Structural and thermal studies on PuTe2O6

Article abstract of DOI:10.1016/S0925-8388(00)00841-0

PuTe₂O₆ was synthesised by the solid state reaction route and characterized by X-ray diffraction and thermal methods. The structure of PuTe₂O₆ was derived by the Rietveld analysis of X-ray powder diffraction data in the monoclinic system with cell parameters a = 0.69937(1), b = 1.10014(2), c = 0.73404(2) nm, β = 107.98(2)°, Z = 4 in the space group P2₁/n. In the structure, each plutonium atom is coordinated to eight oxygen atoms. The structure is made up of zigzag strings of PuO₈ distorted edge-sharing polyhedron parallel to the a axis. Tellurium atoms are coordinated to three oxygen atoms. PuTe₂O₆ melts at 1125 K incongruently and decomposes according to the reaction: PuTe₂O₆(s) → PuO₂(s) + 2TeO₂(g). The kinetics of decomposition under isothermal heating conditions in flowing air were studied to determine the rate constants, activation energy and reaction mechanism.

Full text of DOI:10.1016/S0925-8388(00)00841-0

Journal of Alloys and Compounds 307 (2000) 114–118
L
www.elsevier.com/locate/jallcom
Structural and thermal studies on PuTe2O6
*
K. Krishnan, K.D.S. Mudher , V. Venugopal
Fuel Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
Received 7 January 2000; accepted 22 February 2000
Abstract
PuTe2O6 was synthesised by the solid state reaction route and characterized by X-ray diffraction and thermal methods. The structure of
PuTe2O6 was derived by the Rietveld analysis of X-ray powder diffraction data in the monoclinic system with cell parameters
a50.69937(1), b51.10014(2), c50.73404(2) nm, b5107.98(2)8, Z54 in the space group P21 /n. In the structure, each plutonium atom is
coordinated to eight oxygen atoms. The structure is made up of zigzag strings of PuO8 distorted edge-sharing polyhedron parallel to the a
axis. Tellurium atoms are coordinated to three oxygen atoms. PuTe2O6 melts at 1125 K incongruently and decomposes according to the
reaction: PuTe2O6(s)→PuO2(s)12TeO2(g). The kinetics of decomposition under isothermal heating conditions in flowing air were
studied to determine the rate constants, activation energy and reaction mechanism.
2000 Elsevier Science S.A. All rights reserved.
Keywords: PuTe2O6; Crystal structure; Thermochemistry
1. Introduction
derived by Rietveld profile analysis of X-ray powder
diffraction data, while the data on the rate of decomposi-
tion during isothermal heating experiments were used to
evaluate kinetic parameters, such as activation energy and
mechanism of decomposition.
Crystal structure and thermochemistry of the binary and
ternary oxides of thorium, uranium, plutonium and fission
products formed during the irradiation of oxide fuels is
important in evaluating their performance in a reactor.
Tellurium, one of the highly corrosive fission products that
embrittles the steel claddings in fast breeder reactors [1],
also exists in different chemical states in an irradiated fuel
[2]. Various compounds of tellurium with uranium [3–5]
and other actinides [6–8] have been synthesised and
characterized by X-ray diffraction. In the systematic
studies, we have earlier published the thermochemical,
vaporisation behaviour and the Gibbs’ energy of formation
of tellurium bearing compounds in the M–Te–O system,
where M5Ce, Th, U and Ni [9–12].
In the Pu–Te–O system, Pu2O2Te is a well character-
ized phase and is reported to be isostructural with the
corresponding rare earth oxide tellurides [13]. Though
there is some data regarding the preparation of PuTe2O6
[14], no detailed investigations have been carried out on its
structure and thermochemistry. In the present work, we
report the preparation, crystal structure and thermal studies
on PuTe2O6. The crystal structure of PuTe2O6 has been
2. Experimental
PuTe2O6 was prepared by the solid state reaction route,
by heating well-ground mixtures of PuO2 and TeO2 in 1:2
molar ratio in an alumina boat in static air atmosphere at
975 K for 24 h. The product was removed intermittently,
ground and reheated to obtain pure phase of the compound.
The X-ray diffraction data was collected on a Diano
diffractometer, enclosed in a glove box to handle radioac-
tive material, using graphite monochromatised CuKa1
radiation (l50.15406 nm). The step scanned intensity data
were recorded in the 2u range of 10–1008 with a step of
0.028, with a counting time of 5 s for each step. The X-ray
data were analyzed using the Rietveld analysis program
DBWS-9411 [15] to derive the structure.
The thermal stability of PuTe2O6 was studied by
recording the TG and DTA patterns simultaneously using
the Mettler thermoanalyser enclosed in a glove box. The
experiments were carried out in platinum cups in flowing
dry air, at the heating rate of 10 K/min up to 1673 K.
*Corresponding author.
E-mail address: kdsingh@apsara.barc.ernet.in (K.D.S. Mudher)
0925-8388/00/$ – see front matter
PII: S0925-8388(00)00841-0
2000 Elsevier Science S.A. All rights reserved.

