On the xenon hexafluoride-uranium pentafluoride complex XeF6UF5

2938

Notes

J. inorg,nucl.Chem.. 1972,Vol. 34, pp. 2938-2941. PergamonPress. Printedin Great Britain

On the xenon hexafluoride-uranium pentafluoride complex XeF6UF 5 (First received 30.4ugust 1971; in revised form 29 November 1971) Ir~ THE COURSE of further investigation of properties of the compound XeF~UF5 published elsewhere [1] we tried to collect some more structural data. Since the magnetic and solution properties of the compound indicated its ionic structure, the recorded Raman spectra of the solid and its solution were expected to be interpreted in terms of such supposition. However, in the observed spectra no bands were found which could be unambiguously attributed to the UFo- species. Comparison of these spectra with some others of related compounds were made in an attempt to elucidate the nature of the XeFsUF5 adduct. In addition, thermal decomposition of the compound was investigated [2] in orderto contribute to the knowledge of the reactions in the system xenon fluorides-uranium fluorides [3]. EXPERIMENTAL Materials The pale yellow XeFtUF5 was prepared from xenon hexafluoride and uranium pentafluoride as described elsewhere[l]. Greenish yellow crystalline plates up to 2 mm in size were grown from the greenish blue solutions of the compound in anhydrous hydrogen fluoride by thermal cycling of these solutions. The solutions were prepared in an all KelF apparatus attached to a metal vacuum line. X e F ~ s F s for comparison purposes was prepared as published elsewhere [4, 5].

Solubility In the course of crystal growing the solubility of the compound in anhydrous hydrogen fluoride was estimated by gradual removal of the solvent in vacuum until the first crystal appeared. The weight of the saturated solution and the weight of the solid remaining after the solvent has been distilled off were determined and the result was corrected for the amount of the solvent in the vapour over the saturated solution. Density The density of the crystals was determined picnometrically using low viscosity halocarbon oil as a liquid (Voltalef IS). Weighing of the solid in a glass pycnometer was done in a dry box. The compound was reacting slowly with the oil and with glass and therefore the result is not very reliable. Determination of the density with carbon tetrachloride as the pycnometric liquid was found not to be practical since a slow reaction took place in this case. X-ray measurements Crystals were in a dry box immersed in halocarbon oil of a medium viscosity and kept continually under oil. The single crystals, selected under a microscope, were loaded into quartz capillaries which were prior to that also smeared with oil. The high absorption of the crystals besides their pronounced reactivity made this work difficult. Even the smallest crystals (0.1 mm) absorbed so much that only few Weissenberg patterns were recorded satisfactorily, and most of the symmetry information has been collected from Latie back reflection diagrams. Weissenberg patterns were obtained with CuK~ radiation since no substantial improvement of the quality was observed with MoK~ radiation. Since in many of the powder patterns the presence of fl-UF~ was detected as a consequence of decomposition, powder patterns were recorded until a few with lowest number of lines were obtained. Infrared spectra Spectra of the powdered solid were recorded on Perkin-Elmer 225 spectrometer (400-1300 cm-1), 1. 2. 3. 4. 5.

J. Slivnik, B. Frlec, B. ~ e m v a and M. Bohinc, J. inorg, nucl. Chem. 32, 1397 (1970). M. Bohinc. Part of M.Sc. Thesis submitted to the University of Ljubljana, July, 1971. M. Bohinc and B. Frlec, J. inorg, nucl. Chem. 34, 2942 (1972). H. Selig, Science 144, 537 (1964). K. E. Puilen and G. H. Cady, lnorg. Chem. 6, 2267 (1967).

Notes

2939

on Beckman Model I.R.-9 spectrometer (200-650 cm -1) and on RIIC Fourier spectrometer FS-520 (40-400 cm-l). Silver chloride and polyethylene windows were used, respectively. The solid was sandwiched between the plates in a dry box. Spectra of gases were recorded on Zeiss UR-20 spectrometer. Raman spectra Spectra of the solid, the solid in the equilibrium with the saturated solution in anhydrous hydrogen fluoride and the spectrum of this solution were recorded with the samples in KelF tubes (6 mm O.D.) equipped with all KelF valves. The tubes were filled with the solids in a dry box and the solvent was added on a vacuum line. Coderg Raman spectrometer with a 80 mW H e - N e laser for solids and a blue H e - C d 100 mW laser for solutions was used to record the spectra. Thermal decomposition

