Structural and thermochemical studies on Cr2TeO6 and Fe2TeO6

Article abstract of DOI:10.1016/S0925-8388(00)01508-5

The crystal structure and measurement of the thermodynamic quantities of Cr₂TeO₆ and Fe₂TeO₆ prepared by the solid state reaction were studied. The crystal structure was derived from X-ray powder diffraction, wh

Full text of DOI:10.1016/S0925-8388(00)01508-5

Journal of Alloys and Compounds 316 (2001) 264–268
L
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Structural and thermochemical studies on Cr2TeO6 and Fe2TeO6
*
K. Krishnan, K.D. Singh Mudher , G.A. Rama Rao, V. Venugopal
Fuel Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
Received 3 July 2000; accepted 14 November 2000
Abstract
Cr2TeO6 and Fe2TeO6 were prepared by the solid-state reaction route. The crystal structure was derived for both compounds from
X-ray powder diffraction data. Cr2TeO6 and Fe2TeO6 are isostructural and have the trirutile structure. The Gibbs free energy of formation
(DfG8) for Cr2TeO6 and Fe2TeO6 was obtained from vapor pressure data employing the Knudsen Effusion Mass Loss technique (KEML)
and is given by the relation
DfG8Cr2TeO6(s) 5 (21651.6 1 0.5683T)615 kJ/mol (1014–1100 K)
DfG8Fe2TeO6(s) 5 (21234.3 1 0.4729T)615 kJ/mol (979–1052 K).
2001 Elsevier Science B.V. All rights reserved.
Keywords: Oxide materials; Crystal structure; Thermodynamic properties; Nuclear reactor materials; Solid-state reactions
1. Introduction
study of the crystal structure and measurement of the
thermodynamic quantities of the tellurates of chromium
and iron, namely Cr2TeO6 and Fe2TeO6. The crystal
structure was derived from X-ray powder diffraction data,
whereas thermodynamic quantities such as enthalpy of
vaporization and the standard Gibbs energy of formation
were calculated from vapor pressure measurements over
Cr2TeO6(s) and Fe2TeO6(s).
The structure and thermochemistry of binary and ternary
oxides of uranium, plutonium and fission products formed
during the irradiation of oxide fuels is important in
evaluating their performance in a reactor [1]. Tellurium is
one of the highly reactive fission products, which embrit-
tles the stainless steel cladding components [2] containing
Cr, Fe, Ni, Zr, Nb, etc. used in Fast Breeder Reactors
(FBR). The binary phase diagram of M–Te systems (M5
Fe, Cr, Ni, Mo, Nb, Zr, La, Ru and Ag) have been
compiled and evaluated in the literature [3]. In the
transition metal–tellurium–oxygen system, the formation
of several compounds, through the solid-state reaction
route, such as Cr2Te3O9, Fe2Te3O9, CoTeO3, CoTe6O13,
NiTeO3, NiTe2O5 and Ni2Te3O8, has been reported by
Sokolov et al. [4]. Thermal and structural studies of the
phase transformations of the tellurites of trivalent
chromium and iron have been carried out by Gospodinov
and Gjurova [5]. Recently, we reported the preparation,
characterization and vaporization behavior of compounds
in the Ni–Te–O system [6]. In continuation of our earlier
investigations on the M–Te–O system, we report here a
2. Experimental
2.1. Preparation and characterization of the compounds
Cr2TeO6 and Fe2TeO6 were prepared by the solid-state
reaction route by heating well-ground mixtures of Cr2O3
and Fe2O3 with TeO2 in their respective molar ratios of
1:1 in the form of pressed pellets in an alumina boat in air
at 975 K for 24 h. Samples were reground and refired
twice to obtain single-phase compounds. The formation of
the compounds was confirmed from their X-ray diffraction
patterns recorded on a Diano X-ray diffractometer using
graphite monochromatized Cu Ka1 radiation (l 5 0.15406
nm). The step-scanned intensity data was obtained in the
2u range of 15 to 1008 with a step of 0.028, by counting for
5 s at each step. Refinement of the structure was carried
*Corresponding author. Fax: 191-22-550-5150.
E-mail address: kdsingh@apsara.barc.ernet.in (K.D. Singh Mudher).
0925-8388/01/$ – see front matter
PII: S0925-8388(00)01508-5
2001 Elsevier Science B.V. All rights reserved.

