupied for 40% on"y. In a difference Fourier synthesis several unsharp maxima could bo detected. These positions were

τΜ: •Μ'. -37-

II ·•.'·· filled with statistically distributed cesium atoms, their occupancy •]•"·. i- roughly being based on the peak heights in the difference Fourier synthesis. A least-squares refinement of this structure model, using corrected scattering factors [2,3] resulted in an R-factor of 30%. In the subcell each atom is a superposition of several atoms from the true cell, with coordinates distributed around the subcell atomic position according to the space group symmetry. This distribution of coordinates might be represented by an anisotropic thermal refinement of the subcell atoms, taking into account that these anisotropic parameters have no thermal significance. In applying this procedure to the uranium atoms in the Cs.U.O.. subcell the R-factor immediately dropped to 18.5%, indicating that the constructed model is basically correct. Concerning the reflection set there should be made two remarks: at first the crystals of Cs.U,0._ were of a very poor quality, and secondly some subcell reflections might have suffered from overlap with non-subcell reflections, because of the very large c-axis and the use of the molybdenum radiation. Therefore no refinement calculations have been made in other space groups.

Table 4.1.

Least-squares coordinates of CsJJ.O.- (subcell Cmcm).

Atom x/a σ(x/a) y/b 0(y/b) z/c a(z/c) occupancy

Ul 4c 0.000 - 0.202 0.00! 0.250 - 1.00 U2 4c 0.000 - 0.211 0.002 0.750 - i.00 U3 8d 0.250 - 0.250 - 0.000 - 1.00 U4 8f 0.000 - 0.388 0.001 0.024 0.001 0.40 •o - • Csl 4b 0.000 - 0.500 - 0.000 - 0.40 Cs2 4c 0.500 - 0.591 0.010 0.250 - 0.20 Cs3 8g 0.314 0.003 0.490 0.004 0.250 - 0.60 Cs4 8e 0.328 0.005 0.500 - 0.000 - 0.30

/"; v/fir2 *) F — F |/£F obs calc obs*

•4f^tf^^ -38-

The final coordinates of the Cs.U.O.» structure, resulting from the least-squares refinement of the subcell coordinates in space group Cmcm, have been summarized in table 4.1. A list of observed and calculated structure factors has been included in appendix A2, whereas the powder X-ray pattern is given in appendix C3.

4.4. Crystal data and crystal structure of Cs_U,0 16

A powder sample of Cs.U.O.,, mixed with starch to avoid preferred orientation, was mounted on a Philips PW1150 powder X-ray diffractometer. It was step-scanned from 8.00° up to 65.00 2Θ in steps of 0.02 , using Ni- filtered CuKct radiation. The measured profile was corrected with an estimated background. The pattern could be indexed with a monoclinic C-centered unit cell, with a - 13.465(2) 8, b - 15.561(2) 8, c « 15.928(4) X and β - 92.78(1)°, as followed from a least-squares refinement on Q-values. The systematic extinctions correspond to the space groups C2/c and Co. Omitting some weak reflections, a subcell was found with c' = £c = 7.964(2) A. Due to the symmetry of the large cell the space group of the subcell is C2/m or Cm respectively. A calculated density of 6.822 cor-

responds to 8 molecules Cs_UcO,, in the true cell; and therefore 4 in ζ 5 ID the subcell. It was decided to solve the crystal structure of Cs.U.O,, based on L D ID the subcell with space group C2/m. The X-ray profile was integrated to 95 intensities covering 314 partly overlapping reflections with 26-angles up to 65 . Since Guinier films of Cs.UcO., and Cs„U.O,_ resemble each i D Ιο Ζ 4 13 other very much, hexagonal uranium layers could be constructed in the Cs_U.O subcell. A difference Fourier synthesis revealed several weak *) maximathe subcell, indicatin. Afterg ainclusio statistican ofl thdistributioe cesium atomn osf the cesiuR-factom ratom ons F 'in dropped to 14.8%, indicating the model is fundamentally correct. An attempt to refine the structure in the unit cell with c » 15.928 X and space group C2/c failed to show more distinct positions for the cesium i-m' atoms. In table 4.2 the final coordinates of the subcell refinement are listed. The structure model has not been refined in space group Cm.

7) 2 2 2 R Γ 1Ι Σ η F , - 7. η F , Ι / Ζ η F obs calc ι obs

«tj&'f ••;•'** . 'ι

-39- ! \

A listing of the observed and calculated intensities in the powder Μ- pattern is presented in appendix B4.

table 4.2.

Least-squares coordinates of CsJJJ)-~ (subaell C2/m).

Atom x/a a(x/a) y/b σ(y/b) z/c CT(Z/C) occupancy

Ul 8j 0.013 0.002 0.205 0.001 0.270 0.004 1.00 U2 4e 0.250 - 0.250 - 0.000 - 1.00 U3 4f 0.250 - 0.250 - 0.500 - 1.00 U4 4g 0.000 - 0.389 0.002 0.000 - 1.00 Csl 4i 0.223 0.010 0.000 - 0.196 0.012 0.50 Cs2 4i 0.796 0.007 0.000 - 0.223 0.014 0.50 Cs3 2d 0.500 - 0.000 - 0.500 - 1.00 Cs4 4i 0.147 0.008 0.000 - 0.506 0.016 0.50

4.5. Discussion

From this investigation, it appears that the structures of Cs_U,0 _ and Cs-ILO., are very similar. The skeleton of the uranium atoms in both compounds has been reproduced in figure 4.1. In the structures of CsîÔ.- and CsÛ-O., the pseudo hexagonal uranium layers are linked by bridges consisting of two uranium atoms. In CS.U..0 , these bridges are Z j Jo found more frequently than in Cs„U,O.,. Also the positions of the cesium atoms are not the same in both structures as follows from a comparison of the tables 4.1 and 4.2. Since oxygen atoms have not been located in the structures, conclusions concerning the coordinations could not be drawn. However, in the last chapter the oxygen positions in the double bridges will be discussed. The double bridges linking the hexagonal uranium layers have not been found before, whereas hexagonal uranium layers have been found in other alkali uranates and uranium oxides [4-10].

Due to the structural resemblance of Cs.,U.O,_ and CsoU.0,, it is / 4 IJ ί ο Ιο interesting to examine the corresponding part of the phase diagram of the Cs-U-0 system in air (figure 2.1) more closely. The question arises how the solid solution, which is formed above !000°C between Cs.U.O.. Β&-

y.%;»••• 'iTafrtf;jfr •·=-'•'• *» ίί •^ -40-

I ••·•"• ι •'. •}' r V'

Figure 4.1. Uranium positions in the subeells of CsJ1.0.~ and CsJJ,-O-„. ύ 4 J.O ώ O ΙΌ The blaak spheres form the double uranium bridges.

and Cs„ü,0.,, can be limited by an orthorhombic compound Cs„U,O.„ and a monoclinic compound Cs-ULO.,. Reinvestigation of the high-temperature Guinier films has yielded the following more, precise picture of the phase relationships between Cs.U.O _ and Cs.U.O.,. At room temperature Cs-U.O.» crystallizes in an orthorhombic cell with a c-axis of about 40&, whereas Cs_U_O,, has a monoclinic unit cell with a c-axis of about 16A. / j ίο At 600 C the Cs-U-O., c-axis is halved, the unit cell remaining monoclinic. Above 1000 C the CS2Ö,O.„ c-axis is changed to an axis of about 8A, whereas the crystal system becomes slightly monoclinic, as follows from the splitting of the X-ray lines on a high-teirperature Guinier film. In this smaller cell the structures are statistically disordered. Now a - 4y '-I -

'"'. ••'" #••:' solid solution can exist between Cs_U.O,_ and Cs_U_O,,, as shown in m : 4 Π

' -· -•'' ••

';-MW -41-

. , {·· Ι! .

1100. . Γ.-·

900.

700.

500. .... •.it

0.4 05 Cs/U ratio

Figure 4.2.

Part of the phase diagram of the pseudo-binary Cs-U-0 system in air. The dotted lines represent metastable equilibria in open atmosphere (p(CsJ3)=0). The figure should be compared to figure 2.1.

function of the temperature and the Cs/U ratio. However, at 900°C all samples decompose into Cs.11,0 „ and U_0„ after long heating times. Therefore some lines in figure 4.2 have been dotted because they represent metastable equilibria (compare to figure 2.1). As the CSjU.O.- crystals, used in this study were allowed to cool down slowly the orthorhombic 40A-phase was obtained. Upon cooling down rapidly the Cs.UeO.g sample the monoclinic 8A cell could be quenched in (apart from some very weak lines indicating a transformation to the 16A* cell), The unsharp patterns of both compounds below 600 C can be explained by assuming that the long range order cannot be obtained perfectly at reac- • ,-r * •·*•• ;'«r-:-.-""

-42-

tion temperatures of 500 - 600°C. This fact also accounts for the poor if Ι - • quality of the Cs U,0 _ crystals.

• '& *>". Γ References

[1] International Tables for X-ray Crystallography, Birmingham, (1962).

[2] D.T. Cromer and J.T. Waber, Acta Cryst., U±, 104 (1965).

[3] D.T. Cromer, Acta Cryst., Jj}, 17 (1965).

[4] L.M. Kovba, Radiokhimiya, Jjl, 727 (1972). A

C5] L.M. Kovba, Zh. Strukt. Khimii, J2· 256 0972).

[6] L.M. Kovba and V.I. Trunova, Radiokhimiya, JJ3» 773 (1971).

[7] J.S. Anderson, Chimia, 2!3, 438 (1969).

[8] B.O. Loopstra, Acta Cryst., J_7, 651 (1964).

[91 P.C. Debets, Acta Cryst., 2^, 589 (1966).

[10] C. Greaves and B.E.F. Fender, Acta Cryst., B28, 3609 (1972).

.·•?/ • -43-

. I: '•· CHAPTER V. THE CRYSTAL STRUCTURES OF *) AND Cs2UO4

5.1. Introduction

In chapter IV the crystal structures of two cesium uranates, in which uranium is in the hexavalent state, were described. In this chapter the crystal structures of CsÔjO^, CsÔ^, Cs^gO^ and CsÔ^ are presented. The only cesium-uranate(VI) crystal structure left is that of Cs-U-O-,, which will be dealt with in chapter VI.

5.2. Experimental

Powder samples of Cs-UO,, Cs_U_O__ and Cs_U ,0,, were prepared as described in chapter II. The hygroscopic samples of Cs.UO, were handled in a dry box. The samples of Cs_U,O„„ and Cs_U,,O., always contained im- ί I it i I 3 4D purities like other uranates and U,0_. Single crystals of Cs.UJJ were made in a number of different ways: - reaction of amorphous UO- with molten CsCl yielded yellow transparant

Cs,U,0 ? crystals after 120 hours of heating in air at 700°C in a gold boat; ι •.·'•. - the same crystal needles were grown in a reaction of amorphous U0_ with CSjSO in a platinum cup after 50 hours at 1100°C in air (in the same experiment Cs„U,O.„ crystals were formed, as described in section 3.2); - several experiments starting with CsCl or Cs_S0, and UO, or cesium

uranates like Cs.U 0 , Cs„U5O ,, etc. in molten quartz tubes yielded Cs.U 0 crystals after heating at different temperatures (700-1000°C) for several days. Attempts to grind the needle-shaped Cs.U.O.- crystals in a crystal spherizer yielded strongly ellipsoidal crystals.

5.3. Single-crystal analysis and structure of Cs,U5O)7

A single crystal of Cs.UjO.- with a diameter = 0.1 mm was mounted on the sample support of a Philips computer-controlled single-crystal X-ray

*) This chapter is a revised version of the paper, by A.B. van Egmond, J. inorg. nucl. Chem., in the press. J*·.

il -44-

diffTactometer. The crystal showed an orthorhombic unit cell with cell dimensions a - 18.776(4) X, b - 7.070(1) 8 and c - 14.958(3) 8 (these

cell parameters were refined from a powder diffractogram of Cs,U_0I7); the systematic extinctions (Okl absent for k=2n+l, hOl absent for l = 2n+l and hkO absent for h+k=2n+l) correspond to space group Pbcn uniquely [J], Assuming four Cs.U.O.- units in the cell, the calculated density is 6.669, close to the measured value 6.62. In the range 6.00 < 2Θ < 50.00 1723 reflections were measured with monochromated MoKct radiation, 1553 reflections being greater than three times the counting standard deviation of the intensity. An absorption correction was lot applied to the reflection set. The positions of the uranium and cesium atoms were found using the direct-method programs SINGEN and PHASE of the X-RAY SYSTEM [2]. After subsequent refinement of the position parameters and three-dimensional i Fourier calculations all oxygen atoms could be located in the Cs.U O._ crystal structure. The weighted refinement of position parameters, thermal parameters and scale factor was based on F, with weights from

-il • statistics; scattering factors were taken from Cromer and Waber [3] and

Table 5.1

Least-squares coordinates of Cs.U,0-„.