K. Krishnan et al. / Journal of Alloys and Compounds 307 (2000) 114–118
115
Table 2
Pre-heated alumina was used as the reference standard for
a
Refined atomic coordinates and temperature factor for PuTe2O6
DTA measurements. For evaluating the kinetic parameters,
isothermal heating measurements were carried out at 1193,
1208, 1223 and 1233 K in the same instrument.
2
˚
Atom
x
y
z
Biso(A )
Pu
0.2571(6)
0.2322(9)
0.2623(8)
0.145(3)
0.405(4)
0.063(3)
0.155(3)
0.524(4)
0.182(4)
0.0895(3)
0.2736(5)
0.0065(4)
0.4326(7)
0.3(1)
1.5(3)
1.1(4)
2.6(6)
3.5(8)
2.8(7)
2.1(7)
2.8(8)
2.9(7)
Te1
Te2
O1
O2
O3
O4
O5
O6
20.0742(6)
0.249(2)
20.4607(8)
0.160(2)
3. Results and discussion
0.405(2)
0.388(2)
0.020(2)
0.423(3)
0.483(4)
3.1. Crystal structure
20.297(3)
20.323(2)
20.389(3)
20.101(2)
20.229(3)
The X-ray diffraction data of PuTe2O6 was indexed on a
monoclinic cell. The least squares refined values of the
lattice parameters are given in Table 1. The extinction
conditions corresponded to the space group P21 /n. The
similarity in the crystal data of PuTe2O6 and CeTe2O6
[16] suggests that both the compounds are isostructural.
Wroblewska et al. [14] have reported an orthorhombic cell,
a E.s.d. values are given in parentheses.
Powder cell [17] and is shown in Fig. 2, depicting the
atoms in the unit cell projected along the b-axis. The
structure of PuTe2O6 is similar to that of CeTe2O6. In the
structure, each plutonium atom is surrounded by eight
oxygen atoms at distances ranging from 0.222 to 0.254 nm
(average distance 0.235 nm). The corresponding Pu–O
distances in Pu(SO4)2?4H2O range from 0.231 to 0.241
nm at an average distance of 0.23360.003 nm in an
Archemedian square antiprism geometry [18]. The eight
coordination of Ce41 ion in CeTe2O6 has been described
as trigonal prism bicapped over two rectangular faces [16].
The structure of PuTe2O6 consists of zigzag chains of
PuO8 distorted polyhedra running parallel to a axis. These
polyhedra are linked to each other by the common O2 –O2
and O3 –O3 edges. Each tellurium atom is surrounded by
three oxygen atoms at distances given in Table 3. Of the
three oxygen atoms around Te1 atom, O2 and O3 share
common edge with PuO8 polyhedra and O1 atom is linked
through the corner of other plutonium polyhedra. All the
three oxygen atoms around Te2 atom i.e. O4, O5 and O6
share corners with three plutonium polyhedra. The oxygen
atoms coordinated to tellurium atoms belong to PuO6
˚
a55.60, b510.46 and c511.76 A for PuTe2O6. However,
the present X-ray data could not be indexed with the
reported cell parameters.
The crystal structure of PuTe2O6 was derived using the
atomic parameters reported for CeTe2O6 as the trial model.
Rietveld’s method was used for the refinement of the
X-ray data, using the DBWS 11 program [15]. The variables
include a scale factor, six background parameters, three
half width parameters defining pseudo Voigt profile peak
shape, the unit cell dimensions, atomic positions, thermal
parameters for plutonium and tellurium atoms. Details of
refinement are given in Table 1. The refinement of the
parameters gave final profile agreement factors of Rp 5
10.4%, Rwp 512.6% and Rexp 57.9% and S51.6. The
fractional atomic coordinates and isotropic thermal param-
eters of the atoms in PuTe2O6 are given in Table 2, and
selected atomic distances and interatomic angles are given
in Table 3. The observed, calculated and difference X-ray
powder diffraction patterns are shown in Fig. 1.
chains. Thus, in the structure there are infinite chains of
(PuO6) , which are linked by tellurium atoms.
8n2
The structure of PuTe2O6 was drawn using software
2n
Table 1
Table 3
Details of Rietveld refinement for PuTe2O6
Selected bond lengths (nm) and interatomic angles in PuTe2O6; estimated
standard deviations are given in parentheses
Chemical formula
Formula weight
Space group
a (nm)
b (nm)
c (nm)
PuTe2O6
590.2
P21 /n
0.69937(1)
1.10014(2)
0.73404(2)
107.98(2)
4
7.29
10–100
0.02
0.15406
4500
Pseudo Voigt
10.4
Pu–O1
Pu–O2
Pu–O2
Pu–O3
Pu–O3
Pu–O4
Pu–O5
Pu–O6
0.235(2)
0.235(3)
0.246(2)
0.221(2)
0.254(2)
0.225(2)
0.236(2)
0.227(3)
Te1 –O1
Te1 –O2
Te1 –O3
0.192(2)
0.190(3)
0.184(2)
a
b
c
Te2 –O4
Te2 –O5
Te2 –O6
0.191(2)
0.182(2)
0.192(3)
b (8)
Z
d
e
rcalc.(g cm23
2u range (8)
)
Step size (2u, 8)
Wavelength (nm)
No. of data points
Peak profile
Rp (%)
Rwp (%)
Rexp(%)
S
O1 –Te1 –O2
O1 –Te1 –O3
O2 –Te1 –O3
a
93.9(9)
106.7(1.0)
86.7(9)
O4 –Te2 –O5
O4 –Te2 –O6
O5 –Te2 –O6
105.2(9)
96.0(1.0)
91.9(1.0)
2 1/2 1 x,1/2 2 y, 2 1/2 1 z.
b 1/2 2 x, 2 1/2 1 y,1/2 2 z.
c 1/2 1 x,1/2 2 y, 2 1/2 1 z.
d 1 2 x, 2 y, 2 z.
12.6
7.9
1.6
e 1/2 2 x,1/2 1 y, 2 1/2 2 z.

