Investigations on cesium uranates—I

J inorg, nucl. Chem, t975, V~II37, pp 14a3-1436. PergamonPress Printedin Greal Brilain

INVESTIGATIONS ON CESIUM U R A N A T E S - - I CHARACTERIZATION OF THE PHASES IN THE Cs-U-O SYSTEM E. H. P. CORDFUNKE, A. B. VAN EGMOND and G. VAN VOORST Reactor Centrum Nederland, Petten. The Netherlands

(Received 26 August 1974) Abstract--The Cs-U-O system was investigated by means of X-ray analysis, thermal analysis and phase studies. Eight cesium uranate phases, in which uranium is hexavalent, were found. Their thermal stability in air was investigated and the formation of new phases is described. X-ray powder data of all phases are given.

INTRODUCTION

EXTENSIVE studies have been made on the alkali-metal uranates, especially by Ippolitova et al. [1], indicating the existence of a number of polyuranates depending on the M/U ratio (M = alkali metal) and temperature. But although since that time improved methods of investigation have led to some refinements[2,3], incomplete or conflicting literature data concerning their composition and structure[4] have not yet been resolved. Renewed interest in these compounds as possible reaction products in nuclear fuel elements during fission has given the impetus to reinvestigate these uranate systems systematically. In a previous paper [5] a study of the sodium uranate system has been published. In this paper a detailed investigation of the Cs-U-O system is presented. EXPERIMENTAL

Crystalline samples of the various cesium uranate phases were prepared by heating carefully ground mixtures of amorphous UO3 and cesium carbonate in a golden boat in air at 600°C, until reaction was complete. The atomic ratio's Cs/U ranging from 0.05 to 4.0 were accurately fixed in the starting materials. Since the reaction proceeded only slowly, especially at low Cs/U ratio's, the reaction products were reground several times between the heating periods; progress was checked by X-ray diffraction (Guinier films). In general, equilibrium compositions were obtained after a heating period of 1 week. Since only Cs2UO4 is hygroscopic, it was handled in a dry box. Thermal stabilities were investigated either by static experiments (ignition of the sample at a fixed temperature during various periods of time) or by Differential Thermal Analysis (DTA), using a BDL-apparatus with a heating rate of 9°/rain. Chemical analysis. The uranium content in the uranates was analysed amperometrically after dissolution of the sample in concentrated phosphoric acid. The U(V) content was obtained by direct titration with Fe(II); total uranium was titrated with Fe(ll) after oxidation of the solution[6]. The cesium content was determined by titration of a buffered solution (pH =5) with sodium tetrafenyl borate amperometrically. X-ray analysis. For identification purposes X-ray films were

made with a Guinier focusing camera, using CuKo radiation. High temperature X-ray films were taken with a Nonius Guinier-III camera. For quantitative purposes X-ray diagrams were recorded with a Philips diffractometer, using CuK~ radiation with a Ni-filter. All spectra were recorded with SiO2 as the internal standard. Unit celt constants were calculated from a least-squares refinement mostly based on 20-40 lines. Densities were measured from powder specimens in diethyl phtalate at 25°C after evacuating the pycnometer. Most of the density measurements were carried out in triplo. RESULTS AND DISCUSSION Cesium uranates (VI) From the X-ray patterns distinct cesium uranate phases have been found to exist in air up to = 725°C at atomic ratio's Cs/U = 0.125, 0.286, 0.40, 0.50, 0.80, 1.0 (o~ and/3) and 2,0. This indicates the existence of the compounds Cs2UI6049,

Cs2U7022,

Cs2UsOI6,

Cs2U4OI3,

Cs4U5017,

Cs2U207 (a and/3) and Cs2UO4, since uranium is entirely in the hexavalent state. The cotour of the phases gradually changes from ochre (Cs/U = 0.125) via yellow ( C s / U : 0.8) to orange (Cs/U = 2.0). The X-ray patterns of all cesium uranates could be indexed. The relevant data, including the Q-values of the 5 strongest lines, are given in Table 1. The formation of the cesium uranate phases by the solid state reaction, as described in the Experimental, is very slow, especially at low Cs/U ratio's. Notwithstanding long heating periods (several weeks), it appeared to be impossible to obtain the 0.286 phase in a pure form. It was always contaminated with both the 0.125 and the 0-40 phase. However, since the 0-286 phase is isostructural with the corresponding rubidium and potassium phases (which can easily be prepared in a pure form[7]), it is evident that with cesium the reaction is kinetically hindered. Whereas the 0.125-phase has not been described previously, the 0.286-phase was first found by Efremova