Atom x/a σ(x/a) y/b a(y/b) z/c σ(z/c)

UI 4c 0.5000 - 0.1330 0 0004 0.2500 - U2 8d 0.1035 0.0001 0.0781 0 0003 0.2082 0.0002 U3 8d 0.3118 0.0001 0.0267 0.0003 0.1875 0.0002 Csl 8d 0.3072 0.0002 0.3004 0.0007 0.4458 0.0003 Cs2 8d 0.0548 0.0002 0.2718 0.0007 0.4705 0.0003 01 4c 0.000 - -0.035 0.008 0.250 - 02 8d 0.079 0.002 0.078 0.009 0.113 0 007 03 8d 0.132 0.003 0.109 0.009 0.304 0 006 04 8d 0.212 0.002 0.204 0.005 0.174 0.004 05 8d 0.381 0.002 0.257 0.005 0.203 0.004 06 8d 0.466 0 002 0.145 0.007 0.356 0.004 07 8d 0.322 0 .002 0.051 0.008 0 .081 0.008 08 8d 0.296 0 003 -0.015 0.010 0 .309 0.002 09 8d 0.078 0 002 0.388 0.006 0 217 0.004

FV-V ·..*, *ΐ';' '$""J

-45-

•J 'i, ,.

!!'·"'.• corrected for anomalous dispersion with Af', according to Cromer [4]. In the last stages of the refinement uranium and cesium atoms were allowed to vibrate anisotropically, but, since an absorption correction had been omitted, some temperature factors were non-positive definite. The final R -factor was 21.3% ; the position parameters resulting from the last refinement cycle have been listed in table 5.1. In figure 5.1 the coordination of the uranium atoms by oxygen has been drawn, whereas in figure 5.2 an impression of the crystal structure of Cs,U,O._ is given: the layer composition of Cs.U.O.y has been very clearly expressed. The I., " relevant interatomic distances and angles in the uranyl groups of the Cs.U.O.- crystal structure, as calculated with aid of the BONDLA routine of the X-RAY SYSTEM [2], are summarized in tabia 5.2. A list of observed and calculated structure factors is presented in appendix Al, whereas the powder pattern is listed in appendix C2.

Figure S.I.

The oxygen coordination of uranium in the pseudo hexagonal uranium layer (uranyl oxygen atoms are not shown; the angles are in degrees, accurate to = 2°; distances are in X, accurate to 0.05 %.).

*) *«lc - 'ob. cobs Stereoscopic view of Cs.U/)„ along the b-axis (large spheres designate cesium atoms, small spheres deszgnate uramum atoms)

Table 5.2.

Interatomic distances and angles in uranyl groups of Cs,UrO-„

atoms distances atoms angle

UI-06 1.71(6) 06-UI-06 174 °(3) U2-02 1.50(9) 02-U2-03 171 °(3) U2-03 1.54(8) Ü3-07 1.61(11) 07-U3-08 176 °(3) U3-03 1.87(4)

5.4. The crystal structure of Cs2U7O22

As stated in chapter II Cs2U_O2„ is isostructural with the corresponding rubidium and potassium uranate, RbjU-O-n [5] and KjUyOu» C63 respectively.

A sample of Cs2U7O22 was mounted on a PW1150 powder diffractameter, after mixing the sample with starch to avoid preferred orientation. The diffractogram of the sample was step-scanned from 8.00° to 65.00° 2Θ in steps of 0.02° using Ni-filtered CuKa radiation. The X-ray spectrum fej -47-

could be indexed on an orthorhombic unit cell with a « 6.949(1) A, b = 19.711(2) X and c = 7.3955(8) 8. The space group is Pbam or Pba2 as follows from the systematic extinctions (Okl absent for k*2n+l, hOl absent for h«2n+l). With two Cs.UyO». units in the cell a density of 7.488 is calculated. Since Cs.U-O», samples always contained small amounts of Cs-U-O., and Cs.U.-O,, the profiles of the latter compounds were appropriately subtracted from the Cs.U-O-» profile. Thereafter the diffractogram of Cs-U_O._ was integrated with a computer program to yield 92 intensities covering 219 reflections with 2Θ up to 67°.

A model, based on Kovba's work on K_U7O_. [6], without the oxygen *) atoms, was refined in space group Pbam to an R-factor of 20%. On changing the space group to Pba2 and including uranyl oxygen atoms, the R-factor did not improve. The coordinates, resulting from the last refinement cycle in space group Pbam are listed in table 5.3; the layer structure of Cs„U_O - is shown in figure 5.3. Appendix B5 contains the observed and calculated X-ray pattern of Cs.U-O.-·

Figure 5.3,

Stereoscopic view of CsJJ„0^s along the ,^-tixis (large spheres , •l'ïji-'.ï*-*.'1' designate cesium atoms, small spheres ·'• "innate uranium atoms). ν *> R - Σ Ι Σ η - Ε η / ζ η

Pi-o ,· „ f.;- j ^ Γ "'«'<·'•* *".,'

-48-

5.3.

Least-squaves coordinates of

Atom x/a o(x/a) y/b o(y/b) z/c o(z/c)

Ul 2a 0.000 - 0.000 - 0.000 - U2 4g -0.021 0.006 0.218 0.002 0.000 - U3 4g -0.040 0.005 0.408 0.002 0.000 - U4 4h 0.230 0.005 0.039 0.002 0.500 - Csl Ah 0.174 0.008 0.309 0.003 0.500 -

5.5. The crystal structure of Cs-U.,.0,,

The diffractogram of the cesium uranate with the lowest Cs/U ratio was obtained as described for Cs^O^. The X-ray spectrum could be indexed on an orthorhombic unit cell with a » 14.686(3) A°, b » 13.422(2) X and c = 19.752(3) 8; the space group is Cmca or C2ca (hkl absent for h+k=2n+l, hOl absent for l = 2n+l, and hkO absent for k»2n+l). The profile was

corrected for small amounts of <*-U3Og and Cs2U7O22> whereafter the profile was integrated to 68 non-zero intensities, covering 317 reflections with 2Θ < 61°. Since the b-axis in Cs^^O^g is twice the a-axis in CsJ 0», and

Cs U the res the c-axis in CSJUJJO^ is equal to the b-axis in 2 7°22' P ence of hexagonal layers parallel to the bc-plane in Cs-U.-O,, was obvious. After some Fourier calculations in space group Cmca only 68 "heavy" atoms could be found in the unit cell, which together with the rough composition ratio 0.10 < Cs/U < Q.15 (see chapter II) leads to the formula CSgUj . Assuming 4 units Cs.U 50,- in the unit cell the density is calculated to be 7.800. In further refinements in space group Cmca an indication which atoms are cesium and which are uranium could not be found for the interlayer atoms. The final R-factor on this rather poor model was 28%. The coordinates of the atoms are listed in table 5.4, the structure being drawn in figure 5.4. A list of observed and calculated intensities is presented in appendix B6.

ί* •' *) Fobs Fcalc

jni^uè -49-

Table 5.4.

Least-squares coordinates of Cs2Uib°46

Atom x/a o(x/a)Τ ι y/b o(y/b) z/c |o(z/c) occupancy

UI 16g 0.235 0.005 0.127 0.005 I 0.066 0.003 1.0 U2 I6g 0.285 0.004 0.389 0.005 i 0.149 0.004 I 1.0 j U3 8e 0.250 0.101 0.007 0.250 1.0 J U4 4a I 0.000 0.000 0.000 1.0 I U5/Csl 8f 0. 000 0.288 0.007 0.126 0,006 0.67/0.33 I U6/Cs2 8£ 0. 000 -0.037 0.007 0.206 0.006 0.67/0.33 U7/Cs3 8f 0. 000 0.169 0.007 0.392 0.006 0.67/0.33

,1 Λ

Figure 5.4.

Stereoscopic view of CspU .O.g along the b-axis (large spheres designate statistically distributed uranium/aesiion atoms 2:1, small spheres designate uranium atoms).

5.6. The crystal structure of CsJJO, Since Cs„UO, is very hygroscopic, its X-ray diffractogram had to be a recorded from a sample in plastic foil. The sample had been mixed with a small amount of α-quartz for calibration purposes. The unit cell of

Cs2UO, is tetragonal with parameters a » 4.3917(6) X and c » 14.804(3) 8. The symmetry corresponds to the space group 14/nmrn, as was found pre-· ft viously by other investigators [7-9]. -50-

From the diffractogram 37 intensities were integrated by hand, covering 44 reflections up to 80 2Θ. A model, based on the results of Kovoa [7], was refined with these intensities, omitting two reflections which suffered from overlap with α-quartz reflections. The resulting cesium parameter z/c, as listed in table 5.5, is in good agreement with l· Kovba's results, but the uranyl distance U-0„ is too large: 2.34(1) A. 2 The final R-factor on F was !4.3%. A stereoscopic impression of the structure is given in figure 5.5; the final least-squares coordinates have been summarized in table 5.5. In appendix Bl a list of observed and calculated structure factors is given.

Table 5.5.

Least-squares coordinates of CsJJO..

ΓAtom x/a o(x/a) y/b o(y/b) z/c o(z/c) I

U 2a 0.000 0.000 0.000 Cs 4e 0.000 0.000 0.345 0.005 01 4c 0.000 0.500 0.000 02 4e 0.000 0.000 0.160 0.010

Figure 5.S.

Steveosaopia view of CsJUO, along the b-axis (large spheres designate oesium atoms, small spheres designate uranium atoms).

JV ^*^"Pj^'"' '"', Ϊ-U^l ^ Oα wsMiwtóïlfeSp' ÉÜÉ

-51-

5.7. Discussion

In case of Cs„UlCO,, the X-ray analysis has been a tool to define the Ζ 15 At) composition of the compound. The structure model itself is rather poor, due to impurities in the sample and considerable overlap in the pattern. The same facts affect the structure model of Cs Ü 0 .. In both compounds mixed cesium-uranyl layers have been shown next to (pseudo) he:.agonal uranium layers. Therefore these compounds might be called cesium uranyl uranates, in agreement with the nomenclature of Kovba [6]. In Cs„U,cO,, the exact positions of the cesium atoms could not be established. One could speculate that the cesium atoms in Cs.U .0,, cluster together in short zig-zag chains like in Cs-U-O-o» rather than all cesium atoms being separated by uranyl groups. In this way a kind of short-range order can be obtained. Since the crystal structure of Cs_U0, contains tetragonal uranium layers it is related to Rb.UO, and K„UO, [9], Together with these alkali uranates Cs„U0, belongs to the K.NiF, structure class [10]. As the intensities of Cs_UO, suffered probably from preferred orientation, the results of this structure study are net as accurate as the results of Kovba et al. [6]. The crystal structure of Cs.U 0 - contains endless layers of pseudo hexagonally arranged cesium atoms only between the likewise hexagonally arranged uranium layers. In the uranium layers the uranium atoms are coordinated by six of seven oxygen atoms, two oxygen atoms always be- 2+ longing to the uranyl group U0„ . The oxygen atoms in the uranyl groups link the cesium layers and the uranium layers. The uranium-oxygen distances in the uranyl groups vary from 1.50 A to 1.87 A. In other crystal structures uranium-oxygen distances in uranyl groups are reported to range from 1.70 8 to 1.95 A [6-8,11-13]. In the hexagonal layers uranium is coordinated by four or five oxygen atoms, the uranium-oxygen distances ranging from 2.09 X to 2.51 X.as has been shown in figure 5.1. Although all distances may be somewhat affected by the omission of an absorption correction, the general coordination of the uranium atoms can be supposed to be correct.

References

[1] International Tables for X-ray Crystallography, Birmingham, (1962), [2] See chapter I, reference [23]. ft •:'<-ï& -52-

• J [3] D.T. Cromer and J.T. Waber, Acta Cryst., JjB, 104 (1965). f! [4] D.T. Cromer, Acta Cryst., JJJ, 17 (1965).

[5] L.M. Kovba and V.I. Trunova, Radiokhimiya, JJ, 773 (1971).

[6] L.M. Kovba, Zh. Struct. Khimii, \3_, 256 (1972).

[7] L.M. Kovba, I.A. Murav'eva and A.S. Orlova, Radiokhimiya, \b_, 648 (1974).

[8] H.R. Hoekstra, J. inorg. nucl. Chem., 27_t 801 (1965).

Γ9] L.M. Kovba, E.A. Ippolitova, Yu.P. Simanov and V.I. Spitsyn, Russ. J. Phys. Chem., jSS, 275 (1961).

[10] D. Balz and K. Pliefh, Z. Electrochemie, 59, 545 (1955).

Γ Ml L.M. Kovba, Radiokhimiya, Jjl, 727 (1972).

[i2] L.M. Kovba, Radiokhimiya, J2» 309 ('971)

Γ13] Κ. Ohwada, Spectrochimica Acta, ^4A, 595 (1967). -53-

CHAPTER VI. THE CRYSTAL STRUCTURES OF Cs^O., *^

6.1. Introduction

In chapter II it has been reported that Cs U_O_ exhibits three different phases. Two phases, called a- and B~Cs U„0_, have a reversible phase transition at 300 C. A third diuranate, the γ-phase, was found to be metastable with respect to the β-form at 600°-800°C.

Recently y-Cs„U„O7 has been described by Kovba et al. [1], its unit cell belonging to the hexagonal crystal class. The crystal structure was reported to consist of hexagonal uranium layers and cesium layers, linked by the uranyl oxygen atoms, whereas the remaining oxygen atoms were statistically distributed in the hexagonal uranium layer. In this chapter the cryst.nl structures of the three cesiunrdiuranate phases are reported, completing the description of the cesiunr-uranate(VI) crystal structures.