116
K. Krishnan et al. / Journal of Alloys and Compounds 307 (2000) 114–118
Fig. 1. Observed (dots) and calculated X-ray diffraction Rietveld plot for PuTe2O6. Difference curve is shown at the bottom of the plot.
Fig. 2. Structure plot of PuTe2O6 projected along the b-axis showing the coordination around Pu and Te atoms..

K. Krishnan et al. / Journal of Alloys and Compounds 307 (2000) 114–118
117
3.2. Thermal studies
mechanisms as reported in the literature [19]. The regres-
sion coefficients (g ) calculated for all the kinetic models
using iterative procedure are given in Table 4. The
correlation coefficients for the phase boundary controlled
mechanisms are more consistent with the value approach-
ing unity at all the temperatures among the mechanisms
assumed.
Generally, the mechanism of the decomposition re-
actions resulting in solid/solid and gaseous products could
be controlled either by diffusion or phase boundary
reaction. Our earlier studies on ThTe2O6 and CeTe2O6
[9,10] have indicated similar mechanism as governed by
the phase boundary controlled. Also, the observed linearity
in the nature of vaporisation at all temperatures in the
present study suggests that, the surface area of the reactant
is constant throughout the course of the reaction and the
rate governing equation can be represented by the phase
boundary controlled mechanism as given in Eq. (3)
The thermogravimetric curve shows that PuTe2O6 is
stable in air up to 1173 K, beyond which the compound
starts losing weight due to its decomposition. The weight
loss is complete around 1573 K. The product at the end of
the decomposition was identified by X-ray diffraction as
PuO2. The observed weight loss of 55.1% was in agree-
ment with the expected loss of 54.0% for two moles of
TeO2, during the decomposition of PuTe2O6 according to
the Eq. (1)
PuTe2O6(s) → PuO2(s) 1 2 TeO2(g)
(1)
DTA showed an sharp endothermic peak at 1125 K and
did not show any reversible peak due to solidification,
indicating that the compound PuTe2O6 melts incongruent-
ly. The compound was heated in a furnace up to its melting
temperature and cooled slowly to room temperature. The
X-ray diffraction pattern of the product was identified as a
mixture of PuO2 and TeO2 confirming further the incon-
gruent melting of the compound.
g(a) 5 1 2 (1 2 a)n 5 kt/r0
(3)
where a is the fraction decomposed, n is equal to 1/3, 1/2
and 1 for 3-, 2- and 1-dimensional symmetries; r0 is the
radius of the reactant and g(a) is the function of a. Since
the change in radii of the reactant to the product is
negligible for one-dimensional symmetry, the value of n
becomes 1 and the Eq. (3) reduces to
3.2.1. Kinetic studies
Kinetic parameters such as the rate constant k and
activation energy E were evaluated from the isothermal
data at temperatures 1193, 1208, 1223 and 1233 K. The
weight loss at each isothermal temperature was followed
with respect to time. The fraction decomposed a was
calculated from the weight loss measurements at all the
temperatures using the relation
