Preparation and thermal decomposition of barium uranyl oxalate and of barium plutonyl oxalate

J. Inorg. Nucl. Chem., 1966, Vol. 28, pp. 1111 to 1114. Pergamon Press Ltd. Printed in Northern Lm)and

NOTES

Preparation and thermal decomposition of barium uranyl oxalate and of barium plutonyl oxalate (Received 22 September 1965) Ai"r~m~rs to prepare barium monoplutonate BaPuO4 by precipitation of the hydroxide yielded only art amorphous solid which on heating to 900°C gave ABOI type X-ray diffraction patterns, with most of the plutonium remaining in the VI oxidation state, cx~ By heating an intimate mixture of BaO and PuOt to 1100°C or above, normal BaPuOa, slightly different from the above, was obtained, a) This paper deals with an attempt to prepare relatively simple barium uranyl oxalate and barium plutonyi oxalate with the object of obtaining and studying the ABOs and ABO, compounds of barium with uranium and plutonium formed by the decomposition of the complex oxalates. EXPERIMENTAL (a) Preparation Ammonium oxalate was added to a saturated solution of uranyl oxalate in 1.0 M HNOs, followed by a slight excess of barium nitrate, both solutions in 1.0 M HNOs. The resulting solution was poured into an equal volume of ethanol; on standing, a bright yellow solid of composition, BaUOt(C4Ot)j'6HtO separated. (Found: ratio Bat+: UO2t+: CtO, t- = 1: 0.91: 0.26; number of water molecules by difference = 6.) With the ratio of brainm nitrate to uranyi oxalate greater than 2 the solid which separated had the composition Ba, UOt(CtO,)a'4HtO. (Found, ratio Bat+: 130**+: CtO4t - - - 2: 0.95: 2.98; number of water molecules by difference = 4.) This compound was not further studied. Only uranyi oxalate precipitated when oxalic acid was used in place of ammonium oxalate. For the preparation of the corresponding plutonium compound, ammonium oxalate and barium nitrate (as either solid or saturated sointion) was added to a solution of plutonium (VI) in the molar ratio of Ba: Pu (barium slightly in excess) = 1 : 1 and plutoninm (VI): ammonium oxalate = 1:2. A solid separated on standing. When the added ammonium oxalate was less than in the above ratio, the barium plutonyl oxalate formed was mixed with plutonyi oxalate. In such preparations, the plutonyi oxalate formed first was separated and the supernatant on standing precipitated barium plutonyi oxalate. (Found, average of 3 samples; Ba:Pu:CaO4 ----0'96:1 : 1-99.) The solids obtained were washed twice with dilute HNOs, once with distilled water and finally twice with alcohol. They were either dried in vacuum (,---fewram) or by a jet of pure nitrogen.

Co) Chemical analysis 50-100 mg of barium uranyl oxalate, suspended in water, was decomposed with dilute HISO,. The precipitated BaSO4 was separated, heated to constant weight at 800°C and weighed as such. The supernatant solution was neutralized with dilute ammonia using methyl red indicator. The separated uranate was washed with 2 % ammonium nitrate solution, re-precipitated after dissolution in dilute HsSO4, dried, ignited and weighed as UaO,. The filtrate from the uranium precipitation was concentrated, acidified with dilute H,SO,, and titrated against KMnO, at 70°C. In the case of the plutonium compound, the plutonium (VI) in solution after precipitating BaSO, had to be reduced with a reagent which could be easily and completely removed before the oxalate was estimated. Few drops of H,SOt was added to the supernatant after separating BaSO,, and the (x~D. M. CHACggABUgTrY,N. C. JAYADEVANand C. K. SlVARAMAr.RmHNAN,Acta Cryst. 16, 1060 (1963). 1111

1112

Notes

plutonium (IV) so obtained was precipitated as hydroxide. This was dissolved in dilute HNO8 and plutonium estimated radiometrically. The excess H2SO8 in the supcrnatant was removed by boiling the solution before titrating with KMnO4. The method was tested for plutonyl oxalatos, PuOs (CsO~)'3HsO and found suitable. 10-15 mg samples were used for each analysis. The number of water molecules was arrived at by difference and verified by thermogravimetry. (c) Thermogravimetry For studying the thermal decomposition, approximately 10-20 mg of the dried samples were loaded on a silica spring thermobalance with a sensitivity of 2.5 mm/mg, and the temperature raised at the rate of 5°C/min. The weight changes were observed with an index fibre and a cathetometer which could be read to an accuracy of 0.01 mm. (d) X-ray analysis The complex oxalates, the final decomposition products above 900°C and the intermediates were analysed with a 19 cm Unicam camera using Cu-Kg radiation. In many cases filtered radiation was used. RESULTS AND DISCUSSION (a) Barium uranyl oxalate Barium uranyl oxalate gave diffraction pattern different from that of uranyl oxalate, barium oxalate, ammonium oxalate and oxalic acid. The pattern was characterized by a large number of lines and appeared to be triclinic with large unit cell dimensions. Spectrophotometric analysis of a solution in 6 N HC1 showed that all the uranium was present as UOss+. (Absorption maximum at 415 m# using Beckmann DU Spectrophotometer.)