·•:*.'* 6.2. Experimental

Powder samples of the diuranates were synthesized as described in chapter II. Pure samples of β-Cs.U-O- were obtained on heating mixtures of amorphous UO. and cesium carbonate at 600 C. This phase changes into a-Cs„U„0_ when the temperature is kept below 300 C for several days. The change in the X-ray pattern due to the phase transition is shown in figure 6.I indicating only a small structural change. Upon rapid cooling from 600 C down to room temperature the crystal structure of B-Cs„U-O_ is quenched. The X-ray pattern of y-Cs-U-O- always showed broad vague lines.

The γ-phase also appeared to contain small amounts of (5-Cs?U.0_. The experimental circumstances concerning the γ-diuranate formation have been described extensively in chapter II.

6.3. The X-ray analysis of ct-Cs-UjO-

A powder sample of α-Cs.U.O-, mixed with starch, was mounted on a rotating sample support of a Philips powder diffractometer. The sample

*) This chapter is a revised and extended version of the paper by A.B. van Egmond, J. inorg. nucl. Chem., in the press. :·$&$$•& S I

^^;-^^^

'20

•%',•',: 200-

400-

600-

Figure 6.Ί.

High temperature film of CsJJJD , showing the X-ray patterns of the X-ray patterns of the two monoolinia diuravate. phases and Cs.UJJ.^. ο ->• Β transition at SOO°C. β •* Cs Uβ.- decomposition at - 650°C. (Guinier film taken in air, monochromated CuKa radiation).

MfiSP -55-

contained small amounts of Cs.U.O.-. Its X-ray pattern was step-scanned from 10.00° to 78.74° 2Θ in steps of 0.02°. The pattern could be indexed with a monoclinic C-face centered unit cell with a « 14.528(3) A, b = 4.2638(7) X, c = 7.605(1) X and 6 = 112.93°. Thereafter the profile could be integrated to 56 intensities covering 153 reflections. The space group of the α-diuranate is C2/m, Cm or C2 (hkl absent for h+k = 2n+l). The b/c-ratio of the unit cell allows a nearly ideal hexagonal uranyl oxygen layer parallel to the bc-face of the cell. These (pseudo) hexagonal layers have been established in a number of cesium uranates [chapter IV,V], However, the strong 001 reflection indicates a structure with sheets parallel to the centered ab-face. Nevertheless it was impossible to describe the structure as a (pseudo) hexagonal uranium network parallel to the ab-face due to the centering. A three-dimensional Patterson synthesis, based on 23 reflections, showed the heaviest peak in position 0,i,i. Assuming this peak to be the U-U vector, the Patterson function favours the model with layers parallel to the bc-face. Several configurations according to this model, based on uranium and cesium atoms only, did not result in a satisfying agreement of (summed) observed and calculated squared structure factors. Therefore an electron-microscope study was undertaken to obtain more information about the unit cell. From several powder particles of the diuranate electron diffraction images could be obtained confirming the proposed unit cell. Two examples of the electron diffraction images are shown in figure 6.2. In addition a neutron diffractogram was in accordance with the indexing of the X-ray pattern (see section 6.6). The heaviest Patterson peak could also be regarded to be the superposition of all U-Cs vectors in the unit cell, which allows a non- (pseudo)hexagonal uranium layer parallel to the ab-face. A model based on uranium and cesium atoms only in C2/m rapidly resulted in an R-factor of 11%. A difference Fourier synthesis revealed that all oxygen atoms are situated at reasonable distances from the uranium atoms. In further computations the 002 reflection appeared to have a very heavy weight in the least-squares refinement. After neglecting the 002 reflection the R-factor dropped to 5.8%. Scattering factors were taken from Cromer and Waber [2] and corrected for anomalous dispersion with Af', taken from Cromer [3D. The overall isotropic temperature factor was refined to

} 2 2 2 * R - Σ 1Ι η F , - η F . | / Σ η F , obs calc ' obs

o.-·»;.. ' 'i

-56-

• ί· h'

"3 ,

Ι f. Figure 6.2. Electron diffraction images of CsJJJ}^. Because of the tem- •¥< y< rn'ni'i' increase nf the sampla by electron absorption it is >u> clear to which phase (a or B) the diffraction images refer. The reciprocal axes have been indicated. After calibration with Mo, Al and Nb the following cell constants could be calculated for· Cs^UJ) : a = 14.b %, b = 4.3 %, a = 7.6 %. and β - lib0. Deviations from exact cell constants are caused by non-proper alignment of the crystallites. Electron diffraction images were taken using a Philips EM Z01, operated at 80 kV.

; !--Λ '. -57-

0.72(9) Χ , which is reasonably reliable considering the maximum (sin 6/A)2-value 0.167 S~2. In the final model the 0,-atom is located in a position which is only half occupied, as is shown in figure 6.3a. This fact might be caused by the choice of space group C2/m instead of C2. Since the

»,.._ •· '• R-factor was low, indicating that the uranium and cesium positions are reliable and the structure factors are rather insensitive for a change in the oxygen atom positions, the structure model has not been refined in other space groups. The coordinates of the final refinement have been listed in table 6.1, whereas table 6.2 contains the most relevant data of the coordination of uranium by oxygen. A steroscopic view of the structure has been given in figure 6.4. The observed and calculated

X-ray pattern of ct-Cs U 0? is given in appendix B2.

Table 6.1. Least-squares coordinates of a-Cs J) J)„ from the X-ray data.

Atom x/a σ(x/a) y/b o(y/b) z/c o(z/c) occupancy

U 4i 0.1465 0.0006 0.000 - -0.007 0.002 1.0 Cs 4i 0.3909 0.0009 0.000 - 0.562 0.002 1.0 01 4i 0.204 0.005 0.000 - 0.25 0.01 1.0 02 4i 0.401 0.006 0.500 - 0.27 0.01 1.0 03 4i 0.318 0.005 0.000 - -0.01 0.01 1.0 04 4g 0.000 - 0.241 0.030 0.00 - 0.5

Table 6.2.

Uranium-oxygen distances in a- and &-CsJJ„0„ from X-ray and neutron data.

Distance a Cs U e-cs u o ~ 2 2°7 2 2 7 X-ray neutron X-ray

U-01 1.81(9) 1.81(2) 1.87(7) U-02 1.87(10) 1.88(2) 1.95(6) U-03 2.49(5) 2.34(2) 2.24(4) U-03' 2.19(5) 2.26(2) 2.30(4) ϊ· ν U-04 2.38(7) 2.21(2) 2.15(1) -58-

Fignre 6.3.

• Uranium layers in a-Cs^U^O^ (a) and $-Cs£)£ (b), projected on the ab-face. Small oirales designate uranium atoms; large airoles designate

oxygen atoms; uranyl oxygen atoms are not shown. For a-Cs0U'Or, the 6 2 7 neutron diffraction data have been used, the O.-atom being statistically distributed over tuo positions.

*l'" Figure 6.4. ^m - •:· :. Stereoscopic view of a-Cs„UJ) along the b-axis (large spheres designate Η --^ ? • ^α cesium atoms, small spheres designate uranium atoms; oxygen atoms are not shown). Kfm I

;·:ί! \ ·*. \\

-59-

6.4. The X-ray analysis of e-Cs-U^

Α sample of U-Cs-U-Oy was heated at 600°C for 16 hours. Guinier films after quenching the structure did not show any impurities. The X-ray pattern of %-CsJSJ)-. was recorded as described for the α-phase. The pattern could be indexed with a C-face centered monoclinic cell with a - 14.516(2) X, b - 4.3199(6) 8, c - 7.465(1) % and β - 113.78° (1). Up to 62.00° 2Θ its profile was integrated to 55 intensities covering 82 zero, non-zero and overlapping reflections. The space group of β-Cs-U-O- was also assumed to be C2/m (hkl absent for h+k - 2n+l). For the structure refinement the uranium and cesium coordinates of the α-phase were taken as initial parameters. Oxygen atoms could be located in a difference Fourier synthesis. Again the 002 reflection was omitted from the refinement whereafter the R-factor dropped to 5.3%. The isotropic temperature factor Β » 0.15(15) A is less reliable compared

to the temperature factor of a-Cs_Uo0,, due to the smaller (sin θ/λ) - range. Final coordinates of β-Cs.U-O- are listed in table 6.3. The uranium coordination by oxygen is summarized in table 6.2, and shown in figure 6.3b. A list of observed and calculated structure factors is presented in appendix B3.

Table 6.3.

Least-squares coordinates of B-(7s„ü„CL from the X-ray data.

Atom x/a σ(x/a) y/b a(y/b) z/c a(z/c) occupancy

U 4i 0.1474 0.0006 0.000 - -0.004 0.001 1.0 Cs 4i 0.3978 0.0007 0.000 - 0.584 0.001 1.0 01 4i 0.206 0.004 0.000 - 0.27 0.01 1.0 02 4i 0.399 0.004 0.500 - 0.29 0.01 1.0 03 4i 0.294 0.003 0.000 - -0.04 0.01 1.0 04 2a 0.000 - 0.000 - 0.00 - 1.0

'•\f,.''•''. te

Α»^%ί*. -60-

'\\ ••

6.5. The X-ray analysis of Y-CS2U2O7

Notwithstanding the experimental problems with the synthesis of hexagonal Cs-U.O. a Guinier film (Ni-filtered CuKct radiation) could be obtained, with was clear enough to calculate the unit cell parameters. A least- squares refinement of 25 Q-values yielded a hexagonal cell with a » 4.108(1) & and c * i4.646(5) 8. These dimensions agree reasonably with the results of Kovba et al., who reported a * 4.106(3) X and c - 14.58(2) X [1], Possible space groups for the hexagonal γ-diuranate are P6-/mmc, PE2c and P6_mc (hh2fil absent for l»2n+l). The density, calculated from the X-ray unit cell, is 6.624(5) assuming one Cs.lLOy unit in the cell. The X-ray pattern of y-Cs-U.O- is listed in appendix CI,

6.6. The neutron analysis of a-Cs-U-Oy

Λ pure sample of a-CsJuyO-, was mounted on the neutron powder diffractometer at the Petten High Flux Reactor. The sample was contained in a cylindrical vanadium sample holder of 0.2 mm wall thickness and 20 mm ·••»>. • diameter. Monochromatic radiation with a wave length of 2.5715(4) A was obtained from a copper (111) - plane [4], Soller slits of 10' angular divergence were mounted between the reactor and the monochromator, and in front of the BF_ detector, respectively. At room temperature the neutron profile of the sample was measured frcm 150 to 3850 dmc 2Θ in steps of 2 dmc (10000 dmc - 360°) in about 4 days. The ratio of the peak heights to the background was rather low because of the high absorption of the neutrons by cesium.

The neutron data were used to refine the structure of a-Cs„U207 with a program written by Rietveld [5,6]. Applying the profile refinement method the following quantities were varied: the cell dimensions, the zero point of the 2Θ scale, the atomic position parameters, an overall

• · " £r'. isotropic temperature factor, a scale factor, the half width parameters

P, Q, R from the relation - Ρ tan θ + Q tan θ + R where bQ is the θ width at half maximum of a Bragg peak at angle Θ, a parameter which allows a correction for preferred orientation in the 001 direction and a parameter which corrects for asymmetric deviations of the observed peak from a Gaussian shape at low diffraction angles. In the refinement the Κ*Λ scattering lengths 0.85 * 10 cm, 0.558 χ 10 cm and 0.580 χ 10~l2cm were used for uranium, cesium, and oxygen, respectively [7]. •"* ;«*.'.•.%•

—O I

.I'.

First the structure of a-Cs U_07 was refined in space group C2/m

resulting in an R-factor of 12.9% (R * ΐ w | IQbs - I j | / Σ w Iobs). where the profile is converted to integrated intensities Γ63. Since the neutron intensities are much more sensitive for a change in the oxygen positions than the X-ray intensities are, the a-Cs-U-O, structure has also been refined in space group C2 with the neutron data. This refinement did not result in a better R-factor nor in better standard deviations for the least-squares parameters. Therefore C2/m is concluded to be the better space group for description of the a-Cs_U_O_ structure. Furthermore the following cell parameters resulted from the C2/m neutron refinement: a - 14.528(1) X, b - 4.2676(3) X, c - 7.6026(6) X and β » 112.986(7)°, all in good agreement with the X-ray cell dimensions. The overall temperature factor was 1.6(1) X and the corrections for asymmetry of the peaks and preferred orientation were small. The values of the position parameters are listed in table 6.4 and uranium-oxygen distances are collected in table 6.2. Figure 6.5 shows the fit between

-"·-• v«U observed and calculated profile.

Table 6.4.

Least-squarescoordinates of a-CsJ]J) from the neutron data.