g(a) 5 a 5 kt
(4)
The plot of a against time t for all the temperatures is
shown in Fig. 3. The rate constant k for each temperature
were calculated from the slopes of the curves and then fit
into the Arrhenius equation, to derive the activation energy
of decomposition of PuTe2O6. The rate constants k derived
from the slopes of the linear fit are given in Table 5. The
representation of 2ln k and 1/T (K) in an Arrhenius plot
as shown in Fig. 4, gave an activation energy value of
278610 kJ/mol for the decomposition of PuTe2O6.
a 5 (W 2W0)/(Wf 2W0)
(2)
t
where W0, W and Wf are the initial weight, weight at time t
t
and final weight for the completion of the reaction,
respectively. The a values were fit in the range 0.15–0.80
into various kinetic models for all the diffusion controlled,
nucleation growth and phase boundary controlled reaction
Table 4
Regression coefficient values (g ) obtained for the most commonly used mechanisms in the a range of 0.15–0.80 at different temperatures
Mechanism
g (a)
1193 K
1208 K
1223 K
1233 K
1. Diffusion controlled
Parabolic law
Valensi, two-dimensional
Jander, three-dimensional
Brounshteub–Ginstling
Mampel-unimolecular law
a2
0.9867
0.9767
0.9620
0.9721
0.9944
0.9892
0.9762
0.9547
0.9697
0.9882
0.9848
0.9711
0.9499
0.9465
0.9863
0.9877
0.9741
0.9511
0.9670
0.9873
[(1 2 a) log (1 2 a)] 1 a
[1 2 (1 2 a)1 / 3
]
2
[1 2 2a/3 2 (1 2 a)2 / 3
[2log (1 2 a)]
]
2. Nucleation growth
Two-dimensional
Three-dimensional
2 log (1 2 a)1 / 2
2 log (1 2 a)1 / 3
0.9989
0.9952
0.9987
0.9988
0.9994
0.9991
0.9990
0.9985
3. Phase boundary controlled
One-dimensional
Two-dimensional
a
0.9992
0.9991
0.9983
0.9993
0.9984
0.9960
0.9999
0.9970
0.9943
0.9994
0.9980
0.9955
[1 2 (1 2 a)1 / 2
[1 2 (1 2 a)1 / 3
]
]
Three-dimensional

118
K. Krishnan et al. / Journal of Alloys and Compounds 307 (2000) 114–118
cross-linked with TeO3 units. The structure of PuTe2O6 is
similar to that of CeTe2O6.
PuTe2O6 melts incongruently and decomposes above
1173 K with the loss of TeO2(g) as the vaporizing species.
The kinetics of decomposition studied in flowing air under
isothermal heating conditions showed that the decomposi-
tion of PuTe2O6 is consistent with the phase boundary
controlled mechanism. The activation energy evaluated for
the decomposition of PuTe2O6 was found to be 278610
kJ/mol.
Acknowledgements
The authors are thankful to Shri D.S.C. Purushotham,
Director Nuclear Fuels Group and Shri R. Prasad, Head,
Fuel Development Chemistry Section for their keen inter-
est in this work and Dr. G.A. Rama Rao for helpful
discussion.
Fig. 3. Plots of a vs. time (min) at various isothermal temperatures.
Table 5
Rate constants for the decomposition of PuTe2O6 at different tempera-
tures
(K)
2 ln k
References
1193
1208
1223
1233
4.9275
4.5401
4.2455
4.0172
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PuTe2O6 belongs to the monoclinic crystal system. The
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Fig. 4. Arrhenius plot of 2ln k vs. 1/T (K) for the decomposition of
PuTe2O6(s).