X-ray, thermal and infrared spectroscopic studies on potassium, rubidium and caesium uranyl oxalate hydrates

Article abstract of DOI:10.1023/A:1010112708170

M₂UO₂(C₂O₄)₂·nH₂O compounds (M = K, Rb and Cs) have been prepared and characterized by chemical and thermal analyses as well as by X-ray diffraction and infrared spectroscopy. X-ray powder

Full text of DOI:10.1023/A:1010112708170

Journal of Thermal Analysis and Calorimetry, Vol. 61 (2000) 107–117
X-RAY, THERMAL AND INFRARED SPECTROSCOPIC
STUDIES ON POTASSIUM, RUBIDIUM AND CAESIUM
URANYL OXALATE HYDRATES
N. D. Dahale, K. L. Chawla and V. Venugopal
Fuel Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, India
(Received December 17, 1998; in revised form July 5, 1999)
Abstract
M2UO2(C2O4)2·nH2O compounds (M=K, Rb and Cs) have been prepared and characterized by
chemical and thermal analyses as well as by X-ray diffraction and infrared spectroscopy. X-ray pow-
der data show that the compounds belong to an orthorhombic system. Thermal and infrared studies
show that the compounds decompose to M2UO4 through the formation of alkali metal carbonate and
UO2 as intermediates. K2UO2(C2O4)2·3H2O, and Rb2UO2(C2O4)2·2H2O gave K2UO4, Rb2UO4 at 700
and 600°C respectively, while in the case of Cs2UO2(C2O4)2·2H2O, the intermediate products of de-
composition reacted to yield Cs2U4O13 at 1000°C.
Keywords: caesium uranyl oxalate, decomposition, DTA, potassium uranyl oxalate, rubidium ura-
nyl oxalate, TG, X-ray diffraction
Introduction
The caesium–uranium–oxygen and rubidium–uranium–oxygen systems have
been investigated extensively due to their chemical interactions in (U,Pu)O2 fuels
in fast reactors. Fission products caesium and rubidium have been observed to mi-
grate radially and axially within the fuel pins. These elements can react with fuel
forming corresponding uranates depending on oxygen potential and temperature.
Cladding deformation and breach have been found and attributed to the formation
of low density Cs–U–O and Rb–U–O compounds [1, 2]. Caesium and rubidium
can react with oxide fuel forming hexavalent as well as lower valent uranates de-
pending on the local oxygen pressure. The conflicting opinions center around the
stability of the possible compounds, mainly Cs2UO4 and Cs2U4O12 and conditions
required for their formation. Systematic studies on the preparation, thermal stabil-
ity, phase characterization and thermodynamic properties of such systems are be-
ing carried out at our laboratory.
Thermal decomposition of rubidium and caesium uranyl oxalate gave high purity
phases of Rb2UO4, Cs2UO4, Cs2U2O7, Cs4U5O17 and Cs2U4O13. The preparation of all the
alkali metals and ammonium uranyl oxalates with UO22+ toC2O42− ratio of 1:2 are reported
1418–2874/2000/ $ 5.00
Akadémiai Kiadó, Budapest
© 2000 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