Atom x/a a(x/a) y/b a(y/b) z/c a(z/c) occupancy

U 4i 0.1425 0.0006 0.000 - -0.006 0.001 1.0 Cs 4i 0.3935 0.0007 0.000 - 0.567 0.001 1.0 01 4i 0.1923 0.0007 0.000 - 0.252 0.002 1.0 02 4i 0.4133 0.0007 0.500 - 0.275 0.001 1.0 03 4i 0.3055 0.0007 0.000 - 0.005 0.002 1.0 04 4g 0.0000 - 0.171 0.003 0.000 - 0.5 j

6.7, Discussion

Especially in the investigation on the cesium diuranate structures a great influence of the heat treatment, the reaction time, the inclusion of impurities and the particle size of the materials on the structure of the sample was found. For example, a second sample of 6-Cs_U_0_ yielded a unit cell with a - 14.512(2) X, b - 4.2967(3) X, c - 7.535(1) and •fes. ••'ν.;'ί.·> '.'^V

- 1D8D ι fCPHB-CS2U207, 300 DEG K. LflMBDfi = 2.571514)

OBSERVED PROFILE CSLCULBTED PROFILE 880-j

480 Η

280 4 to

10 2 ° 30 40 SO 60 70 1Ï0 UO n£Tfl( DEGREES)

Figure 6.S. Neutron powder profile of

.-/'• •" . 'ΐ -βί-

ο - 113.35° on indexing the X-ray pattern [8], which is significantly different from the cell constants, reported in section 6.4. The γ-diuranate formation might also be influenced by the above mentioned facts, as may be concluded from the description in section 2.3.

Due to the poor crystallinity of γ-CsJJJO7 the unit cell reported in section 6,5, can be a subcell. In the next chapter it will be seen that K.U.O- and Rb„U„O_ samples show a splitting of the X-ray lines, causing these structures to be monoclinxcally deformed. The same might be true for the y-CsJJ.Oy crystal structure. However, the structure of γ-ϋβ,ϋ,Ο-, as described by Kovba et al. [1] will not be grossly incorrect: it consists of hexagonal uranium layers and hexagonal cesium layers, linked by uranyl oxygen atoms. The remaining oxygen atoms in the hexagonal uranium layer might well be ordered positions, in agreement with the description

U and Rb U in the sections for K2 2°7 2 2°7 7.3.3 and 7.5. When tables 6.1 and 6.4 are compared, the X-ray refinement and the

neutron refinement of the a-Cs2U.O7 structure result in the same coordinates. From the neutron data the oxygen coordinates are obtained more precisely as could be expected from the neutron scattering lengths. Both refinements result in a good agreement of the U-0 distances, which are listed in table 6.2. From the crystal structures presented in this paper the phase transformation from the α-diuranate into the β-diuranate can be understood as follows: the 0,-atom, which is statistically disordered in the .- ί α-structure (figure 6.3.a) occupies a fixed position in the fj-structure (figure 6.3.b). This results in a small rearrangement of all atoms in the diuranate structure modifying also the cell parameters. The most surprising result of this study is the arrangement of the uranium atoms in the diuranate structures. The other cesium polyuranates contain pseudo-hexagonal uranium layers [chapter IV,V] and the mono- uranate contains tetragonal uranium layers [chapter V]. The roonoclinic diuranate crystal structures are built up from layers which contain hexagonal as well as tetragonal arrangements of uranium atoms. This kind of uranium arrangement in layers has never been reported before. •?**

References

[1] L.H. Kovba, I.A. Murav'eva and A.S. Orlova, Radiokhimiya, 16, 648 (1974). τ >'

-64- I..-!: [2] D.T. Cromer and J.T. Waber, Acta Cryst., J£, 104 (1965).

[3] D.T. Cromer, Acta Cryst., JJ3, 17 (1965). '45 'ji. CA] B.O. Loopstra, Nucl. Instrum. Methods, 44^ 181 (1966). ι.':, [53 H.M. Rietveld, Acta Cryst., n, 151 (1967). [6] H.M. Rietveld, Α Program for the Refinement of Nuclear and Magnetic Structures by the Profile Method, update 1972 (1969).

[7] G.E. Bacon, Acta Cryst., A28, 357 (1972).

[8] E.H.P. Cordfunke, A.B. van Egtnond and G. van Voorst, J. inorg. nucl. Chem., 37, 1433 (1975).

9 . · '

V^ :**• ο, Jtivi'-IHX&R:-. i'éL'^jsüi;.' -65- I. :

CHAPTER VII. POTASSIUM AND RUBIDIUM URANATES

7.1. Introduction

In the foregoing chapters the crystal structures of the cesium uranateu have been described. To examine the influence of the alkali-metal ion radius on the composition and the crystal structures of the uranates structural information on the lithium, sodium, potassium and rubidium uranates is needed. Recently systematic investigations on lithium and sodium uranates have been published [1,2], A literature study of the potassium and rubidium uranates yielded several incomplete and even conflicting data. Therefore it was decided to investigate the K-U-0 and Rb-U-0 systems systematically. The results of this study are presented in this chapter.

7.2. Experimental

In general the experimental methods described in chapter II for the cesium uranates were used in this study. Carefully ground mixtures of amorphous U0_ and alkali carbonate reacted completely at 700 C within about 50 hours. The metal/uranium atomic ratios were accurately fixed in the starting mixtures as 4.0, 2.0, 1.5 and from 1.0 down to 0.1 in steps of 0.1. All samples were handled in a dry box, although only the M.UO, samples are rather hygroscopic. The uranium content was analysed in some cases according to the procedure described by Eberle et al. [7], i

7.3. Hexavalent uranates

7.3.Κ The potassium uranate system

From Guinier films of the reacted mixtures it was concluded that four distinct phases at K/U ratios 2.0, 1.0, 0.5 and 0.286 exist in air up to at least 700°C. Since uranium is entirely in the hexavalent state, the formulae of these compounds are K-UO,, KpU„O_, Κο^Δ^ιι an(* ^9^7^99

*) This chapter is a revised version of the paper by A.B. van Egmond and E.H.P. Cordfunke, J, inorg. nucl. Chem., in the press. e»?»«Bfflisaaw)Rs«5»a

7.1. Crystal data of potassium and rubidium wcanates(VI) **'

+) AXES j a.b.c d) CRYSTAL SPACE ***> } FORMULA ζ d calc ANGLES: α,β,γ (degrees) CLASS GROUP Μ * 4.3319(3) 4.3319(3) 13.185(1) K2UO4 90 90 90 TETRAG. 14 /nnnm 2 5.103(1) 96 6.9252(5) 7.9729(9) 6.9920(7) K2U2O7 90 109.623(8) 90 MONOCL. P2,/m 2 6.085(1) 44 14.307(1) 14.307(1) 13.998(2) HEXAG. P6 /m 8 6.629(1) 27 Wl3 90 90 120 3 6.9500(6) 19.525(2) 7.2121(6) K2Ü7°22 90 90 90 0.RHOMB. Pbam 2 7.114(1) 45 4.3548(3) 4.3548(3) 13.869(2) Rb2UO4 90 90 90 TETRAG. I4/mmm 2 5.972(1) 99 6.9472(5) 8.0175(6) 7.3276(8) Rb2U2O7 90 108.635(8) 90 MONOCL. P2,/m 2 6.519(1) 29 14.339(1) 14.339(1) 14.311(2) R HEXAG. P6 /m 8 6.939(1) 30 Wl3 90 90 120 3

R 6.9586(4) 19.609(1) 7.2781(5) W>22 90 90 90 0.RHOMB. Pbam 2 7.320(1) 59

*) See ref. [33. **) The complete X-ray patterns are published in the appendices. ***)Space group with highest symmetry is indicated [4], t) Unit cell parameters have been calibrated with α-quartz. A single crystal of α-quartz showed on a PW1100 single crystal diffractometer a hexagonal unit cell with a * 4.913(2) 8 and c « 5.405(1) 8. liüüi'·'^

-67-

respectively; their colours vary from yellow to orange. The X-ray patterns could be indexed from Guinier film data of samples which were heated at 700°C for at least 100 hours. The unit-cell parameters, which were calibrated with α-quartz, and the calculated densities of the potassium uranates(VI) are listed in table 7.1. The X-ray pattern of a mixture with K/U ratio 4.0 showed some unidentified weak lines at the initial stages of the calcination, but these lines disappeared after continued heating, indicating that a compound K.UO, is not stable at 700°C in air. The X-ray spectra of the potassium uranates are given in the appendices C12 - C16.

This system is very similar to the potassium uranate system. Thus, the

uranates(VI) RbJDO,, Rb„U_O7, Rb-U.O.. and Rb2U7O„2 have been found to exist, their colours varying from yellow to orange. The formation of the ·.· -1 rubidium uranates takes also place in air at 700°C. The results of the indexing of the X-ray powder patterns are given in table 7.1. It is evident that all rubidium uranates are isostructural with the corresponding potassium uranates. The X-ray patterns of the rubidium uranates are presented in the appendices C7 - CI 1.

7.3.3. The crystal structures of M_U,0 and M-U-O (M=K,Rb)

The crystal structure of K-U.O.» has been investigated by powder dif- fractometry. According to the systematic extinctions the space group can be P6_/m, P6, or P6.22 [4]. From a Patterson synthesis, based on 29 reflections, a layer structure with sheets perpendicular to the c-axis and consisting of 13 uranyl groups could be found. This model excludes the P6.22 space group. The remaining uranium atoms and the potassium atoms could not be located. Structure models with different interlayer positions for uranium and potassium never resulted in converging refinements. In all least-squares calculations, using scattering factors published by Cromer and Waber [5,6], a rather large shift in one of the uranium atoms was found, as indicated in figure 7.1. Assuming the interlayer uranium atoms to be grouped in uranyl groups the hexagonal layer composition can be deduced to be (UO-^QO-Q, as already suggested by Kovba and Trunova [28]. 11 ι-, > j IS ' 'Λ '. Κ -68-

.' ;/•$

I- - ih V. i I

'-'1 •3 Figure 7.1. 51

Uranium positions in pseudo hexagonal (UO )-J)^^ layers in ίίηί/.Ο . Ideal hexagonal positions are given by the lattice (a = b = 14.Z07U) &, space group P6_/m; the following

positions have been plotted: (.OO,.OO3.OO)} (.32,.24,.02) and (.60,.16,.01)).

The unit cells of potassium and rubidium diuranate have been reported : :·.ν -1 to be rhombohedral [13,15,17,21,24], From our results it follovs that both diuranates crystallize in the monoclinic crystal system with space group P2./m or P2 . The strong subcell reflections (hkl : k= 2n) lead to the following positions for the metal atoms in P2./m: uranium in 2a and 2e (x = 0.50, z = 0.00), potassium (or rubidium) in 2d and 2e (x = 0.00, z=0.50). In agreement with the sodium diuranate structure C23] the oxygen atoms in the layers may be located in 2b, 2e (x = 0.21, z = 0.00) and 2e (x = 0.79, z = 0.00) and the uranyl oxygen atoms in 4f (x = 0.10, = 0.00, z = 0.28) and 4f (x = 0.60, y = 0.25, 0.28) Mi •-

H-.i :·?. :. -69-

7.4. Pentavalent uranates

The formation of the pentavalent KUO, and RbUO, by the reaction of U02

with equimolar amounts of K?UO, and RbjUO, respectively at about 800 C in inert atmosphere has already been described by RÜdorff et al. [9,10]. • The authors mention that they could not prepare CsUO, in this way. This

is confirmed in our experiments, the compound Cs2U,O12 being formed instead [chapter II,III]. On the other hand, heating Rb„U,0 „ or mixtures

of Rb2UO, and U,0„ in the molar ratio Rb/U = 2.0 in inert atmosphere does not yield RbJl.O.,» but a mixture of RbUO, and UO,,. The formation takes place via several intermediate metastable phases. Although KUO, and RbUO, can also be obtained by thermal decomposition

of K2U2O or Rb2U2O7 at 850°-900°C, the reaction is very slow, U02 being formed gradually by the simultaneous decomposition of KUO, or RbUO, into

UO2< Decomposition into U02 takes place rapidly above 1050 C. Heating of

the uranates(VI) in hydrogen yields U02 at already low temperatures (~ 600°C). The X-ray patterns of both KUO, and RbUO, can be indexed with a perovskite-type cell, space group P23, in agreement with the observations by RÜdorff et al. [9,10], The data are summarized in table 7.2.

Table 7.2.