K. Krishnan et al. / Journal of Alloys and Compounds 316 (2001) 264–268
265
Table 2
out by the Rietveld profile method using the computer
program DBWS-9411 [7] for deriving the structure.
Atomic parameters for Cr2TeO6 and Fe2TeO6. Estimated standard
deviations are given in parentheses
Atom
Site
x
y
z
B
2
(A )
2.2. Thermal and vapor pressure measurements
˚
Cr2TeO6
Cr
Te
O1
O2
The thermal stability of Cr2TeO6 and Fe2TeO6 was
studied by recording the thermogravimetric (TG) patterns
in air in a Mettler thermoanalyser at a heating rate of 10
K/min. The experiments were carried out in platinum cups
in flowing dry air up to 1673 K.
Mass loss measurements were carried out in a Cahn
Vacuum microbalance by the Knudsen Effusion Mass Loss
(KEML) technique. A boron nitride cell with a knife edge
orifice of approximately 1.0 mm diameter at the center of
the lid was used as the Knudsen cell. The vapor pressures
of TeO2(g) over Cr2TeO6(s) and Fe2TeO6(s) were mea-
sured in the temperature range 1014–1100 and 979–1052
K respectively. The experimental conditions were similar
to those reported in our earlier studies on the Ni–Te–O
system [6].
4e
2a
4f
0
0
0
0
0.3338(3)
0
0
0.67(6)
1.03(4)
1.01(16)
1.41(31)
0.3058(13)
0.3082(8)
0.3058(13)
0.3082(8)
8j
0.3359(8)
Fe2TeO6
Fe
Te
O1
O2
4e
2a
4f
0
0
0
0
0.3368(5)
0
0
0.74(7)
0.96(5)
1.11(18)
1.32(29)
0.2937(24)
0.3097(15)
0.2937(24)
0.3097(15)
8j
0.3346(16)
basis of the trirutile structure by Rietveld profile analysis
of the step-scanned X-ray intensity data. The variables
include a scale factor, six background parameters, three
half-width parameters defining the pseudo-Voigt profile
peak shape, the unit cell dimensions, atomic positions,
thermal parameters for chromium (or iron) and tellurium
atoms. Details of refinement and agreement factors are
given in Table 1. The structural parameters including
fractional atomic coordinates and the isotropic thermal
parameters of the atoms and selected inter-atomic distances
for Cr2TeO6 and Fe2TeO6 are given in Tables 2 and 3,
respectively. The observed, calculated and difference X-
ray powder diffraction pattern for Cr2TeO6 is shown in
Fig. 1. The structures of Cr2TeO6 and Fe2TeO6 are similar
and the structure of Cr2TeO6 is shown in Fig. 2. In the
structure, each Cr and Te atom is surrounded by six
oxygen atoms in an octahedral coordination. The cation
oxygen octahedra form edge-sharing chains which are
occupied alternately by TeO6 and CrO6 octahedra in the
ratio of 1:2.
3. Results and discussion
3.1. X-ray studies
The X-ray diffraction data for these compounds were
indexed on a tetragonal cell and the least squares refined
values of the lattice parameters are given in Table 1. The
X-ray powder diffraction data for Cr2TeO6 and Fe2TeO6
are in good agreement with the data reported in the
literature [8]. The similarity in the X-ray powder patterns
suggests that both the compounds are isostructural.
Oxides with the general formula A2BO6 are known to
exist in the trirutile structure [9]. The crystal structure
parameters for Cr2TeO6 and Fe2TeO6 were refined on the
3.2. Thermal studies
Table 1
a
Rietveld refinement data for Cr2TeO6 and Fe2TeO6
The TG curves in Figs. 3 and 4 show that both Cr2TeO6
and Fe2TeO6 are stable in air up to 1123 K. Cr2TeO6
decomposes rapidly in the range 1250–1350 K and the
mass loss is complete at 1623 K. The decomposition of
Fe2TeO6 takes place over a wider temperature range,
1200–1550 K, and is complete at 1673 K. The slow
Chemical formula
Formula weight
Space group
a (nm)
b (nm)
c (nm)
Cr2TeO6
327.6
Fe2TeO6
335.3
P4/mnm
0.45453(1)
0.45453(1)
0.90119(3)
2
P4/mnm
0.46057(2)
0.46057(2)
0.90923(4)
2
Z
r(cal) (g cm23
)
–
–
2u range (deg)
Step size (2u, deg.)
Wavelength (nm)
No. of data points
Peak profile
Rp (%)
Rwp (%)
Rexp. (%)
S
15–100
0.02
0.15406
4250
Pseudo-Voigt
8.79
11.15
15–100
0.02
0.15406
4250
Pseudo-Voigt
10.45
8.53
Table 3
˚
Main interatomic distance (A) for M2TeO6 (where M5Cr or Fe)
Cr2TeO6
Fe2TeO6
Te–O1 32
Te–O2 34
M–O1 32
M–O2 32
M–O2 32
M–M
1.966 (6)
1.925 (6)
1.950 (4)
1.964 (6)
1.981 (4)
2.996 (5)
3.008 (3)
1.913 (11)
1.949 (11)
2.002 (8)
1.991 (8)
2.017 (8)
2.968 (9)
3.062 (5)
8.64
1.29
7.81
1.33
a Rp 5 100 3 ouyobs 2 ycalu/ouyobsu;
R
wp 5 100 3 h[ow(yobs 2 ycal )2]/
M–Te
[ow(yobs)2]j1 / 2
.