108
DAHALE et al.: URANYL OXALATES
[3]. X-ray structural analysis on (NH4)2UO2(C2O4)2 [4] and K2UO2(C2O4)2·3H2O [5] is
available. In both the compounds the linear uranyl ion has five oxygen atoms forming a
plane perpendicular to the O–U–O axis. The ammonium salt has the polymeric anion
[UO2(C2O4)2·H2O]2nn– in which all the five oxygen atoms are from three oxalate groups,
none of which are planar [4, 6]. The infrared spectrum of K2UO2(C2O4)2·3H2O has been
discussed in terms of simple penta coordinated arrangement about the UO22+ ion,
K2[UO2(C2O4)2·(H2O)]·2H2O involving two bidentate oxalate groups and one bonded wa-
ter molecule [9]. This seems to be a substantially realistic approach and many of the hy-
drated salts have been written in this form, but no X-ray structural data are actually avail-
able to confirm it. The anhydrous compounds decompose when heated, forming metal
uranates. The anhydrous potassium salt is reported to rehydrate stepwise to the
monohydrate in air and to dihydrate, but not to the trihydrate on standing in a closed ves-
sel over water for 12 h [3].
In previous studies X-ray and thermal analysis on Li2UO2(C2O4)2·5H2O and
Na2UO2(C2O4)2·4H2O were reported [6]. The results of X-ray, thermal and infrared
studies on Rb2UO2(C2O4)2·2H2O and Cs2UO2(C2O4)2·2H2O are discussed here. Potas-
sium uranyl oxalate trihydrate has been prepared to compare its results with rubidium
and caesium systems.
Experimental
Saturated solutions of rubidium or caesium oxalate and uranyl oxalate were mixed in
equimolar proportions, heated to boiling and allowed to cool slowly. The yellow
crystals separated were washed with ice cold water, alcohol and then dried in air. The
method employed for the preparation of these compounds has been described earlier
[3]. The X-ray diffraction (XRD) patterns were recorded on a Siemen’s diffracto-
meter using CuKα radiation and graphite monochromator. The infrared absorption
spectra (IR) were recorded on a Perkin Elmer Model 180 spectrophotometer with the
samples dispersed in KBr discs. Simultaneous thermogravimetric (TG) and differen-
tial thermal analysis (DTA) were carried out on ULVAC Thermal Analyzer in flow-
ing stream of dry air and argon atmosphere at the rate of 10°C min–1. All the experi-
ments were carried out in platinum crucible with preheated alumina as reference
material for DTA. The gases used were of high purity.
Analysis
The compounds were analyzed for their uranium, oxalate, carbon, hydrogen and wa-
ter contents. Uranium was estimated potentiometrically using the Davies and Gray
method [7] and oxalate by volumetric titration with KMnO4. The carbon and hydro-
gen analysis were carried out on a Carlo Erba elemental analyzer. Water content was
determined from the thermogravimetric curves. The results of the analysis are shown
in Table 1. The analysis result confirmed the compounds to be Rb2UO2(C2O4)2·2H2O
and Cs2UO2(C2O4)2·2H2O.
J. Therm. Anal. Cal., 61, 2000

DAHALE et al.: URANYL OXALATES
109
Table 1 Chemical analysis in mass %
Rb2UO2(C2O4)2·2H2O
Cs2UO2(C2O4)2·2H2O
Compound
found
calcd
36.45
27.10
7.35
found
calcd
31.31
23.62
6.40
Uranium
Oxalate
Carbon
36.11
26.95
7.40
31.18
24.30
6.35
Hydrogen
0.60
0.61
0.53
0.53
Results and discussion
Thermal studies of Rb2UO2(C2O4)2·2H2O
TG and DTA curves were recorded up to 1000°C. Figures 1 and 2 show the thermal
decomposition of Rb2UO2(C2O4)2·2H2O in flowing atmosphere of dry air and argon
Fig. 1 TG and DTA curves of Rb2UO2(C2O4)2·2H2O recorded at 10°C min–1 in air.
Mass of the sample 36.6 mg
J. Therm. Anal. Cal., 61, 2000

110
DAHALE et al.: URANYL OXALATES
Fig. 2 TG and DTA curves of Rb2UO2(C2O4)2·2H2O recorded at 10°C min–1 in argon.
Mass of the sample 35.83 mg
gas. Four steps are clearly seen in TG curves. The corresponding DTA also showed
exothermic or endothermic DTA peaks indicating that the mass loss of compound oc-
curred in four stages to form the monouranate. In both atmospheres two water mole-
cules were lost in a single step, corresponding to endothermic DTA peaks centered
around 160 and 155°C respectively. The analysis of the TG curves indicated that the
water molecules were lost at 100 to 180°C. Isothermal heating at 180°C for few h
gave the exact water loss of the compound. The loss in mass of the compound corre-
sponds to the loss of the two water molecules both under argon and air. The X-ray dif-
fraction pattern of anhydrous compound Rb2UO2(C2O4)2 was found to be diffuse. An-
hydrous rubidium uranyl oxalate was stable up to 300°C as indicated by TG curve.
The second and third mass loss steps start after 300°C.
The thermal decomposition of anhydrous compound is highly exothermic and very
fast in dry air atmosphere, while comparatively slow in argon atmosphere. This is due to
oxidation of carbon monoxide to carbon dioxide in air. The reaction in air gave two exo-
thermic DTA peaks at 330 and 395°C and one endothermic peak at 475°C. A definite
plateau was observed after 475°C. From 475 to 620°C there was no mass loss. The fourth
mass loss started after 620°C accompanied by an endothermic DTA peak at 630°C. In
J. Therm. Anal. Cal., 61, 2000