Crystal data of KUOV and o

cubic cell coiistant (X) X-ray compound colour ζ RÜdorff [9,10] this study density

KUO3 greybrown 4.290 4.2966(3) 1 6.806(1) RbUO. redbrown 4.323 4.3275(3) 1 7.635(1)

7.5. Discussion

In the literature many investigations on the potassium and rubidium uranates are found. The following potassium uranates(VI) are reported to

exist: K2Ug025 [19], KÔ^ [18,27], KÔjg [13,16,21,26], K2U4°13 CI3,I6,26.273, K U3°io C 11,16,19,24], K^^ [ 11-19,2», 23-273, 2 1 K2UO4 [14,16,17,19] and KÛOg [12,16,20]. However, in this investigation no indication was found for a compound Sift'

'J- •11 -70-

KnU_O-,. or K„0. nUO- (n>8) as mentioned by Anderson [19]. Kovba [27] : K U an( t ie u =r i 'i; reported by unit cell parameters of 2 7^22* * * ^ "·^ ystal structure of this compound some years later [183. Kovba's results are con-

firmed by ours, K,.U_0„2 being isostructural with Cs.U 0.. [chaptsr V], The phase region K„O. η UO„ (3 Sη i 6) has been a source of much confusion. Kovba and Churbakova [21] proposed a solid solution with limits

K2O(UO3) and K,U,0.-, called "tri-hexauranate". Recently the existence of this solid solution has been denied by Kovba [27], However, Toussaint and Avogadro [24] yet reported a triuranate in agreement with Kovba's earlier results [21]. Efremova et al. [16] reported the compounds K„U,0 ^,

υ Ο but Anderson 19 K„ü,0 and Κ2 3 ΐ0 E ^ interpreted the infra-red spectrum ϋ and a n her of K.U.O _ as a superposition of the spectra of Κ2 3°10 *8 uranate K.l) 0 ,. Furthermore Anderson [19] announced a single-crystal analysis of Ko^Vin w^^cn» however, was not completed [32]. Allpress et al. [13] described the decomposition of potassium halogeno-uranates into

K2U6°19 and K2U4°13' but Lucas ^" ^ detected K2U3°l0 in sim^lar decomposition experiments. Kepert [26] also described LOO . and K_U,0._ to be formed in the potassium uranate system. Our results are in contradiction with most of these statements: besides K„U,0 the only stable potassium polyuranate found is K_U,O._. The indexed unit cell is in agreement with the results of Kovba [27],

U0 lavers to be Kovba and Trunova [28] suggested ( 2^n°20 present in the Rb.U.O _ crystal structure, which is in agreement with our results for Wl3· As already shown' KJ3J)- crystallizes in the monoclinic crystal system. Anderson [19] already suggested that the previously assumed rhombohedral cell must be a subcell, which was confirmed by Hoekstra [14]. The subcell indicates that the metal atoms form pseudo-hexagonal layers, but the oxygen atoms in the U-0 layers are no longer forced to be distributed in fractionally occupied positions in an eightfold uranium coordination, as was assumed by Kovba [23,27]. No indication for a phase transformation in the diuranate structure [8] has been found.

s The compound K2^4 ^ well-known in the literature. It has a tetragonal uranium layer structure and belongs to the K.NiF, class [8,30]. From our high-temperature X-ray work no indication has been found for a phase transition of K-UO, into a cubic form as described by Kovba et mm al. [20]. Probably these authors observed the decomposition of K„U0, into KÜ0-, as may follow from their reported cell constant. PW?;·

-71- ft' -?·:

The existence of K.UO , as reported by Efremova et al. [12,16,20], could not be confirmed, which is in agreement with results of Kepert [26] and Hoekstra and Siegel [25], As in the case of the potassium uranates several rubidium uranates have been mentioned in the literature. Our results agree with a recent publication of Kovba and Trunova [28], except for the unit cell of rubidium diuranate. A compound Rb.U.O.-, described as the final product of the decomposition of Rb.UO-Cl, [29] could not be prepared by us. Probably the final product of these decomposition reactions is Rb„U,0 „, as indicated by Allpress et al. [I3J. Finally, the raonoclinic indexing of potassium and rubidium diuranate, given in this paper, is close to the indexing of Kovba [23] and Cordfunke and Loopstra [2] for the sodium diuranate, However on the basis of the primitive monoclinic unit cell for potassium and rubidium diuranate, it is still not possible to index all the reflections given for Na.U.O- in the latter paper.

References

[1] J. Hauck, J. inorg. nucl. Chem., _36. 2291 (1974).

[2] E.H.F. Cordfunke and B.O. Loopstra, J. inorg. nucl. Chem., 33, 2427 (1971).

[3] P.M. de Wolff, J. Appl. Cryst., ]_, 108 (1968).

[4] International Tables for X-ray Crystallography, Birmingham (1962). I [5] D.T. Cromer and J.T, Waber, Acta Cryst., Jj5, 104 (1965). [6] D.T. Cromer, Acta Cryst., JJJ, 17 (1965).

[7] A.R. Eberle, M.W. Lerner, C.G. Goldbeck and C.J. Rodden, NBL-report no. 252, (1970).

[8] Investigations in the field of Uranium Chemistry, V.I. Spitsyn, editor, ANL-report Trans-33 (1961).

[9] W. Rüdorff, S. Kemmler-Sack and H. Leutner, Angew. Chem., 74, 'r' 429 (1962).

JL' J . : [10] S. Kemmler-Sack and W. Rüdorff, Ζ. anorg. allg. Chem., 354, 'ί ' ''\ 255 (1967). = " :f Τ [11] J. Lucas, Rev. Chim. miner., I, 479 (1964). f, '1

*«·#··!--.-Ui,

.'Λ , ei' -.--- · if- -72-

[12] K.M. Efremova, E.Α. Ippolitova and Yu.P. Simanov, Vestnik Moskovsk. Univ., Khim. Set., 24, 57 (1969).

[13] J.G. Allpress, J.S. Anderson and A.N. Hambly, J. inorg. nucl. Chem., 30, 1195 (1968).

[14] H.R. Hoekstra., J. inorg. nucl. Chem., ^7, 801 (1965).

[15] N.C. Jayadevan, K.D. Singh Mudher and D.M. Chackraburthy, BARC- report-726, India, Bombay (1974). [16] K.M. Efremova, E.A, Ippolitova, Yu.P. Simanov and V.I. Spitsyn, Dokl. Akad. Nauk. SSSR, j24_, 1057 (1959).

[17] L.M. Kovba, E.A. Ippolitova, Yu.P. Simanov and V.I. Spitsyn, Dokl. Akad. Nauk. SSSR, J_20, 1042 (1958).

[18] L.M. Kovba, Eh. Strukt. Khim., _Π, 256 (1972).

[19] J.S. Anderson, Chimia, £3, 438 (1969).

[20] L.M. Kovba, E.A. Ippolitova and Yu.P· Simanov, Zh. Strukt. Khim., 2, 211 (1961).

[21] L.M. Kovba and T.I. Churbakova, Zh. Strukt. Khim., 2, 585 (1961).

[22] L.M. Kovba, Radiokhimiya, J_3, 309 (1971).

[23] L.M. Kovba, Radiokhimiya, JU, 727 (1972). • • β [24] C.J. Toussaint and A. Avogadro, J. inorg. nucl. Chem., 3£, 781 (1974). - ' 'ή [25] H.R. Hoekstra and S. Siegel, J. inorg. nucl. Chem., ^6, 691 (1964).

[26] D.G. Kepert, Thesis, University of Melbourne (I960); cited in [25].

[27] L.M. Kovba, Radiokhimiya, J_2, 522 (1970).

[28] L.M. Kovba and V.I. Trunova,' Radiokhiraiya, J^, 773 (1971).

[29] M.P. Vorobei, A.S. Bevz and O.V. Skiba, Zh. Fizich. Khim., 4£, 2434 (1974).

[30] L.M. Kovba, E.A. Ippolitova, Yu.P. Simanov and V.I. Spitsyn, Russ. Jrnl. Phys. Chem., 35_, 275 (1961).

•"· '·>ν-.'*- [31] W.H. Zachariasen, MDDL-report, 1152 (1946).

[32] J.S. Anderson, private communication (1975),

τ.- -73-

CHAPTER VIII. CRYSTAL CHEMISTRY OF THE ALKALI URANATES

8.1. Introduction ι?

In the foregoing chapters of this thesis the crystal structures of most of the cesium uranates, which were introduced in chapter II, have been described. In addition, some attention has been paid to some regions of V'f the Cs-U-0 phase diagram, for instance the phase transition a + B-Cs-U^ and the phase region Cs-U.O.- - Cs-U,-0]/:· F°* convenience the most 1 relevant structural information of the unit cells of the cesium uranates is listed in table 8.1. It also contains information on two compounds,

which were given the formulae Cs„U.0.o and Cs_U„O„., in chapter II. These z o is ί y ii compounds have not been investigated in detail because the influence of the (partial) oxygen pressure on their formation is not known accurately. Rubidium and potassium uranates have been discussed in chapter VII. The Na-U-0 system has been investigated systematically by Cordfunke and Loopstra \T\. Compounds with formulae Na.UO , Na.UO, (a and β), Na„U„C· (α,β and γ) and Na,U_O were found to exist as distinct sodium ι ζ. ι b / uranates(VI). Upon reduction of the hexavalent sodium uranates two uranates(V) with formulae NaUO, and Na_UO, are formed. The lithium-uranate system is more complicated than had been assumed for a long time [3]. Recently, Hauck Γ4] has reinvestigated the Li-U-0 system and surveyed the literature on lithium uranates. In the phase diagram of the pseudo-binary system Li.O-UO, the following uranates can be distinguished: Li,UO, (a and 0), Li.UO., Li„UO., Li,U O,_ (α,β and v), ob Hj i. t\ ο jr Ιο

Li„U_O _ and Li?U,O.q. Upon reduction of the lithium uranates(Vl), according to Kemml^r-Sack [51, a number of stable compounds is found:

Li7UO,, Li.UOj, LiUO.j, Li„U,0 . and a compound with composition 0.15 . In figure 8.1 the various compositions of the hexavalent alkali uranates are summarized. In this chapter the alkali uranates will be discussed from a crystallographic point of view.

'.•Λ 11 8.2. Structural characteristics of uranates(VI)

In 1960 Kovba Γ6] published a study entitled: "the regularities in the structures of uranates and their relation to the properties of the uranates". He proposed three characteristic types of uranate motifs, ,' • ';· •' m

mn iiiiiwiTW»r Table 8.1. Crystal data of ceeiwn uranates.

Crystal Space +) Axes : a.b.c &) "20 20 Formula ζ d Class group Angles: α,Β,γ (degrees) see Γ1 ] calc meas

Cs2U04 tetragonal 14 Aram 4.3917(6) 4.3917(6) 14.804(3) 39 2 6.604 - 90 90 90

a-Cs2U2O7 raonoclinic C2/m 14.528(1) 4.2676(3) 7.6026(6) 50 2 6.535 - 90 112.986(7) 90 ) B-Cs2U2O7* monoclinic C2An 14.516(2) 4.3199(6) 7.465(1) 51 2 6.619 6.52(12) 90 113.78(1) 90

Ï-CS2U2O7 hexagonal P6.,/mcc 4.108(1) 4.108(1) 14.646(5) - 1 6.624 - 90 90 120 r orthorhombic Pbcn CS4U5°17 18.776(4) 7.070(1) 14.958(3) 25 4 6.669 6.62(15) 90 90 90 C-2°*°I3"> orthorliombic Cracm 13.494(2) 15.476(2) 7.911(2) 33 4.8 6.879 6.82(17) 90 90 90 monoclinic C2/m 13.465(2) 15.561(2) 7.964(2) 30 4 6.822 - 90 92.78(1) 90 Cs2U7°22 orthorhombic Pbam 6.949(1) 19.711(2) 7.3955(8) 37 2 7.488 90 90 90 CS2U,5°46 orthorhombic Cm» a 14.686(3) 13.422(2) 19.752(3) 23 7.800 - 90 90 90

a-Cs2U40!2 rhombohedral R3m 10.9623(6) 10.9623(6) 10.9623(6) 44 4 7.110 7.16(20) 89.402(7) 89.402(7) 89.402(7) 6-Cs2U4°.2*) monoclinic P2, 7.886(1) 8.002(1) 10.793(2) 38 2 6.882 - 90 92.62(1) 90

Y-CS2U4O12*> cubic Fd3m 11.2295(6) 11.2295(6) 11.2295(6) 88 4 6.613 - 90 90 90 monoclinic CS2U6°18 4.137(1) 13.471(1) 8.089(1) 60 1 7.300 - 90 90.37(1) 90

I CS2U,O27 orthorhombic primitive 14.956(4) 10.571CJ) 3.9856(7) 16 7.484 90 90 90

*) unit cells of 3-Cs2U2O? at 600°C, g-Cs2U4Ol2 at 660°C, y-Cs^O^ at 880 C. } For Cs U.0, and Cs„U O,, subcells are given; CsJJ.O., at 800°C, Cs/U-ratio was 0.42. .» 2o 4 13o 2 5c 16 Ζ 5 ID t) Unit cell parameters have been calibrated with α-quartz, see note at table 7.1.

MMa^ 8.0 4.0 2.0 1.0 05 025 0.125 M/U-ratio

Figure 8.1.

The compositions of alkali uranates(VI).

all with composition [(U02)0_]: 1. hexagonal uranium layers, 2. tetragonal uranium layers, 3. infinite uranium chains. Especially in the alkaline-earth uranates(VI) Kovba pointed out a systematic influence of the metal-ion radius on the crystal structure of MUO, compounds. However, in case of the alkali uranates such a regularity was not obtained. Some years later Keller [7] gave a systematic description of crystal structures and bonding in uranates(VI) in his Habilitationsschrift "Uber die Festkörperchemie der Actiniden-Oxide". After an extensive literature survey Keller distinguished six types of uranium-oxygen motifs, also present in other metal-actinide-oxygen compounds:

fatfe -76-

(a)

a. •

Λ. s

Figure 8.2. Uranium-oxygen motifs according to Keller [7]j see section 8.2: (pseudo)-hexagonal layers of (UOgJOg-cubes with composition i(UO2)O2l (a), (pseudo)-tetragonal layers of (W^^O^-octahedra with composition [(UO2)C>2~} (b), endless chains of (U02)0^-oatahedra with composition [Ci/OgJOg] (°)> and endless chains of (UO4)02-octahedra with composition L(UO4)O1 (d). The black spheres are more or less in one plane; the large ones designate oxygen atoms, the small ones uranium atoms. The open spheres designate oxygen atoms above and below the plane of the black spheres^ forming the uranyl groups in (a), (b) and (c), and forming part of the "Og-square" in (d).