266
K. Krishnan et al. / Journal of Alloys and Compounds 316 (2001) 264–268
Fig. 1. Observed (?) and calculated (———) X-ray diffraction Rietveld plot for Cr2TeO6. The difference curve is shown at the bottom of the plot.
decomposition of Fe2TeO6 in comparison to that of
Cr2TeO6 suggests that the activation energy associated
with the decomposition of Fe2TeO6 is more than that of
Cr2TeO6. The final products of the decomposition of
Cr2TeO6 and Fe2TeO6, due to the combined loss of 1 mol
of TeO2 and 0.5 mol of O2, were identified by XRD as
Cr2O3 and Fe2O3, respectively. During decomposition, the
observed mass losses of 53.9% for Cr2TeO6 and 52.8% for
Fe2TeO6 are in good agreement with the expected losses
of 53.6 and 52.4% as represented by Eqs. (1) and (2):
1
]
Cr2TeO6(s) → Cr2O3(s) 1 TeO2(g) 1 2 O2(g)
(1)
(2)
1
]
Fe2TeO6(s) → Fe2O3(s) 1 TeO2(g) 1 2 O2(g)
Fig. 3. Thermogravimetric curve for the decomposition of Cr2TeO6(s)
under non-isothermal conditions in air at a heating rate of 10 K/min.
Fig. 2. Structure of Cr2TeO6 showing the coordination around Cr and Te.