DAHALE et al.: URANYL OXALATES
111
Fig. 3 TG and DTA curves of Cs2UO2(C2O4)2·2H2O recorded at 10°C min–1 in air.
Mass of the sample 32.48 mg
Fig. 4 TG and DTA curves of Cs2UO2(C2O4)2·2H2O recorded at 10°C min–1 in argon.
Mass of the sample 74.75 mg
J. Therm. Anal. Cal., 61, 2000

112
DAHALE et al.: URANYL OXALATES
both atmospheres the final product of decomposition after heating upto 700°C was iden-
tified as Rb2UO4 and therefore, identical mass loss was observed.
In argon atmosphere the decomposition of anhydrous compound occurred below
400°C with endothermic peak at 330°C. The fourth mass loss occurred after 480°C.
The mass loss at 600°C corresponded to the formation of RbUO3. When the com-
pound was heated up to 800°C the mass loss was not complete. XRD pattern of
heated product showed that the XRD patterns matched that of RbUO3. Isothermal
heating at 800°C for 2 to 3 h gave Rb2UO4. The observed mass loss of 27.48% is in
agreement with that calculated for conversion to Rb2UO4 (27.52%). In order to iden-
tify the intermediate products formed in the thermal decomposition the samples were
heated in air and argon at 350°C for few h. The product obtained in argon gave UO2
and Rb2CO3. The formation of UO2 was confirmed by XRD pattern, while that of car-
bonate by the IR spectra. The mass loss was in agreement with the formation of
Rb2CO3 and UO2 as intermediates which reacted at 650°C to form Rb2UO4. Heating
in air, however, did not show the presence of UO2 probably because in air it reacted
immediately with Rb2CO3. When the compound was heated up to 1000°C the product
correspond to the formation of Rb2U2O7.
Thermal studies of Cs2UO2(C2O4)2·2H2O
TG and DTA curves were recorded up to 1000°C for Cs2UO2(C2O4)2·2H2O. The ther-
mal decomposition in flowing atmospheres of air and argon are shown in Figs 3 and 4
respectively. In both atmospheres the compound lost two water molecules below
150°C in a single step which was accompanied by endothermic DTA peaks at 145
and 150°C respectively. The anhydrous compound remained stable up to 280°C. The
product obtained at 650°C was identified from its XRD pattern to be Cs2UO4. Al-
though the nature of mass loss in both atmospheres were almost identical but the
DTA curves were different. While the reaction between 280 and 450°C in air was ac-
companied by two exothermic peaks at 300 and 400°C and in argon had an endother-
mic peak at 310 and one exothermic peak at 330°C. Above 450°C there was a weak
endothermic peak around 790°C.
In the absence of any clear plateau in the TG curve where DTA indicated a reaction,
identification of the intermediate reaction products were tried by heating the sample at
350°C for long duration. The product obtained in argon at 350°C gave the XRD pattern
of UO2 whereas that in air diffuse pattern was obtained. The mass losses were in agree-
ment with the formation of Cs2CO3 and UO2. (Found 20.84%; calculated 20.33%.)
The formation of M2UO4 even under low oxygen potential conditions as in the
case of Cs2UO4 [7] is a measure of the stability of this structure.
It is observed that when Cs2UO2(C2O4)2·2H2O was heated at 400 to 450°C for
10–15 h in furnace, the mass loss corresponds to the formation of Cs2UO4. This com-
pound is very hygroscopic and therefore very difficult to record XRD pattern. Cs2UO4
decomposes at 650°C during which Cs2U2O7 and Cs4U5O17 are formed successively. The
latter compound in turn decomposes very slowly into Cs2U4O13 at 1000°C.
J. Therm. Anal. Cal., 61, 2000

DAHALE et al.: URANYL OXALATES
113
Fig. 5 TG and DTA curves of K2UO2(C2O4)2·3H2O recorded at 10°C min–1 in air. Mass
of the sample 37.9 mg
Fig. 6 TG and DTA curves of K2UO2(C2O4)2·3H2O recorded at 10°C min–1 in argon.
Mass of the sample 25.7 mg
J. Therm. Anal. Cal., 61, 2000