: ιΐ'ρι-'·:·'•:[;&/•:. *'..:„'y' ."ί"'ν.'·.'.ι.;.,. . ''•Λ' - .-V--..> iafeij^ -77-

1. (pseudo)-hexagonal layers of (UO_)O,-cubes with composition [(UO.JO^] 2. (pseudo)-tetragonal layers of (UO,)0,-octahedra with composition

[(UO2)O2]; 3, endless chains of (UO-)O.-octahedra with composition [1(02)02]; 4. endless chains of (UO,)0.-octahedra with composition [(UO.)O]; 5. isolated UO,-octahedra, linked by metal ions; o ,6+ 6. regular UO,-polyhedra with a statistical distribution of U and t> metal ions. It should be noted that the last type referred to a cubic compound a-Na,UO_, which, however, does not exist, as was shown by Cordfunke and

Loopstra Γ2]. Also the other actinide compounds a-Na,XO5 (X*Np.Pu.Am), which were mentioned by Keller, have not been published in the literature. The first four types are shown in figure 8.2. Keller also extended the group of alkaline-earth uranates(VI), described by Kovba [6], to a group of bivalent-metal uranates(VI). In this way he found that the uranium-oxygen bonding type in these compounds is determined by the radius of the metal ion, as is shown in table 8.2,

Table 8.2.

The influence of the cation radius on the uranium-oxygen bonding type, according to Keller i7l. The type numbers are •A • I explained in section 8.2.

cation cation uranate(VI) type uranate(VI) type radius (A) radius (A)

Li UO 0.68 1+2 MgU0 0.74 3 2 4 4

Na U0 0.98 2+3 CdUO^ 0.99 3+1 2 4

K UO 1.33 2 CaU0 1.04 1 2 4 4

Rb2UO4 1.49 2 SrU04 1.20 1+2 2 PbU0 1.26 Cs2UO4 1.65 4 2

BaUO4 1.38 2

Both, Kovba and Keller, emphasized in their studies that some minimum uranium-uranium distance corresponds to each type of uraniutn-oxygen bonding.

'-.' w •**ίκ •„Siari.'-isi VJLL^ •'•'·*

-78-

Independently from Keller Prigent and Lucas [8] distinguished three types of crystal structures for alkali mono- and diuranates, similar to those proposed by Kovba Γ63.

8.3. Uranium(-oxygen) motifs in alkali uranates(VI)

The alkali-uranate(VI) crystal structures have been restudied since a decade and have been established fairly well at present. Only the lithium uranate structures have not yet been elucidated entirely Γ A]. Neverthe- less it is possible by now to characterize the alkali-uranate(VI) crystal structures along similar lines as was done by Keller in the case of the alkaline-earth uranates. This characterization of the crystal structures is given in figure 8.3.

1.0 0.5 025 0.125 M/U-ratk)

Figure 8.3.

Struatuval characteristics of alkali uranates(VI); (for explanation of symbols, see section 8.3).

• ït-':'\ The following types have been assumed to be characteristic: : J I.* ' 1. L, stands for a (pseudo)-hexagonal arrangement of the uranium atoms in mutually isolated layers; 2. L means a (pseudo)-tetragonal arrangement of the uranium atoms in mutually isolated layers; -79-

3. CJJQ o stands for mutually isolated endless chains of (U0_)0,- octahedra, sharing oxygen edges, with composition [(U0_)0_]; the chains are linked by metal ions; 4. CjjQ o i-s short for mutually isolated endless chains of (UO^i^- octahedra, sharing corners, with composition C(UO,)O]; the chains are

•u···" linked by metal ions; 5. ISOL means a structure, which contains mutually isolated UO,-octahedra, linked by metal ions; 6. 3D stands for a structure type, which can be regarded as an ot-U_Og three-dimensional network of uranium-oxygen bonds, in which alkali- metal ions have been introduced. In the first two types no mention is made of the oxygen atoms in the (pseudo)-hexagonal or (pseudo)-tetragonal layers, which makes these two types not entirely comparable to the corresponding types of Keller,

J that were mentioned in section 8.2. Keller used the CaUO, crystal structure as an example of the hexagonal layers, in which the uranium atoms

. '· •/ are coordinated by eight oxygen atoms, forming (IK^O^-cubes. However,

•il % this coordination is not found in any of the (pseudo)-hexagonal layers of the alkali-uranate(VI) structures. In these structures the uranium atoms are coordinated by six or seven oxygen atoms. The types 3, 4 and 5 introduced here are comparable to the corresponding Keller types, but type 3D is not the same as type 6 of the Keller classification, which is not encountered in any real crystal structure, as was mentioned in section 8.2. In figure 8.3 several of these type symbols have been introduced for one compound, when that compound exhibits two or more phases

(CsJ 0?, Na-U.O-, Na.UO,, Li-UO,) with essentially different crystal structures. From figure 8.3 it is evident that the crystal structures of the alkali uranates(VI) can be classified by the following rules: 1. with increasing alkali-metal radius a layer structure is preferred; for large alkali-metal ions chain structures or isolated-octahedra structures are not found; 2. in layer structures a hexagonal arrangement of the uranium atoms is found for M/U < 2.0, while tetragonal layers occur for a ratio M/U » 2.0; only in the case of cesium a mixed tetragonal-hexagonal layer structure is found to be the stable phase (in figure 8.3 the

symbol L is used); , J*, \- • ;·

' -".•''» •.! ·· 1 - 3. small alkali ions can be introduced in an ct-U_OQ lattice. J O -80-

The first rule can be understood from the fact that small ions prefer low coordination numbers, whereas large ions prefer high coordination numbers. On comparison of the figures 8.2a and 8.2b rule 2 is more or less self-evident: the area covered in a tetragonal uranium motif is larger than the area covered by a hexagonal uranium motif, given a fixed number of uranium atoms. Consequently the tetragonal layers leave more space for penetration of the alkali-metal ions into the layer. The third rule follows from the neutron study of the ot-UO, crystal structure by Greaves and Fender [9]. These authors established the a-U0_

structure to be a uranium-deficient a-U-0o structure. These rules will be illustrated by a detailed discussion of figure 8.3 in the following sections.

In this region of figure 8.3 two almost complete series with formulae Μ UO, and M_U 0 are present.

Starting with the hexagonal layer structures of Na2U„Ü7 [10] and of

Κ U 0 and Rb.U 0? [chapter VII], in which the interlayers have been completely filled with alkali ions, it is plausible that, on increasing I the M/U-ratio and preserving a layer structure, the uranium layers have to form tetragonal motifs.

In the case of cesium a metastable hexagonal phase Y-CS_U.O7 is found. Due to the large radius of the cesium ion the average U-U distance in the hexagonal uranium layers has been increased to about 4.1 X

(in Na2U2O?, K^O.,, Rb^O^ 3.95 - 4.00 R). This causes the introduction of tetragonal motifs in the stable phases of Cs U_0_, resulting in layers with both hexagonal and tetragonal motifs. A stable (pseudo)- hexagonal structure is also possible when cesium is removed from the interlayer, i.e. the Cs/U ratio is lowered. This explains the existence of the (pseudo)-hexagonal crystal structure of Cs.OJ) . Since β-Na.UO, is a metastable phase [2], the formation of tetragonal layers in Li_UO, seems doubtful. However, there is evidence Μ that the lithium ions are in the tetrahedral interstices rather than in the ninefold coordination of the K_NiF,-structure [11]. Both β-Na.UO, and Li Ü0, are orthorhombically deformed, which is due to the tendency for low coordination numbers for sodium and lithium ions. This tendency % m -81-

also makes plausible that the chain structure of a-Na.UO, [12] is stable with respect to β-Na.UO, [2]. Whereas in the case of cesium the Cs/U-ratio is lowered from 1.0 to

0.8 to form the (pseudo)-hexagonal structure of Cs.U.O 7> the lithium ion is so small that additional lithium oxide can be introduced in the hypothetical lithium-diuranate structure, increasing the Li/U-ratio to

1.2 in the compound Li,UcO1Q (= LiU- Q-0,) [4], On the other hand one could assume uranium to be introduced in the interlayer of the sodium diuranate structure, to form Na.U 0,,, but evidence for this suggestion is not found in the literature [2].

Since lithium and sodium prefer tetrahedral coordination by oxygen the chain structures of Li.UO- and Na.UO can be understood. Potassium, rubidium and cesium are too large for tetrahedral coordination, explaining why the meso-uranates(VI) of these elements have never been found (see

\''•).•?' chapter VII, [13-15], chapter II). In Li,UO, (a and g) so many lithium ions are present in the structure, that the chains are broken up and isolated UO.-octahedra are found o in the structure [16]. The arrangement of the UO,-octahiidra in Li,U0, is different from the arrangement in Ba_U0,, which was used by Keller [7] as an example of an isolated ϋθ,-octahedra structure (the crystal structure of Ba,UO, might be slightly more complex than was assumed by Keller [17]).

The small lithium ion can be introduced in the ci-U„0o structure without disturbing the three-dimensional uranium-oxygen lattice [4]. The crystal structure of a-U.0 is shown schematically in figure 8.4a. 3 iso For sodium the same structural fact was established by Greaves et al. [18], who prepared a sodium uranate(VI) with the formula Na u|_v/6°3» °* ^x 0.165. By electron diffraction they found the sodium ions to occupy the uranium vacancies in the a-UO. structure [9,18]. Β Because of the variable Na/U-ratio this compound has not been included in figure 8.3. In the LiJJfp.n structure [3,4] the lithium ions are probably statistically distributed, whereas Li-U.O.» can be regarded as a three- te loft

" 7

:'"*v •'• •'•',••.:" ·': " l' v 4 ! • •••·•' '..•;-^fe;;„ .'^.ΐ.,'.-τ'.-^^ίΐ'^νν^ *;i/;. - •'·' r^..;S£'' '-- ."" '' K" ·; '•" " ^"'/Ail> , ,.«.,;< k*^^taa^ii-iix£'i I«*J f •• "Ki •• ν'

-82-

•'S ü:'.'. 'I .•'S •••». •m ι

• (a) · (b) Figure 8. 4.

•Schematic view on the orientation of the uranyl groups in the crystal i :

structures of a-U„Co (a) and'• &-U0- (b). In a-U-O. 120-221 a three- o O o o o dimensional uranium-oxygen network is obtained by -O-U-0-U-O-chains connecting the hexagonal uranium layers; in B-UO [231 the layer o character is preserved by the orientation of the uranyl groups in the interlayer. dimensional uranium-oxygen network, in which lithium is introduced in an ordered way [19]. However, little is known with certainty about the lithium positions in these structures, because lithium cannot be located by X-ray powder diffraction and neutron diffraction studies concerning these compounds have not been performed.

.3. A. and the series (M = K,Rb,Cs)

Cesium can also be introduced in a uranium-oxide structure in a low concentration (Cs/U = 0.133), but the cesium ion is too large to fit in the

a-U30„ lattice (figure 8.4a). Therefore the host structure for Cs.U 50 , is the β-uo., structure, which is built up from (pseudo)-hexagonal uranium layers, connected by uranyl groups, as is shown in figure 8.4b.

The M.U7O22 structures are related in a similar way to the β-UO, structure. The interlayers of these uranate structures contain uranyl groups and alkali ions in equal amounts (see chapter V, VII, [24]). When the crystal structures of Cs.U.O _ and Cs' U_0„. are compared, a "simple" structural relation is found: half of the cesium positions in Cs.U.O _ have been occupied by uranyl groups in Cs.UyO-.. This relation

is shown in figure 8.5. Applying this procedure once more to the Cs_U70

i >

-•'•'V : ' 'ét .··'•-'t' I -83-

• fi

••'/

Figure 8.5.

The interlayers in Cs^ü^O^ (a) and Cs8U?Og2 (b). The Cs^^O^ inter-

layer is formed by oesi:m atoms only; the interlayev in the Cs U?0^ structure consists of cesium atoms and uranyl groups (compare to

figures b.P, and 6.3). Cs;:,U„(),,„ has been assumed to be isostruotuval

with κφ?0νΑ \'λ4Λ.

structure, transformed to the large unit cell of the cesium uranate

with the lowest Cs/U-ratio, a formula "Cs„U,fi0 " can be deduced [25]. From the fact that the formula of this compound is found to be CS-U..O.

2+ 2 15 4 [chapter V], one has to assume that a uranyl group (UO„) occupies a slightly larger area in the interlayer than a cesium atom.

§i2i5i_The_tetra-uranate_region

In K-U.O.™ and Rb„ü,O._ the interlayer is occupied by alkali ions and •'•'•• é' uranyl groups in the ratio 8/3 (chapter VII, [26.]). Therefore the distance between the uranium layers is mainly determined by the radius of the

•'"•&,

-Jfc . * i»'.Tig»grrW>l'' '-TT 'r7fCli' n.", :

-84- !*•• · Ί) ..... t." EP; i

alkali ion. Apparently when the potassium or rubidium atoms are replaced I by cesium atoms, the distance between the uranium layers becomes too large for interlayer uranyl groups to form a stable coordination with If the oxygen atoms of the uranyl groups in the uranium layers. Therefore a double uranium bridge is introduced (figure 4.1), which stabilizes the Cs_U,O._ structure after folding the hexagonal uranium layers. Cs„U,.O., has a similar structure and forms a solicd solution with Cs.U.O.» at elevated temperatures (see section 4.5). 2 4 13

8.4. Uranium-uranium distances and uranium-oxygen bonding

Kovba [6] and Keller [7] suggested a connection between the uranium-oxygen bonding type and the smallest uranium-uranium distance in a crystal structure. However, their assumptions were based on incomplete and sometimes even erroneous data, as will be shown. The diuranate structures were believed to contain perfect hexagonal uranium layers. Then the smallest U-U distance can be calculated from the

unit cell parameters only [7]. Since monoclinic deformations for Na„U_07

[2,10], K2U"2O and Rb^Oj [chapter VII] and Cs2U2C>7 [chapter VI] have been established by now, the smallest U-Ü distances can only be calculated when the complete crystal structure or at least the metal positions are known. Keller reported the shortest U-U distance for hexagonal layers to be 3.91 A. But on inspection of the cesium-uranate structures it is

found that Cs.U 0 ? contains U-U distances as small as 3.56. X (figure 8.6c), whereas in the Cs.U ,0,, structure the U-U distance is even 3.51 8. In table 8.3 the ranges of U-U distances in some typical

Table 8.3.

