Investigations on cesium uranates—VI

Z inorg, nucl. Chem.. 1976, Vol. 38, pp. 2105-2107. Pergamon Press. Printed in Great Britain

INVESTIGATIONS

ON

CESIUM

URANATES--VI

T H E C R Y S T A L S T R U C T U R E S O F Cs2U20~ A. B. VAN EGMOND Reactor Centrum Nederland, Petten, The Netherlands (Received 1 March 1976) Abstract--The crystal structures of Cs~U207are described. The structure of the a-compound has been refined from X-ray and neutron powder diffraction data. The structure of/3-Cs~U~O7has been solved by means of X-ray powder diffraction. Besides the a and 0-cesium diuranate, both belonging to the monoclinic crystal class, a metastable hexagonal y-diuranate exists. INTRODUCTION IN FOREGOING papers of this series[I-4] the crystal structures of Cs2UO4, Cs, UsO~7, Cs2U40~3, Cs2UsO,6, Cs2U7022, Cs2UlsO46 and Cs2U4Ot2 have been described. In the first paper[I] monoclinic unit cells were given for a and /3-Cs2U2OT, two phases, which have a reversible transition at about 300°C. In addition Kovba et al. recently described a cesium diuranate with a hexagonal unit cell[5]. For this reason the formation of Cs2U207 was reinvestigated and the crystal structures were studied. EXPERIMENTAL Powder samples of monoclinic diuranate were synthesized as described previously[l]. Pure samples of /3-Cs2U207 were obtained on heating mixtures of amorphous UO3 and cesium carbonate at 600"C. A phase transition into a-Cs2U207 occurs when the temperature is decreased to 300°C for several days. Upon rapid cooling from 600°C down to room temperature the structure of/3-Cs2U207 is frozen in. The hexagonal diuranate, hereafter referred to as y-Cs2U2OT, which has been reported by Kovba et a/.[5], could also be synthesized at 600°C. However, it had partly reacted to /3-Cs~U2OT.At 8000C the hexagonal y-phase rapidly transformed to the g-form, indicating that the hexagonal diuranate is metastable and that its synthesis is merely fortuitous. For the synthesis of the /3-diuranate amorphous UO3 obtained by decomposition of hydrated UO,, had been used, while the UO~, reacting to the y-phase, had been formed on heating uranyl nitrate. In succeeding experiments the reaction time, the temperature and the concentration of impurities like the amount of nitrate, were varied, but the hexagonal y-diuranate could not be synthesized in a pure form. Whereas Kovba et al. mentioned the hygroscopic character of the y-diuranate[5l and O'Hare et al. described the hygroscopic character of monoclinic diuranate--probably the B-form[13], no reaction with water vapour nor with carbon dioxide could be established at room temperature in case of any of the three diuranate structures. RESULTS AND DISCUSSION

The X-ray analysis o/a-Cs2U~07 A powder sample of a-CszU207, mixed with starch, was mounted on a rotating sample support of a Philips powder diffractometer. The sample contained small amounts of Cs4UsOt7. Its X-ray pattern was step-scanned from 10.00 to 78.74 ° 20 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)A, c = 7.605(1)/~ and/3 = 112.93°[1]. Thereafter the profile could be integrated to 56 intensities covering 153

reflections. The space group of the diuranate is C2/m, Cm or C2. The b/c-ratio of the unit cell allows a nearly ideal hexagonal uranyl oxygen layer parallel to the be-face of the cell. These (pseudo) hexagonal layers have been established in a number of cesium uranates [3, 4]. 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, ½, ~. 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. In addition a neutron diffractogram did not contradict the indexing of the X-ray pattern. 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% (R=X[nF~,s- n F 2~cl/EnF~s). 2 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[6] and corrected for anomalous dispersion with Af', taken from Cromer [7]. The overall isotropic temperature factor was refined to 0.72(9) ,~2, which is reasonably reliable considering the maximum (sin0/A)2-value 0.167 ,g, 2. In the final model the O4-atom is located in a position which is only half occupied. 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

2105

2106

A.B. VAN EGMOND

atom positions, the structure model has not been refined in other space groups. The coordinates of the final refinement have been listed in Table 1, whereas Table 2 contains the most relevant data of the coordination of uranium by oxygen.