Thermal decomposition of samples in vacuum at 50°C was followed by weighing a nickel container containing about 0.5 mmole of the compound every hour. The container was kept under dynamic vacuum and the volatiles were collected and periodically examined in a 10 cm absorption length i.r. cell with a nickel body fitted with silver chloride windows. Samples (4 mmoles) were also decomposed in a closed, 120 ml nickel vessel which was kept at 40°C for 100 hr. The volatile decomposition products were pumped off and spectroscopically examined and the weight loss was determined. The vessel was then kept at 50, 60 and 70°C for 45 hr at each temperature and the procedure was repeated each time. Thermal decomposition of the samples was also studied in a flow of argon (61/h) using the Mettler thermoanalyser with the heating rate 6°/min and the TD-I type of crucible support. The platinum crucibles were loaded in a dry box, weighed in a closed container and mounted into the apparatus under auxiliary flow of dry argon. RESULTS The compound XeFrUF5 does not form a solvate with anhydrous hydrogen fluoride and its solubility in this solvent amounts, at 23°C, 15.3 g[ 100 g HF. The crystals were found to be orthorhombic with the following unit cell dimensions: a = 9.39 A _+ 0.02, b = 19.86 A _ 0.04 and c = 8.41 ,~ -+ 0.02, the a and c axes being in the plate plane and the b axis normal to it. The calculated density of 4.70g/cm 3 for Z = 8 is substantially higher as compared with the measured density 4.06_+0.1 g/cm z. We believe that this deviation is caused by the slow reaction of the material with the pycnometric liquid. The observed selection rules: hkl hko okl hol

no extinction k = 2n l = 2n not yet determined

hoo ool oko

no extinction (l=2n) (k=2n)

indicate two possible space groups: Pcca with a and b permutation (no. 54; hoi ! = 2n) or Pbcm with b and c permutation (no. 57; hol no extinction) [6]. It should be mentioned here that previously reported X-ray powder diffraction data[l] should be corrected. They represent a pattern of a sample which was partly decomposed due to the reaction with a capillary glass wall. In the i.r. spectrum of the solid XeFeUF5 the following absorption bands have been observed (in cm-J): 872(w), 648(vs), 612(s,sho), 570(s,b) and 467(m,b). Both i.r. and far i.r. spectra, which have been recorded also, however, cannot be interpreted without a better knowledge of the structure of the compound, which would enable a factor group analysis. Results of the Raman scattering studies are presented schematically in Fig. 1 and are compared with some other results on expectedly related compounds or their solutions in hydrogen fluoride. Relative intensities are given though they are not directly comparable and are significant only for a given spectrum. By way of comparison it is not possible to establish the presence of U F r - ionic entities 6. The corresponding indexed X-ray powder diffraction data are available from the authors upon request.

2940

Notes

I]: II [

XeF6UF5 solid

x2

,,

600 565

I

t,28 '-2~

XeFsUF5 saturated HF sotution

,

585

1,30

XeF6

655 636

I

,

~,0~,

583

I? 2o~ b) NOUF6 solid

300

¢,~ z,-z 1,95

I

,II

,

I

568

H

225 206

t,~l

x2

,I,,,

I

71,1 725 691 663 625 673

5~5

a) solid

&~2 383 365 397 375

I

299

XeFsAsFs solid

'',Io

215 1 ~ 5

XeFs AsFs saturated HF solution 622

1~56

]P

300

[ ,

c) AsFs tiquid AsF~ d)

i 690-560

700

a. b. c. d.

t

I 590-570

600

I ~ v~

I

I

300

200

390-370

500 L,O0 FREQUENCY [crn -1]

Fig. 1. Raman spectra of XeF6UF5 and related compounds. E. L. Gasher and H. H. Claassen, lnorg. Chem. 6, 1937 (1967). M. Drifford and R. Bougon, CEA-N-1409 (1971). L.C. Hoskins and R. C. Lord, J. chem. Phys. 46, 2402 (1967). K. O. Christe and C. J. Schack, lnorg. Chem. 9, 2296 (1970).

either in the solid XeF6UF5 or its solution in anhydrous hydrogen fluoride. Similarly, there is not a clear evidence that AsF6- species are present in X e F ~ s F 6 the spectra of which were recorded for comparison. These spectra also do not bear any resemblance to the spectra of the ionic XeF~+RuF6 and XeFs+PtF6-[7, 8]. Though the later compound is also orthorhombic, there is no structural relation with XeF6UFs. The absence of scattering bands characteristic for octahedrally symmetrical species cannot, however, be used as an argument against their possible ionic structure, as pointed out by K. O. Christie and C. J. Schack [9] discussing a similar system, because of the possibility of ionic structures containing anions of lower symmetry. On the other hand, the Raman scattering spectrum of XeF6UF5 allows a speculative comparison with the spectrum of solid xenon hexafluoride in its tetrametric form. A structural similarity with the monoclinic phase of xenon hexafluoride [ 10] can be seen in cell dimensions if neglecting the small 7. 8. 9. 10.