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600 in °C

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Notes

1113

Uranyl oxalate exists in solutions of oxalates as [UOs(C,O,)~]-s ions.(s) Sincethe barium compound was readily formed omiyusing ammonium oxalate and not oxalic acid, it could be assumed that ammom'um oxalate favoured the formation of the negatively charged ion, which could be precipitated as the barium salt. The thermal decomposition was studied on 15-20mg samples in a silica thermogravimetric balan~ (Fig. 1). The sampt¢ lost weight up to 100°C and there was very little change from 100°C to 300°C. The decomposition of the oxalate started above 300°C and was complete at 600°C; from 600°C to 950°C, there was no weight change. 24

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TemperQture * C F~G. 2. The empirical formula of the ttnal decomposition product was found to be BaUO4 from chemical and X-ray analyses. All the uranium was present as uranium (VI) with Ba: U = 1 : 1. The pattern corresponds to the data by SAmos and SheENcs~and compound was fully characterized as Ba(UOOOs from structural consideration by ZnCHnnU~N.~) No crystalline pattern could be obtained for the products obtained by decomposition of barium uranyl oxalate heated between 120°C and 600°C. (b) Barium plutonium oxalate The formation of barium plutonyl oxalate was frequently accompanied by plutonyl oxalate. This was avoided by the addition of large amounts of ammonium oxalate before the addition of BaS+ ions. The water washed samples were dried with a jet of nitrogen gas. Washing with alcohol was avoided in several preparations, as it was thought it would reduce plutonium (VI). However, if the solid was separated quickly from alcohol and dried in vacuum, there was very little reduction. (l~ H. R. HO~KSTRAand J. J. KATZ, The Actinide Elements (Edited by G. T. SEAnORC;and J. J. KATZ) NNES Vol. 14-A, p. 170 McGraw-Hill, New York (1954). (a) S. SAMSONand L. G. SILLEN,Ark. KemL Min. Geol. 25, No. 21 (1947). (4) W. H. ZACraARIASEN,Acta Cryst. 7, 795 (1954). 13

1114

Notes

The composition of the compound was BaPuO2(C,O4)ocHtO. The water of hydration could not be ascertained directly by chemical analysis. The thermogravimetric analysis of the sample at two different rates of heating (300°C/hr and 200°C/hr) gave identical thermograms (Fig. 2) with no indication of any stable intermediate. From X-ray diffraction patterns, the final product at 900°C was identified as cubic ABOs, compound of plutonium (Pu was found to be mainly in the VI state by spectrophotometry) identical to that reported earlierJ 1~ This was accompanied by a small amount (10-15 Vo) of PuO,. The number of water molecules is roughly 6-8. The X-ray diffraction pattern of the baHnm plutonyl oxalate is different from BaC~O4 or PuO,(C, O4)3H, O and the compound is not isostructural with the corresponding uranyl salt. Its thermal decomposition was also different. It is therefore evident that the decomposition products of the barium uranium salt was Ba(UO2)O, and that of the plutonium salt was mainly of the ABO5 (perovskite) structure type. Thermal decomposition in case of plutonium did not yield BaPuO,.

Aeknowledgement--Thanks are due to the Head, Radiochemistry and Isotope Division, and to Dr. A. S. GHOSHM ~ A R ,

for his helpful discussion.

Radiochemistry and Isotope Division, Atomic Energy Establishment, Trombay, Bombay

D. M. CHACZRABURTTY N. C. JAYADEVAN

J. Inorg.Nuc].Chem.,1966.Vol.28, pp. 1114to 1116. PergamonPressLtd. Printedin NorthernIreland

Correlation of M 6 s s b a u e r p a r a m e t e r s with infra-red and magnetic data

(Received 14 September 1965) ALTHOUOH several chemical applications of the M6ssbauer effect have been reported over the last five yearsy ~the potentialities of this technique for investigating chemical problems have not yet been fully exploited. Here we report work which illustrates two new situations in which the technique has proved useful. These are (i) metal complexes in which there is asymmetry in the electron environment of an atom in an octahedral field, and (ii) where two possible stereochemical structures can be unequivocally distinguished.

1. Deviationsfrom octahedralsymmetry We have investigated the series of compounds [Fepy~X,] where py = pyridine and X = C1, Br, I, CNS and CNO using the following techniques: (a) visible spectroscopy, (b) infra-red spectroscopy and (c) magnetic moment measurement. These results, summarized in Table I, indicate that (i) all the compounds studied were of high spin, but had magnetic moments greater than that calculated for spin contributions only (4.9 B.M.), (ii) from the IR and UV-visible spectra, the shift in the line for the Fe--X vibration indicates a change in the interaction of X with the iron atom, and (iii) the band for the d-d transition in the UV-visible spectrum which is very sharp for compounds of high p [Fepyd,], broadens and finally splits for compounds of low p [Fepy4(CNO)2]. These data suggest that as X is changed the electron anisotropy about the iron atom is changing. These techniques are not, however, sensitive to small changes in energy. A parallel investigation using the M6ssbauer technique to determine the parameters AEe (quadrupole spli0 and d (chemical shift) for 5~Fe showed (Table 1) that this method was more satisfactory. Since d depends on the value of the s-electron wave-function only at the nucleus we do not expect it to reflect changes in X, as was observed. However, AEq, which depends on the electron anlsotropy at the iron atom, is affected by X. In this case a five-fold change in AE~ (accurate to :t:0.05 ram/see) was obtained. This was confirmed by theoretical calculation of both AEe and p for different degrees of axial distortion from octahedral symmetry. The results (Fig. 1) show that for a small degree of distortion from octahedral symmetry AEq is a more sensitive parameter than/~ (and conversely for large distortion, although this is of less experimental importance). tl~ j. F. DUNCANand R. M. GOLDING,Quart. Rev. XIX, No. 1, 36 (1965).