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et al.[8], who gave it the formula Cs2U6019. Since a difference in chemical composition between 0.286 and 0.33 can readily be detected on the X-ray films, we decided to give it the formula Cs2UyO::.This is supported by the fact that our Cs:U7022is isostructural with K2U702z of which a detailed structural work has been published by Kovba[9]. The structure of the new phase Cs2UI6049 is closely related to that of Cs:U7022. The indexing of the X-ray patterns leads to a doubling of the a and b axes of Cs2U7022. It should be noted that the X-ray pattern always contains lines of U308 and Cs2UTO22, even after long heating periods. The compound Cs2UsO~6 has not been found previously. Below 700°C it crystallizes poorly; its X-ray pattern is then always slightly contaminated with the lines of Cs2U7022 and Cs2U4OI3. Above 700°C crystallization occurs, together with a small change in structure as will be shown below. The compound Cs2U40~3 has been reported previously [8]. From the X-ray pattern it was found that CsaU#O~3 has an orthorhombic unit cell (Table 1). There are probably 5 molecules in the cell, which corresponds with a calculated density of 7.163(6). The measured density is much lower, i.e. 6.8. It should be noted, however, that CszU40~3 crystallizes very poorly as follows from the unsharp reflections in the X-ray pattern. Cs4U~O~7 is a bright-yellow phase which has not been described previously. Instead, a phase with the composition Cs2U30~0 was found by Efremova et al.[8]. This phase could not be detected in our investigation. The indexing of the X-ray pattern of Cs4UsO~7was simplified by the fact that single crystals of it could be made. The results of this work will be published separately[10]. Cs2U207 has a yellow-orange colour. When heated at about 300°C it transforms very slowly into a slightly different phase that can easily be frozen in at room temperature upon rapidly cooling. The structures of the low-temperature phase (a) and the high-temperature form (/3) are closely related, as can be seen from the data in Table 1. The orange-coloured Cs2UO4 is very hygroscopic. For this reason its X-ray pattern had to be recorded on the diffractometer with the sample contained in plastic. The X-ray pattern could be indexed with a tetragonal unit cell, space group I4/mmm, in agreement with previous observations by Spitsyn et al. [11] and Hoekstra[12].

place at a temperature which depends on the oxygen pressure of the system. In air dissociation occurs at 1040°C; Cs:U4On then formed is stable in air to at least 1250°C. Both Cs2U40~ and Cs2U40~2 are instable upon prolonged heating (Fig. 1). The stability of Cs2U4On strongly depends on oxygen pressure and temperature. For instance, at very low oxygen pressures (po2< 10-1° atm) decomposition of Cs~U40~, according to: Cs2U40~. ~ 4UO2+x+ 2Cs(g) + (2 - 2x)O2

takes place at temperatures as low as 600°C. The oxygen potential of this equilibrium has been measured as a function of temperature and will be published as Part II of these series[13]. The formation of Cs2U40~2 has been observed by Efremova et al. [14]. The authors obtained this product as a result of prolonged heating of "cesium mesouranate" (Cs,UOs)--a phase not found in our investigation--in air at 1200°C.In agreement with our observations, they found a reversible transition into Cs:U40~3 when it is cooled in air down to 500°C. Their X-ray pattern of Cs~U#O~=, however, does not agree with ours. From high-temperature X-ray Guinier films, taken in inert atmosphere, it was found that Cs2U4On exhibits two phase changes, before decomposing into UO~. The rhombohedral o~-phase transforms reversibly into monoclinic /3-Cs:U40~2 at 625°C, which in turn reversibly transforms into cubic 7-Cs2U4Ol: at 695°C. The latter phase slowly decomposes into UO2+~ at a temperature, depending on the oxygen pressure as mentioned before. A single crystal structure study of a-Cs2U4On, together with structure data of /3- and 7-Cs2U40~2, will be published separately[10]. Compositions with Cs/U atomic ratio <0.5. When heated in air above about 730°C, the phase relationships in the Cs-U-O system (Fig. 1) become somewhat less complicated. At low Cs/U ratio's a new cesium uranate is formed with the approximate Cs/U ratio of 0.22, as determined by X-ray analysis from films with different Cs/U compositions. A chemical analysis of this composition shows the presence of U(IV). The U(IV)/U(VI) ratio (0.126) together with the cesium content, leads to the formula Cs2UgO27 (Cs2Us6+U4+O27 or Cs2U76+U25÷O27). The relevant X-ray data of this uranate are collected in Table 1. It should be noted that Cs2U~O27is also stable up to Thermal stability 875°C. When heated in an open atmosphere above that Compositions with Cs/U atomic ratio >0.5. When temperature a new cesium uranate can be formed with the heated in air above 750°C decomposition of the cesium atomic ratio Cs/U =0.33 as determined by chemical uranates with Cs/U > 0.5 occurs at a rate which depends analysis. From the U(IV)/U(VI) ratio, also determined by on the temperature and on the Cs/U ratio. Cs2UO4already chemical analysis (0.20), the formula Cs2U60~s could be decomposes at 650° during which Cs2U207 and Cs4U~O17 derived. Its unit cell is closely related to U3Os in which it are formed successively. The latter compound in turn easily decomposes under evaporation of Cs~O. In Fig. 1 decomposes very slowly into Cs2U40~3 at 1000°C. A the dotted lines represent the phase relationships obtained section in air of the pseudo-binary system is shown in Fig. which, however, in an open atmosphere (pcs:o= 0) are 1, illustrating the phase relationships discussed in this metastable. As already mentioned, Cs2U~O~6begins to crystallize at paper. Thermal stability of Cs2U40~3.When Cs2U4OI3is heated about 725°C; this takes place simultaneously with a slight above 600°C, a reversible dissociation into Cs:U40~: takes change in its structure. The phase then formed appears to