The X-ray analysis of/3-Cs2U207 A sample of/3-Cs2U207 was heated at 600°C for 16 hr. Guinier photographs after freezing in the structure did not show any impurities. The X-ray pattern of /3-Cs2U207 was recorded as described for the a-phase. The pattern could be indexed with a C-face centered monoclinic cell with a = 14.516(2),~, b=4.3199(6),~, c=7.465(1)A and /3= 113.78°(1). Up to 62.000 20 its profile was integrated to 55 intensities covering 82 zero, non-zero and overlapping reflections. The space group of /3-Cs2U207 is also assumed to be C2/m. For the structure refinement the uranium and cesium coordinates of the a-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 Boo = 0.15(5) ,~2 is less reliable compared to the temperature factor of ot-Cs2U207, due to the smaller (sin 0/k)2-range. Final coordinates of /3-Cs2U207 are listed in Table 3. The uranium coordination by oxygen is summarized in Table 2.

The X-ray analysis of hexagonal ' y - C s 2 U 2 0 7 Notwithstanding the experimental problems with the synthesis of hexagonal CszU207 a Guinier photograph (Ni-filtered CuK~ radiation) could be obtained, which was Table 1. Least squares coordinatesof a-Cs2U207 from theX-ray data At )m

x/a

z/c

ylb

occupancy

U

42

0.1465 (6)

0.000

(-)

-0.007

(2)

Cs

4i

0.3909 (8)

o.ooo

(-)

0.562

(2)

1.0

01

4£

0.204

(5)

o.ooo

0.25

(1)

1.0

0.27

(i)

1.0

-o.ol

(~) (-)

0.5

02

4£

0.401

(6)

o.50o

(-) (-)

03

4L

0.318

(5)

o.ooo

(-)

04

4g

0.000

(-)

0.241

(30)

o.oo

1.0

1.0

Table 2. Uranium oxygen distances in cesium diuranate from X-rayand neutron data Distance

a-Cs2U207 X-ray

~-Cs2U207 neutron

U-03

].81(9) 1,87(10) 2,49(5)

1.81(2) 1.88(2) 2.34(2)

U-03'

2.19(5)

2.26(2)

U-04

2.38(2)

2.21(2)

U-O I U-02

6-Cs2%07 X-ray

1.87(7) 1.95(6)

2.24(4) 2.30(4) 2,15(I)

Table 3. Least-squarescoordinatesof fl-Cs2U207from theX-ray data Atom

y/b

x/a

z/c

U

41

0.1474

(6)

o,ooo (-)

-0.004 (I)

Cs

4£

0.3978

(7)

o,ooo (-)

0.584 (1)

1.0

01

4£

0.206

(4)

0.oo0 (-)

0.27

1.0

(1)

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 = 14.646(5),~. These dimensions reasonably agree with the results of Kovba et al., who reported a=4.106(3)A and c = 14.58(2) ~, [5]. Possible space groups for the hexagonal T-diuranate are P63/mcc, P62c and P63mc. The density, calculated from the X-ray unit cell, is 6.624(5) assuming one Cs2U207 unit in the cell.

The neutron analysis of a-Cs2U207 A pure sample of o / - C s 2 U 2 0 7 w a s mounted on the neutron powder diffractometer at the Petten High Flux Reactor. The sample was contained in a cylindrical vanadium sample holder of 0.5 mm wall thickness and 20 mm diameter. Monochromatic radiation with a wave length of 2.5715(4)/~ was obtained from a copper (Ill)--plane[8]. Soller slits of 10' angular divergence were mounted between the reactor and the monochromater and in front of the BF3 detector respectively. At room temperature the neutron profile of the sample was measured from 150 to 3850 dmc 20 in steps of 2 dmc (1004)0dmc = 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-Cs2U207 with a programme written by Rietveld[9, 10]. Applying the profile refinement method the following quantities were varied: the cell dimensions, the zero point of the 20 scale, the atomic position parameters, an overall isotropic temperature factor, a scale factor, the half width parameters P, Q, R from the relation be2= P tan20+Q tan 0 + R where be is the width at half maximum of a Bragg peak at angle 0, a parameter which allows a correction for preferred orientation in the 001 direction and a parameter which counts for asymmetric deviations of the Gaussian peak shape at low diffraction angles. In the refinement the scattering lenghts 0.85 x 10-12, 0.558× 10 12 and 0.580× 10-12cm were used for uranium, cesium and oxygen respectively[Ill. First the structure of a-Cs2U207 was refined in space group C2/m resulting in an R-factor of 12.9% (R= Xw Ilobs-Icalcl/XWIobs), where the profile is converted to integrated intensities [10]. Since the neutron intensities are much more sensitive for a change in the oxygen positions than the X-ray intensities are, the a-Cs2U207 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-Cs2U207 structure. Furthermore the following cell parameters resulted from the C2/m neutron refinement: a=14.528(1)/~, b = 4.2676(3) ~,, c =7.6026(6)A and /3 = 112.986(7)°, all in good agreement with the X-ray cell dimensions. The overall temperature factor was 1.6(1)~2 and the corrections for asymmetry of the peaks and preferred orientation were small. The values of the position parameters are listed in Table 4 and uranium oxygen distances are collected in Table 2. Figure 1 shows the fit between observed and calculated profile.