These spectra were kindly supplied by N. Bartlett. N. Bartlett, F. Einstein, D. F. Stewart andJ. Trotter, J. chem. Soc. (A) 1190 (1967). K. O. Christe and C. J. Schack, lnorg. Chem. 9, 2296 (1970). P.A. Agron, C. K. Johnson and H. A. Levy, lnorg, nucl. Chem. Lett. 1, 145 (1965).

Notes

2941

angular distortion of xenon hexafluoride and doubling the b period: ~ a = 9"33A XeFrtb=21.92A ( c = 8.95A.

/3=91"9 °

XeFrUF~

t!= 9'39A = 19.86A / 3 = 9 0 °. ~= 8.41A

According to Sturgeon et al. [11] the yellow colour of uranium(V) complexes indicate coordination 8 and not 6 around uranium atoms in the solid. The facts at hand lead to an interpretation of the structure of XeF~U F5 rather in terms of an insertion of UF5 units in a tetrametric xenon hexafluoride lattice. The thermal decomposition of the compound at 50°C in vacuum proceeds without any pronounced steps on the T G A curve. On the basis of the observed total weight loss, the spectra of the volatiles and the X-ray powder pattern of the solid remaining, the decomposition can be described by the following overall equation: UF~XeF~ ~ / 3 U F ~ + XeFr.

(1)

The course of the T G A curve indicates that the compound is at this temperature in a dissociation equilibrium which is, of course, because of the conditions used, completely shifted to the right. Since we could not detect the presence of uranium hexafluoride or lower xenon fluorides in the volatile products of the decomposition, it is evident that oxidation does not take place at these conditions. Weight-loss vs. time curves have been recorded for 50 ° and 60°(2. The decomposition was found to be as expected a reaction of zero order. The two decomposition rate constants were estimated as 2-79 x 10-3 mg/sec + 3 % and 1-2 × 10-2 mg/sec + 9 % for 50 ° and 60°(2 decomposition temperature, respectively. From these data the activation energy for the dissociation was calculated. It amounts 30 kcal/mole with the relative error estimated to 5%. The analogous decomposition in a closed system proceeds, as it was expected, differently. Uranium hexafluoride, xenon hexafluoride and xenon tetrafluoride are only volatile products of the decomposition at all temperatures tested and no residual solid was found at 70°C. The decomposition rate increases regularly as the decomposition temperature is raised. The decomposition, given by the following overall equation, 2UF.~XeF6 ~ 2UF6 + XeF6 + XeF~

(2)

is very rapid above 70°(2. In flow of argon the decomposition proceeds in two steps. It starts at 60°(2 with the maximum rate of weight-loss at 113°C. This step is described by the Equation (1). The second step, which follows immediately and is distinguishable only on the D T G curve, has the maximum rate of weight-loss at 124°C. As shown by the weight-loss and X-ray powder pattern of the solid remaining it corresponds to the decomposition of/3-UF5 yielding U2F9: 3UF5 ---* UzFg+ UF6.

(3)

Such decomposition was reported by Agron to occur within the temperature range 100-152°C [ 12]. The thermal decomposition of XeFrUF5 can be utilized for a simple preparation of pure/3-UFs. The proposed method involves the fluorination of uranium tetrafluoride with excessive xenon hexafluoride at room temperature, the pumping off the volatiles and the decomposition of the reaction product in vacuum at 50°C.

Acknowledgements--Part of this work was financed through the Boris Kidri6 Foundation. The help of R. Bougon who recorded the i.r. spectra is gratefully acknowledged. Institute "Jo~ef Stefan" 6100 Ljubljana Yugoslavia D(partment de Physico-Chimie Centre d'Etudes Nucl~aires de Saclay B.P. No. 2, 91 Gif-sur-Yvette France

B. F R L E C M. B O H I N C P. C H A R P I N M. D R I F F O R D

11. G. D. Sturgeon, R. A. Penneman, F. H. Kruse and L. B. Asprey, lnorg. Chem. 4, 748 (1965). 12. P. A. Agron, AECD 1878 (1948).

$ I N C Vol. 34 no. 9 - - I