Ranges of uranium-uranium distances in some (pseudo)-hexagonal uranium layers.

compound range A

Cs4U5O17 3.56 - 4.42

CS2Uy022 3.66 - 4.38 CS2U.5°46 3.51 - 4.38 a-U-Oj, [20-22] 3.75 - 4.18

3-UO3 [23] 3.76 - 4.08

';••• ;;>,'-7T$: • -Υ-..'•-! κ'ï4'";-;f -«ίΤ-'-;: ff .*• -;p^t0?M^ if ' \ "': ; '•* ^^JSg f'

-85- y., r: * (pseudo)-hexagonal uranium layers are collected. Similar ranges can be 1%·,:·. obtained from K^O^ [chapter VII], K^O^ [24] and Na2U2O7 [10]. For the tetragonal layer structures Keller reported the smallest U-U distance to be 4.36 X. However, the tetragonal parts of the ot- and ,! •-••.! B-Cs-ILO. uranium layers (figure 8.6a and 8.6b) contain distances of 4.18 X and 4.30 X, respectively.

(a) (b) . a.

Uranium-oxygen layers in a.-Cs^U2O? (a), $-Cs2U2O? (b) and Cs.UJ) (a). For the numbering of the atoms see figures 6.3 and S.I (small spheres designate uranium atoms, large spheres designate oxygen atoms). The values in the figures are mutual uranium-uranium distances (in R).

Finally the shortest U-Ü distances in Na.UO,. and Li.UO are 4.64 X and 4.46 X [13], in contradiction with the values given by Keller [7].

h •• The 5.40 X Ü-U distance in the ISOL structure of a-Li,U0, [16] is much smaller than the minimum 6.21 A for isolated-octahedra structures, reported by Keller [7]. It can be concluded that the assumed connection between the shortest v U-U distance and the uranium-oxygen bonding type in alkali-uranate(VI) ii -86- 'ΐ' I.

structures is less pronounced than suggested by Kovba [6] and Keller [7], although of course in general the U-U distances in an ISOL structure are greater than in the other five structure types. From the values given and those listed in table 8.3 it is evident that the ranges of U-U distances found in each of the types L , L , o > Cyo 0 an(* ^ overlap. Therefore it is worthwhile to examine the positions of the oxygen atoms in connection with the U-U distances. It appears that the oxygen coordination polyhedra of the uranium atoms with a short mutual U-U distance (3.5 - 3.7 A) share an edge, whereas for longer U-U distances (4.3 - 4.5 A) a corner is shared. This fact is also illustrated in figure 8.6.

8.5. The composition of the uranium layers

After the work of Zachariasen on the crystal structures of a-UO_ [273 and CaUO, [28] it has been believed for a long time that uranium(VI) ions in hexagonal layers are coordinated by eight oxygen atoms normally. Two of these oxygen atoms form the linear uranyl group, the other six are arranged in a puckered ring perpendicular to the uranyl-group axis. Such perfect hexagonal layers (figure 8.2a) have the composition [(UC>2)0~], which can also be expressed by an O/Ur-ratio of 2.0. Layers in which 0/Ur<2.0 were believed to contain statistical oxygen vacancies [6,7,8,29,30,31]. Several investigators [9,10,20-24,32,33] have shown the latter hypothesis to be wrong. Indeed many uranate(VI) structures have been established, which contain (pseudo)-hexagonal layers with a variety of compositions, the uranium(VI) ions being coordinated in the hexagonal plane by four or five oxygen ions in an ordered way. The variety of 0/Ur-ratios has been expressed in table 8.4. In the column "layer notation" the layer structure has been summarized. For example the notation Cs (U0,>) (U0_) Og in the case of Cs U 0 „ means a layer structure built from sheets with two cesium atoms and two uranyl groups and sheets consisting of five uranyl groups bonded in the layer by eight oxygen atoms. Most of these layer notations have been derived from the foregoing chapters or from the literature. The layer notation of Cs-U,0 _ and Cs.U 0 ,, as listed in table 8.4, needs some further explanation. In figure 8.7 a detailed picture of the double uranium bridges in Cs U,0 and Cs_U,0 , is given. Since the U-U ».- -" -nimjlwnriVT\r~

-87- R ••'••',·

I1 r

Oxygen-uranyl ratios in uranium layers.

compound layer notation type M/U **> 0/Ur +^

a-U3O8 120] (u3o3)o5 hex. 0.0 1.67 β-UO [23] hex. 0.0 1.67

CS U Cs (UO )5.(UO ) O hex. 0.133 1.60 2 ,5°46 2 2 2 10 16

CS U Cs„(UO,)„ . (U0,),0 0.286 1.60 2 7°22 2 2 2 2 5 8a hex. 0.4 1.75

CS U 4[Cs,(U 0~) . (U0„)._0 _] hex. 0.5 1.70 2 4°13 9 CS U Cs . (UO ) O hex. 0.8 1.40 4 5°17 4 2 5 7

CS U Cs_ . (U0_)~0, mixed 1.0 1.50 2 2°7 Cs_ . (U0„)0„ tetr. 2.0 2.00

M U M (UO ) . (UO ) O hex. 0.286 1.60 2 7°22 2 2 2 2 5 7

M U 1[M (ÜO ) .(UO ) O ] hex. 0.5 1.54 2 4°13 8 2 3 2 13 20

M U hex. 1.0 1.50 2 2°7 M2 . (UO2)O2 tetr. 2.0 2.00

**) t) *) Μ = K,Rb Μ = K,Rb,Cs in the uranium layer

distance in this double uranium bridge is about 3.5 A [chapter IV] the oxygen polyhedra share an edge. Together with the oxygen atoms of the (pseudo)-hexagonal layers the uranium atoms in the bridge can form stable coordinations. From this structure model the layer notation in table 8.4 can be derived. From the column "layer notation" the 0/Ur-ratios of the layers can easily be calculated. From table 8.4 a relationship between the layer- composition, i.e. the 0/Ur-ratio, and the type of the uranium layer is not found. Apparently the radius of the metal ion and its "concentration" play a more important role than the 0/Ur-ratio, as was'stated earlier in section 8.3, rule 2. A relationship between the M/U-ratio and the composition of the layer is not found either. As a next point the ranges of U-U distances in the layers, as listed in table 8.3 can be considered. Confining ourselves to the (pseudo)- hexagonal uranium layers, a relationship between the U-U distances and the 0/Ur-ratios is not found.

Μ- : -j -88- i.-: iS

if. • «:

•·§

Figure 8.7.

Bonding in the double uranium bridges in the CaJJ.O.^ and CsJJO1R structures (compare to figure 4.1). The small black spheres form the double uranium bridges; the large open spheres designate oxygen atoms. The black uranium atoms can form wcanyl bonding with the uranyl oxygen atoms bonded to the shaded vacanivm atoms in the (pseudo)-hexagonal layers.

Therefore it must be concluded that it is not possible to formulate a general rule concerning the coordination of uranium by oxygen in relation to the layer composition. Overlooking the (pseudo)-hexagonal layers of Cs,U-O._, Cs.UyU-- and oc-U~0„, as reproduced in figure 8.8, an increase of 4-coordinated uranium atoms (in the layer plane) and 3-coordinated holes seems to be met when the O/Ur-ratio decreases. However, this tendency does not hold when the (pseudo)-hexagonal layers of a-U_O and g-UO_ are compared (figures Jo j 8.8c and 8.8d): the layers in these compounds have the same composition but the coordination of uranium is different, whereas the mean U-U distances in these layers are almost equal, as follows from table 8.3. The same conclusion can be drawn upon comparison of the layers in a-U_O„ and

6-U3Og [21,22].

1 ti 4 -89-

ι ;:

(α) f Λ 'I " •' ·

Figure 8.8. Oxygen coordinations in (pseudo)-hexagonal „\-mium layers in Cs UJ) (a), Cs2U7°22 ^> a~U3°8 ^ an^ ^~U03 ^> · The >.vts designate the uranium positions in the layers, the thick lines give 'he unit cells of the structures; 3-coordinated "holes" in the layers have been shaded.

Esfe -90-

8.6. Uranium layers in Cs^U.O., and CsJ Ο ,

In the (pseudo)-hexagonal layers, which were studied in section 8.5, sixfold and sevenfold coordinations of uranium by oxygen were always

found. In the a-U,08 uranium layers, as shown in figure 8.8c, all space between the uranium atoms has been occupied by oxygen atoms, all uranium atoms being sevenfold (5+2) coordinated. This suggests that the O/Ur- ratio 1.67 of ot-U.O- is a limit for (pseudo)-hexagonal uranium layers. 5 o However, the 0/Ur-ratios in CsJJ.O,- and Cs.U.O,, are 1.70 and 1.75, ί ή 1J /3 1ο respectively. The assumption that eightfold coordinations (6+2) are present in these hexagonal layers is obvious, but there are more ways possible to

explain the high 0/Ur-ratios in Cs2U,0 _ and Cs U O . Firstly the interlayer might contain oxygen atoms in addition to the double uranium bridges and cesium atoms, but this has never been observed in any uranate structure. A similaSecondlr facy t thhaes hexagona been observel uraniud imn layerthe a-U0s migh_ structuret be uranium-deficient, which is a . uranium-deficient a-U-0 structure [9]. As a consequence of the uranium deficiency and because of the charge neutrality also some oxygen must be missing in the Cs.U.O.- structure. In addition the Cs/U-ratio should be somewhat higher than 0.5. The uranium deficiency in the (pseudo)-hexagonal

layers is suggested by the chemical relation between Cs.U.O and Cs_U,0]2 according to the equilibrium reaction

which was discussed in section 2.4.2. Although this chemical relation between Cs„U,0 „ and Cs-U.O.- does not necessarily imply a structural relation between these two compounds, it is striking that in the γ-Cs.ü.O . structure also hexagonal uranium layers exist, in which part of the uranium atoms is missing, as indicated in figure 8.9. An increase of the Cs/U-ratio in Cs.U.O.j has been observed (see next paragraph), but carries little weight because of the low accuracy of the cesium analysis. Finally one could imagine the (pseudo)-hexagonal uranium layers in Cs.U.CL, and Cs_U,0., to have an O/Ur-ratio of J.67 by oxygen deficiency, which implies that part of the uranium atoms is in the pentavalent state. This possibility is supported by the colours of Cs„U,0._ and Cs-U.O.,: they are much darker (yellow-brown, sometimes even deep-red) than the colours of the other cesium uranates. Supposing an 0/Ur-ratio of 1.67 mm 11 . 1 :'- -91-

VI V VI V

in the layers the formulae Cs6U);)U2O47 and CsgUJ7U 05g are derived for "Cs-Uj-O ," and "Cs^U.O " respectively. A chemical analysis of a Cs„U,0 sample (see table 8.5) led to the formula

Cs„uY* ,Ίϋη n,0,_ _., which implies that this explanation for the high O/Ur-ratios should be rejected, where the accuracy of ••he cesium analysis is over-estimated.

Table 8.5.

Chemical analysis of CSJJ\0

element content (%)

Cs 18.78 U(tot) 66.57 U(IV) 0.277

Figure 8.9.

Hexagonal uranium layers in y-CspU 0 „. The plane drawn in (b)} has been indicated in cube (a), which should be compared to figure 3.2a.

8.7. Influence of the metal radius on the interplanar distance in layer structures of uranates(VI)

The influence of the alkali-metal ion is clear from the interlayer distances of the crystal structures, i.e. the distance between the succeeding hexagonal or tetragonal uranium layers. In figure 8.10 the Interlayer distance (A)

«5

0] ro «9 Co I

Cl Cb

g·

s 3 f! -93-

; 'ft

interlayer distances of the alkali uranates(VI) are collected. In general the interlayer distance increases with the alkali-metal ion radius, as could be expected. In the case of potassium and rubidium the interlayer distance decreases upon introduction of additional alkali ions in the interlayer· This might be understood by considering the negative charge on the oxygen atoms and the positive charge on the alkali ions. Since potassium and oxygen have similar radii, there will be an increase of interlayer distance on substitution of an alkali ion by an uranyl group, because in the former case there is alkali-metal/oxygen attraction and in the latter case there is oxygen/oxygen repulsion. In the case of cesium the metal radius is so large that the interlayer distance will depend largely upon the cesium ion. In Cs_U 0 , and Cs U.O., the double bridges cause a very large int^rlayer distance. Since the penetration of alkali ions in the tetragonal motif is greater than for the hexagonal motif, the interlayer distance in a- and

B-Cs„U„O7 is rather short. This tentative structure description is illustrated in figure 8.11. From figure 8.10 it also follows that at low M/U-ratios the uranate(VI) interlayer distances approach that of β-ÜO..