114
DAHALE et al.:URANYL OXALATES
Thermal studies of K2UO2(C2O4)2·3H2O
TG and DTA curves were recorded up to 900°C. The thermal decomposition of
K2UO2(C2O4)2·3H2O in flowing atmosphere of dry air and argon gas are shown in Figs 5
and 6 respectively. In both atmospheres the compound lost three water molecules below
190°C in two steps accompanied by endothermic DTA peaks at 50 and 140°C. In dry air
the anhydrous compound remained stable up to 290°C above which mass loss continued
up to 750°C. The product obtained at 700°C was identified from its XRD pattern to be
K2UO4. The reaction between 300 and 480°C in air was accompanied by two exothermic
DTA peaks at 350 and 415°C with shoulder at 395°C. Above 450°C there was weak en-
dothermic peak at 690°C corresponding to the formation of K2UO4.
In argon atmosphere the anhydrous compound remained stable up to 300°C
above which mass loss continued up to 770°C. The reaction between 300 and 500°C
in argon atmosphere was accompanied by two endothermic DTA peaks at 350 and
480°C. In argon atmosphere the reaction is comparatively slow and isothermal heat-
ing at 800°C for 2 to 3 h gave K2UO4. The observed mass loss of 34.98% is in agree-
ment with that calculated for conversion to K2UO4 (34.24%).
It therefore follows that the thermal decomposition of potassium, rubidium and
caesium uranyl oxalate can be represented as:
~300°C
M2UO2 (C2O4 )2 nH2O 100 to 160º→C M2UO2 (C2O4 )2
→
–nH O
2
300 to 400ºC
UO2 +M2C2O4 –CO →UO2 +M2CO3 700 to 800º→C M2UO4
CO+1/2O2 air→CO2 (exothermic)
(M=K, Rb and Cs)
The decomposition of oxalates is known to proceed with the evolution of CO gas
which is endothermic reaction in inert atmosphere. In air, the oxidation of CO to CO2
gives exothermic DTA peak during the thermal decomposition of the compounds.
X-ray and infrared spectroscopic studies
The X-ray diffraction patterns of Rb2UO2(C2O4)2·2H2O and Cs2UO2(C2O4)2·2H2O are
shown in Tables 2 and 3 and could be indexed on orthorhombic unit cells. The cell
parameters are also given in Tables 2 and 3. The density of the compounds were de-
termined by pycnometric method using toluene. The measured and calculated densi-
ties are in good agreement and the number of formula units per cell is 4.
The infrared spectra of the two oxalates were similar. Both have absorption
bands from the water molecules and coordinated oxalate groups as shown in Table 4.
The infrared spectrum of K2UO2(C2O4)2·3H2O has been discussed in terms of a simple
penta co-ordinated arrangement involving two bidentate oxalate groups and a water
molecule [5, 9]. As a result this compound loses water molecule in two steps. The
similarity of the IR bands with those of the potassium salt suggest similarity in struc-
ture.
J. Therm. Anal. Cal., 61, 2000

DAHALE et al.: URANYL OXALATES
115
Table 2 X-ray diffraction pattern of Rb2UO2(C2O4)2·2H2O, λ=1.5418 Å, a=10.959(3) Å,
b=17.176(2) Å, c=7.671(3) Å, Z=4. Density calcd=3.00 g cm–3. Density obsd=
3.02 g cm–3
dobsd
dcalcd
6.76
h
1
1
2
2
0
2
0
0
3
1
3
0
0
1
0
4
3
3
4
2
1
0
4
1
4
2
2
1
5
k
2
1
1
0
4
3
4
2
2
5
1
3
6
4
6
0
5
5
3
1
6
4
2
8
5
7
5
7
6
l
I/Io
10
80
20
50
10
45
20
40
100
70
70
10
15
10
20
40
10
15
30
10
20
25
80
10
10
25
25
20
20
6.76
0
1
0
1
0
0
1
2
0
0
1
2
0
2
1
1
0
1
1
3
2
3
2
0
1
2
5.90
5.90
5.22
5.22
4.46
4.46
4.30
4.30
3.96
3.96
3.75
3.75
3.50
3.50
3.36
3.36
3.28
3.28
3.24
3.24
3.19
3.19
2.86
2.86
2.767
2.68
2.767
2.68
2.58
2.58
2.50
2.50
2.38
2.38
2.353
2.296
2.244
2.200
2.157
2.109
2.062
1.933
1.922
1.747
1.741
2.352
2.296
2.245
2.197
2.158
2.108
2.063
1.934
1.921
1.748
1.740
3
3
0
J. Therm. Anal. Cal., 61, 2000