1.O

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(4)

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1.0

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0.294

(3)

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-0.04

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1.0

DISCUSSION In part I of this series a unit cell with a = 14.512(2),~, b =4.2967(3),~, c =7.535(1)~ and /3 = 113.35°(1) was reported for /3-Cs2U2OT, which is significantly different from the unit cell reported in this paper. This is probably

2107

Investigations on cesium uranates--VI ~o8o / a - C S 2 U 2 0 7 ,

300°K,

;k:2.5715(4)

........ Observed profile C(~uloted profile

[ .o i

.=_ 1

,8°i "

[

! i

~0

s°G

. . . .

,0

"'" 20

~

40

60

50

70

BO

90

I00

I10

120

150

140

2 - 0 (degrees)

Fig. 1. Neutron powder profileof a-Cs2U207. Table 4. Least squares coordinates of a-Cs2U207 from the neutron data z/c

y/b

x/a

Atom

I occupancy

U

4i

0.1425

(6)

o.ooo

(-)

(D

[

1.0

Cs

4i

0.3935

(7)

0.000

(-)

0.567 (1)

[

1.0

Ol

4i

0.1923

(7)

0.0oo

(-)

0 . 2 5 2 (2)

[

1.0

02

4i

0.4133

(7)

0.500

(-)

0.275 ( I )

[

1.0

03

4i

0.3055

(7)

o.ooo

(-)

0 . 0 0 5 (2)

I

1.0

04

4g

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(-)

O.171

<3)

o.ooo (-)

I

0.5

-0.006

(

crystallinity of the obtained ~/-Cs2U207the here reported hexagonal unit cell can be a subcell, just like for K2U207 and Rb2U2OT[12]. The time of formation of cesiumuranates starting with UO3 from heating uranyl nitrate, is much shorter than with UO3 obtained from decomposition of hydrated UO4. When Tables 1 and 4 are compared, the X-ray refinement and the neutron refinement of the a-Cs2U207 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-O distances, which are listed in Table 2, whereby the neutron data are more accurate than the X-ray data. From the crystal structures presented in this paper the phase transformation from the a-diuranate to the /3diuranate is also more clear: the O4-atom, which is statistically disordered in the a-structure (fig. 2(a)), occupies a fixed position in the /3-structure (Fig. 2(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 arranged uranium atoms in the uranyl oxygen layers[3,4] and the mono-uranate contains tetragonal arranged uranium atoms in the uranyl oxygen layers[4,5]. The monoclinic 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

[a]

[b]

Fig. 2. Uranium layers in ~-Cs2U207 (a) and /3-Cs2U207 (b), projected on the ab-face. Small circles designate uranium atoms; large circles designate oxygen atoms; uranyl oxygen atoms are not shown. For a-Cs2U207 the neutron diffraction data have been used, the O,-atom being statistically distributed in two positions. caused by a different heat treatment of the samples. The formation of the cesium uranates is greatly influenced by heat treatment, reaction time, the inclusion of impurities and particle size of the starting materials. These facts influence the formation of the hexagonal 3,-diuranate, which however can be regarded to be metastable, because it always transformed into the monoclinic phase independently of the calcination temperature. Due to the poor

1. E. H. P. Cordfunke, A. B. van Egmond and G. van Voorst, J. Inorg. Nucl. Chem. 37, 1433 (1975). 2. A. B. van Egmond, J. Inorg. Nucl. Chem. 37, 1929 (1975). 3. A. B. Van Egmond, J. lnorg. Nucl. Chem. 38, 1645 (1976). 4. A. B. Van Egmond, J. Inorg. Nucl. Chem. 38, 1649 (1976). 5. L. M. Kovba, I. A. Murav'eva and A. S. Orlova, Radiokhimiya (Eng. Tr.) 16, 638 (1974). 6. D. T. Cromer and J. T. Waber, Acta Cryst. 18, 17 (1965) 7. D. T. Cromer, Acta Cryst. 18, 17 (1%5). 8. B. O. Loopstra, Nucl. lnstrum. Methods 44, 181 (1966). 9. H. M. Rietveld, Acta Cryst. 22, 151 (1%7). 10. H. M. Rietveld, A program for the refinement of nuclear and magnetic structures by the profile method, (1969), update (1972). 11. G. E. Bacon, Acta Cryst. A28, 357 (1972). 12. A. B. van Egmond and E. H. P. Cordfunke, in press. 13. P. A. G. O'Hare and H. R. Hoekstra, J. Chem. Thermodynamics 7, 831 (1975).