Figure 8.11. Schematic view of the interlayers of some alkali uranates(VI). The blaak bars designate the (pseudo)-hexagonal uranism layers, whereas the open bars designate the (pseudo)-tetragonal uram'-.-n layers. The two concentric circles denote a projection of the uranyl groups in the interlayers. The large open spheres denote cesium and potassium atoms3 respectively. In the figure a penetration of the alkali ion into the tetragonal layer is suggested, in contrast to the hexagonal layer. The double bridges make the situation in CsÛ,£>]•£ and CsÛÔ-jg rather camples. Mw· -94-

1 > 8.8. Uranates(V) with the perovskite structure 'Λ, In the foregoing sections of this chapter much attention has been paid to the crystal structures of alkali uranates, in which uranium is in the hexavalent state. In this section some alkali uranates(V) will be considered. In the introduction of this chapter the compounds LiUCL and NaUO» have been mentioned. Together with KUO_ and RbUO- (section 7.4) a series is formed, in which the influence of the alkali-metal ion radius can be studied. A compound CsUO- has not been found, neither in this investigation (chapter II) nor by Kemmler-Sack et al. [5], The crystal structure of LiUO_ is rhombohedral, isostructural with LiNbO [34], The compounds NaUO,, KUO. and RbUO, have perovskite-type structures. The structures of KUO and RbUO- are cubic (section 7.4) whereas the NaUO structure is orthorhombically deformed [35]. Spitsyn et al. [36] and Kemmler-Sack [5] have discussed the use of the Goldschmidt criterion [37] for uranate structures. The Goldschmidt tolerance factor t for MUO. compounds has to be taken as 1i '-' 'ή , + r ,2- 3 .. l u5+ o1- in which r stands for the respective ion radii. Perovskite structures are stable when t lies in the range 0.9 - 1.1 [38]. Other investigators [39] have pointed out that at the limits of this range results are rather unreliable. However the radii of the respective ions are not accurately known. Since a long time the Pauling radii [40] have been in use, but recently Narayan and Ramaseshan [41] have published values which are much greater than the alkali Pauling radii. In addition the radius of the uranium(V) ion is not known accurately [5]. Therefore the Goldschmidt criterion can hardly be applied in case of the MUO- series. Another method to discuss the possible existence of the CsUO, perovskite structure is to estimate its unit cell edge, whereupon the Cs-0 distance can be calculated and compared to literature values. The perovskite cell parameter of CsUO- can be estimated from the r ι values for KUO, and RbUO-, as given in section 7.4. In figure 8.12a a linear extrapolation of these cell constants is given, based on the

i-ka SI ν H'\. :

-95- 11: m Η

«t 435

A (a)

Figure 8.12.

Estimate of the oubie cell parameter of the hypothetic CsUO„ perovskite, according to the metal ion radius (a) and the cell parameter of the tetragonal MJJO. structures (b).

Pauling radii of Κ ,Rb and Cs . Since the configuration in the tetra- 4 gonal layers in M.UO, compounds (M = K,Rb,Cs) is very similar to the configuration in the cubic perovskite structures, the. unit cell parameter of CsUO_ can also be estimated by comparison of the tetragonal cell constants of M.UO,, as is shown in figure 8.12b. From both estimates the unit cell parameter for CsUO_ is found to be 4.38(3) X. From this a Cs-0 distance of 3.10(2) % can be calculated. From the International Tables [42] a Cs-0 range 3.28 - 3.42 X is found for 12-coordinated cesium atoms. Therefore the conclusion must be that a perovskite structure with formula CsU0_ is not stable.

References

[1] P.M. de Wolff, J. Appl. Cryst., y 108 (1968).

[2] E.H.P. Cordfunke and B.O. Loopstra, J. inorg. nucl. Chem., 33_, 2427 (1971).

[3] L.M. Kovba, Russ. J. inorg. Chem., J_6, 1639 (1971).

[4] J. Hauck, J. inorg. nucl. Chem., 3i6, 2291 (1974).

[5] S. Kemmler-Sac.k and W. Rüdorff, Ζ. anorg. allg. Chem., 354, 255 (1967). at

.*ü:m -96-

[6] L.M. Kovba, Izvest. Vyssikh. Ucheb. Zavedinii, Khim. i Khim. Tekhnol., 2» 219 (I960); see Chem. Abstr., 54, no. 21900 (1960).

[7] C. Keiler, Habilitationsschrift, T.H. Fredericiana, Karlsruhe, p.135, (1964); also KFK-225, Karlsruhe, (1960).

[8] J. Prigent and J. Lucas, Bull. Soc. Chim. France, 1129 (1965).

[9] C. Greaves and B.E.F. Fender, Acta Cryst., B28, 3609 (1972).

[10] L.M. Kovba, Radiokhimiya, U^, 727 (1972).

[II] L.M. Kovba, E.A. Ippolitova, Yu,P. Simanov and V.I. Spitsyn, Russ. J. inorg. Cbsm., 21* 275 (1961).

[12] L.M. Kovba, Vestn. Mosk. Univ., Khim. Ser., \2_, 489 (1971).

[13] H.R. Hcikstra and S. Siegel, J. inorg. nucl. Chem., 2£, 693 (1964).

[14] H.R. Hoekstra, J. inorg. nucl. Chem., 2J_, 801 (1965).

[15] D.G. Kepert, Thesis, University of Melbourne; cited in [13] and [14].

[16] J. Hauck, Z. Naturforsch., 28b, 215 (1973).

[17] H.M. Rietveld, Acta Cryst., 20, 508 (1966).

[18] C. Greaves, A.K, Cheetham and B.E.F. Fender, Inorg. Chem., J_2, 3003 (1973).

[19] L.M. Kovba, Zh. Strukt. Khimii, J^, 458 ('972).

[20] B.O. Loopstra, Acta Cryst., V^, 651 (1964).

[21] B.O. Loopstra, Acta Cryst., B26, 656 (1970).

[22] B.O. Loopstra, J. Appl. Cryst., J}. 94 (1970).

[23] P.C. Debets, Acta Cryst., 2JU 589 (1966).

[24] L.M. Kovba, Zh. Strukt. Khimii, ^3, 256 (1972).

[25] E.H.P. Cordfunke, A.B. van Egmond and G, van Voorst, J. inorg. nucl. Chem., 37, 1433 (1975).

[26] L.M. Kovba and V.I. Trunova, Radiokhimiya, JTJ, 773 ('973).

[27] W.H. Zachariasen, Acta Cryst., ±, 265 (1948).

[28] W.H. Zachariasen, Acta Cryst., JU 281 (1948). L L29] L.M. Kovba, Radiokhimiya, JL2, 522 (1970). (-.Trifj-i ,ΐ,^ϊνΖ*'-

-97-

[30] L.M. Kovba, Ι.Α. Murav'eva and A.S. Orlova, Radiokhimiya, _[6_, 648 (1974). [31] L.M. Kovba, E.A. Ippolitova, Yu.P. Simanov and V.I. Spitsyn, Dokl. Akad. Nauk. SSSR, ^20, 1042 (1958). [32] A.F. Andresen, Acta Cryst., U_, 612 (1958). [33] B. Chodura and J. Maly, Second Int. Conf. Peaceful Uses Atomic Energy, 28, 223 (1958). [34] S. Kemmler, Z. anorg. allg. Chem., 338, 9 (1965). [35] S.F. Bartramm and R.E. Fryxell, J. inorg. nucl. Chem., 32, 3701 (1970). [36] V.I. Spitsyn, E.A. Ippolitova, Yu.P. Simanov, L.M. Kovba, in Investigations in the Field of Uranium Chemistry, see chapter II, ref. [2], p.4-16. [37] V.M. Goldschmidt, Srk. Akad. Oslo, I. Mat. nat. KI., 2_, 97 (1926). [38] W.H. Zachariasen, Srk. Akad. Oslo, I. Mat. nat. KI., U, (1928). [39] D. Balz and K. Plieth, Z. Elektrochemie, 59, 545 (1955). [40] L. Pauling, Proc. Roy. Soc, London A114, 181 (1927). [41] R. Narayan and S. Ramaseshan, J. Phys. Chem. Solids, 21» 395 (1976). [42] International Tables for X-ray Crystallography, Birmingham (1962).

'. · 'i

MNBBau*^r (BStf

3 ! -98-

APPENDICES

Remarks

I. OBS and CALC in Α-appendices are F . and F . KK obs calc (see section 1.2.1)·

OBS and CALC in B-appendices are summations of n. F . and 2 J j F η. cajc : (see section 1.2.I).

Negative OBS means no contribution of intensity to the least-squares structure refinement.

2. Q-OBS in B- and C-appendices are observed Q-values.

3. I/IO in B- and C-appendices are intensities of powder diffraction lines relative to the strongest line (= 100).

1! ····· x-BA» DATA OF ALKALI UPANATES

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SAMENVATTING

Dit proefschrift beschrijft onderzoekingen aan de kristalstrukturen van cesiumuranaten met behulp van voornamelijk Rontgendiffractie. Hoofdstuk I noemt twee redenen voor onderzoek van cesiumuranaten. Deze verbindingen spelen een belar.grijke rol in het gedrag van splijtstofstaven in kernreaktoren. Daart.aast is het interessant om na te gaan welke de invloed van de alkali-ionstrueï is in de kristalstrukturen van de alkali-uranaten, (uraanzouten van de eenwaardige metalen lithium, natrium, kalium, rubidium en cesium). In hoofdstuk II wordt aangegeven welke cesiumuranaten stabiel zijn in het Cs-U-0 systeem, zowel in lucht als bij lage zuurstofdrukken. Uit deze fasenstudie blijkt dat de verbinding Cs.U.O „ de grootste rol zal spelen in de splijtstofstaven van snelle kweekreaktoren. De drie kristalstrukturen van Cs-U,O.„ worden beschreven in hoofdstuk III, Het zwellen van de splijtstof bij het ontstaan van "cesiumuranaat" blijkt verklaard te kunnen worden door het verschil in soorte- lijke dichtheden van Cs-U.O.. and Ü0-. In de hoofdstukken IV, V en vi worden de kristalstrukturen van de zeswaardige cesiumuranaten, te weten Cs„UO,, Cs^U-O-, Cs,U,O._, CSjU.O.,, Cs„U_O.g, Cs„UyO„2 en Cs.U.-O,,, beschreven. Slechts van twee verbindingen, Cs-U^O.- en Cs„U,O.,,, kan dit gedaan worden met behulp van Rontgen- diffractie aan éénkristallen; van de overige wordt de kristalstruktuur bepaald met behulp van Rontgendiffractie aan poederpreparaten. In go·, al

van Cs„U_07 (hoofdstuk VI) wordt tevens gebruik gemaakt van neutronen- en elektronenciffractie. Hoofdstuk VII beschrijft de ' alium- en T.'ubidiumuranaat systemen. Omdat de literatuur veel tegenstrijdigheden aangaande daze uranaten bevat, zijn de K-U-0 en Rb-ü-0 systemen onderzocht, zowel in lucht JIS in inerte atmosfeer. Beide systemen zijn aanmerkelijk eenvoudiger dan het Cs-U-0 systeem. Alle kaliumuranaten zijn volledig ifostructureel met de desbetreffende rubidiumuranaten. In hoofdstuk VIII wordt tenslotte een vergelijk getrokken tussen de alkali-uranaten. De kristalstrukturen van de verschillende uranaten blijken te kunnen worden beschreven op basis van een zestal struktuur- karakceristieken; (pseudo) hexagonale uraniumlagen, (pseudo) tetragonale -, J uraniumlagen, ketens met samenstelling (U02)0„, ketens met samenstelling (U0^)0, geïsoleerde uraanomringingen en driedimensionale uraan-zuurstof Liaari—Wf f' rs"i •"•jte

-123-

roosters. De grootte en de "concentratie" van het alkali-metaal ion blijken doorslaggevend voor de gevonden kristalstruktuur. Een duidelijk verband tussen de omringing van uranium door zuurstof en de samenstelling van de uraniumlagen (uitgedrukt in de verhouding zuurstof/uranyl groepen) is niet aan te geven. De grootte van het alkali-metaal ion wordt ook teruggevonden in de afstand tussen de diverse uraniumlager. in de 1agenstrukturen. Tenslotte wordt verduidelijkt waarom een verbinding CslIO- met perovskietstruktuur niet gevonden wordt in het cesiumuranaat systeem.

il s! - - Ö -124-

CÜRRICULUM VITAE

Andrê van Egmond, geboren 19 juni 1950 te Hengelo (0), behaalde het HBS-b diploma aan het Ichthus College te Enschede in 1967. In datzelfde jaar begon hij sijn studie aan de Technische Hogeschool Twente. Het baccalaureaatsexamen chemische technologie legde hij af in januari 1971. In juni 1973 werd het ingenieursdiploma (richting chemische fysica) behaald. Tijdens zijn afstudeerperiode verbleef hij gedurende zes maanden in het IBM Research Laboratory te San JosS (Calif.), waar enkele labora- toriumautomatiseringssystemen bestudeerd werden. Sinds augustus 1973 is hij als gast-medewerker in dienst van het Reactor Centrum Nederland, waar het onderzoek, beschreven in dit proefschrift, werd verricht.