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Fig, 1. The phase diagram of the pseudo-binary Cs-U-O system at Po2 = 0.2 atm. The dotted lines represent metastable equilibria at pcs2o= 0. have a homogeneity range. The phase width as determined from the X-ray films of different Cs/U compositions, which were heated at various temperatures between 750 o and 1000°C, ranges from 0.40-0.435 (at 750°C), 0.375-0.45 (at 800°C) to 0.375-0.475 at 950°C. At - 1000°C the "Cs2UsO~6-structure" has just reachedthe limiting composition Cs/U=0.5, before decomposing into Cs2U40,2 at 1040°C. Discussion The Cs-U-O system is rather complicated, compared with the corresponding sodium, potassium and rubidium systems [5, 7]. A large number of compounds can exist which, in general, are formed at a low rate of formation. Moreover, several phases in the system crystallize very poorly. A noticeable point is the structural resemblance of the phases in the Cs-U-O system with each other, and more general with U308. An exception is Cs2UO4, which has tetragonal symmetry, and Cs2U40~2 which has the UO2-structure. The other phases all contain (pseudo)hexagonal layers of the composition UO2(O2) with the additional oxygen and cesium atoms, and for the low Cs/U ratio's probably also uranium atoms[9], between these layers. The large volume and the great polarizability of the cesium atom may cause the diversity of phases which makes the Cs-U-O system so complex. A detailed crystallographical study on these phases is underway [10].

Acknowledgements--The authors wish to thank Prof. B. O. Loopstra for valuable disucssions. The experimental assistance of Mr. P. van Vlaanderen with the X-ray work and of the analytical group of RCN that performed the chemical analyses is gratefully acknowledged. REFERENCES

1. Investigations in the Field of Uranium Chemistry (Edited by V. I. Spitsyn), ANL-Trans-33 0%1). 2. L. M. Kovba, Soviet Radiochem. 12, 486 (1970). 3. L. M. Kovba, Soviet Radiochem. 14, 746 (1972). 4. J. G. Allpress, J. S. Anderson and A. N. Hambly, J. Inorg. Nucl. Chem. 30, 1195 (1968). 5. E. H. P. Cordfunke and B. O. Loopstra, J. Inorg. NucL Chem. 33, 2427 (1971). 6. A. Tolk and J. G. van Raaphorst, In Analytical Methods in the Nuclear Cycle, Proceedings of a symposium, 1971, p. 175. I.A.E.A., Vienna (1972). 7. E. H. P. Cordfunke, unpublished results. 8. K. M. Efremova, E. A. Ippolitova and Yu P. Simanov, Ref.[1], p. 59. 9. L. M. Kovba, Zhur. Strukt. Khim. 13, 256 (1970). 10. A. B. van Egmond, J. lnorg. Nucl. Chem. (1975), in the press. I1. V. I. Spitsyn, Ref.[1], p. 4. 12. H. R. Hoekstra, J. Inorg. Nucl. Chem. 27, 801 (1965). 13. E. H. P. Cordfunke, In Thermodynamics o[ Nuclear Materials, Proceedings of a symposium, 1974, I.A.E.A., Vienna (1975). 14. K. M. Efremova and Yu P. Simanov, Vestnik Mosk. Univ. Khim. Set. 24, 57 (1%9). 15. P. M. de Wolff, J. AppL Cryst. 1, 108 (1968).