116
DAHALE et al.: URANYL OXALATES
Table 3 X-ray diffraction pattern of Cs2UO2(C2O4)2·2H2O, λ=1.5418 Å, a=9.315(2) Å,
b=15.999(1) Å, c=12.109(3) Å, Z=4. Density obsd=2.80 g cm–3. Density calcd=
2.75 g cm–3
dobsd
6.07
5.07
4.47
4.19
4.03
3.61
3.36
3.22
3.02
2.61
2.51
2.42
2.29
2.15
2.08
2.01
1.72
dcalcd
6.07
5.08
4.47
4.19
4.03
3.61
3.36
3.22
3.02
2.61
2.51
2.42
2.29
2.15
2.08
2.01
1.72
h
1
1
2
2
2
1
1
0
1
0
2
1
4
1
1
2
3
k
2
0
1
1
2
1
2
3
5
6
1
5
0
3
7
6
4
l
I/Io
90
5
0
2
0
1
0
3
3
3
0
1
4
3
1
5
2
3
5
10
45
100
15
10
30
60
25
15
30
15
30
30
60
50
Table 4 Infrared spectra of alkali metal uranyl oxalate hydrates (cm–1)
Assignment
νOH
K [9]
Rb
Cs
3630, 3390
1715, 1685, 1655
3410, 3630
1715, 1655
νasCO
1727, 1718, 1655
νsyCO
1455, 1420, 1288
1270
1450, 1405, 1380,
1295, 1250
1435, 1405,
1280, 1250
νasO–U–O
νsyO–U–O
O–C–O
927
925
835
785
920
830
785
840
787, 740
The thermal decomposition scheme discussed above is further confirmed by the
IR spectra of these compounds heated to about 350°C in argon atmosphere. Spectral
bands due to carbonate appear in the range 1100–1055 cm–1 and uranyl bands in the
range 950–920 cm–1 disappear as expected. In argon atmosphere the reactions are
comparatively slow and formation of UO2 could be observed. When heated in air for-
mation of intermediate compound UO2 is not observed.
* * *
J. Therm. Anal. Cal., 61, 2000

DAHALE et al.: URANYL OXALATES
117
The authors are thankful to Dr. D. D. Sood, Director, Radiochemistry and Isotope Group for his keen
interest and constant encouragement during the course of this work.
References
1 D. C. Fee, I. Johnson, S. A. Davis, W. A. Shin, G. E. Staahl and C. E. Johnson, ANL-Report,
No. 76–126 (1977).
2 D. C. Fee and C. E. Johnson, J. Inorg. Nucl. Chem., 40 (1978) 1375.
3 I. Chernyaev, Complex compounds of Uranium, Israel Programme of Scientific Translations,
Jerusalem 1966, p. 114.
4 N. W. Alcock, J. Chem. Soc. Dalton Trans., part II, (1973) 1614.
5 N. C. Jayadevan, K. D. Singh Mudher and D. M. Chackraburtty, National Symp. on Crystal-
lography, Kharagpur 1982, category 04.2.
6 N. D. Dahale, K. L. Chawla, N. C. Jayadevan and V. Venugopal, Thermochim. Acta., 293
(1997) 163.
7 W. Davies and W. Gray, Talanta, 11 (1964) 1203.
8 E. H. P. Cordfunke, Proc. Symp. on Thermodyn. Nucl Mater, IAEA, Vienna Vol. 2, 1975,
p. 185.
9 R. N. Shchelokov and W. W. Karasev, Russ. J. Inorg. Chem. (1974) 766/9 quoted in Gmelin
Handbook of Inorganic Chemistry, Uran. Supplement Vol. 13, 1983, p. 199.
J. Therm. Anal. Cal., 61, 2000