K. Krishnan et al. / Journal of Alloys and Compounds 316 (2001) 264–268
267
Table 4
Gibbs energy of formation of Cr2TeO6(s) at different temperatures
Temp.
(K)
p(TeO2)
(Pa)
p8(TeO2)
(Pa)
p(O2)
(Pa)
DfG8Cr2TeO6(s)
(kJ/mol)
1014
1021
1028
1034
1040
1048
1053
1059
1066
1072
1079
1092
1100
0.447
0.627
0.771
1.003
1.294
1.719
2.093
2.700
3.152
3.951
4.881
5.739
7.657
18.226
21.695
25.764
29.799
34.408
41.573
46.722
53.672
62.973
72.098
84.268
111.980
132.950
0.100
0.140
0.173
0.224
0.290
0.385
0.460
0.605
0.706
0.884
1.093
1.284
1.714
21076.9
21071.8
21068.3
21064.2
21060.2
21055.4
21052.2
21048.0
21045.0
21041.2
21037.3
21033.2
21028.1
Fig. 4. Thermogravimetric curve for the decomposition of Fe2TeO6(s)
under non-isothermal conditions in air at a heating rate of 10 K/min.
log p (kPa) 5 (215920.14/T
1 12.408)60.04 (1014–1100 K)
(7)
3.3. Vapor pressure measurements
The standard Gibbs free energy of the reaction shown in
Eq. (1) can be represented as
3.3.1. Cr2TeO6
The equilibrium pressures of TeO2(g) and O2(g)
1
2
]
1
measured in the temperature range 1014 to 1100 K over
Cr2TeO6(s) and Cr2O3(s) were due to the combined loss of
]
DrG8 5 DfG8Cr2O3(s) 1 DfG8TeO2(g) 1 2 DfG8O2(g)
2 DfG8Cr2TeO6(s)
(8)
the gaseous products as represented in Eq. (1). The total
1
]
mass loss due to TeO2(g) and O2(g) was apportioned in
2
Substituting DrG8 5 2 RT ln K and rearranging Eq. (8),
terms of their individual masses using their mole fractions
we obtain
for the reaction given in Eq. (1). The mathematical
derivation for the rate of mass loss (in g/s) is similar to
that adopted for Ni3TeO6 [6] and is
DfG8Cr2TeO6(s) 5 DfG8Cr2O3(s) 1 DfG8TeO2(l)
1
]
1 2 RT ln pO2(g)
1 RT ln( pTeO2 /p8TeO2)
(9)
dw/dt 5 dw1 /dt 1 dw2 /dt
(3)
(4)
where p8TeO2 is the partial pressure of TeO2(g) over
TeO2(l). The values of the Gibbs free energy of formation
for Cr2O3(s) and TeO2(l) are taken from the literature
[10,11]. Using the above values, the Gibbs free energy of
formation of Cr2TeO6 was calculated and is given in Table
4, and can be represented by the relation
dw2 /dt 5 (dw1 /dt)(32/159.6n)
where ‘‘dw/dt’’ is the experimentally measured combined
rate of mass loss of TeO2(g) and O2(g), ‘‘dw1 /dt’’ and
‘‘dw2 /dt’’ are the individual rates of mass loss of TeO2(g)
and O2(g), respectively, and ‘‘n’’ is the ratio of number of
moles of TeO2(g) to O2(g), which is equal to 2 in the
present study. Substituting the value of ‘‘dw2 /dt’’ in Eq.
(3) and rearranging, we obtain
dw1 /dt 5 (dw/dt)(159.6/175.6) 5 (dw/dt)0.9089
(5)
By substituting the individual mass loss of TeO2(g) and
O2(g) in the rate mass equation:
p (Pa) 5 dw/dt[1/kA](2.28 3 1023)œT/M
(6)
the individual partial pressures contributed by TeO2(g) and
O2(g) were determined and are given in Table 4. The
corresponding least squares fit between log p and 1/T for
TeO2 is shown in Fig. 5 and can be represented by the
relation
Fig. 5. Temperature dependence of the vapor pressure of TeO2(g) over
Cr2TeO6(s) and Cr2O3(s).

268
K. Krishnan et al. / Journal of Alloys and Compounds 316 (2001) 264–268
Table 5
Substituting DrG8 5 2 RT ln K and rearranging the above
equation:
Gibbs energy of formation of Fe2TeO6(s) at different temperatures
Temp.
(K)
p(TeO2)
(Pa)
p8(TeO2)
(Pa)
p(O2)
(Pa)
DfG8Fe2TeO6(s)
(kJ/mol)
DfG8Fe2TeO6(s) 5 DfG8Fe2O3(s) 1 DfG8TeO2(l)
1
]
1 2 RT ln pO2(g)
979
983
992
1.218
1.427
1.490
1.766
1.819
2.187
2.450
3.197
3.948
6.115
7.204
7.646
7.346
8.177
0.273
0.319
0.333
0.395
0.407
0.489
0.549
0.716
0.884
1.370
1.612
1.712
2769.8
2767.4
2765.9
2763.1
2762.2
2760.4
2757.4
2752.9
2749.2
2743.1
2739.5
2737.6
1 RT ln( pTeO2 /p8TeO2)
(13)
10.372
12.126
13.443
14.890
16.902
21.165
25.764
28.394
36.949
45.648
998
where p8TeO2 is the partial pressures of TeO2(g) over
TeO2(l). The values of the Gibbs free energy of formation
for Fe2O3(s) and TeO2(l) are taken from the literature
[10,11]. The values of the Gibbs free energy of formation
for Fe2TeO6 are given in Table 5 and can be represented
by the relation
1002
1006
1011
1020
1028
1032
1043
1052
DfG8Fe2TeO6(s) 5 (21234.3 1 0.47286T)
615 kJ/mol (979–1052 K).
(14)
DfG8Cr2TeO6(s) 5 (21651.6 1 0.56833T)
615 kJ/mol (1014–1100 K)
(10)
Acknowledgements
3.3.2. Fe2TeO6
The individual mass loss of TeO2(g) and O2(g) over
The authors thank Shri D.S.C. Puroshotham, Director,
Nuclear Fuels Group, and Shri R. Prasad, Head, Fuel
Development Chemistry Section, for their keen interest in
this work.
1
2
]
Fe2TeO6 and Fe2O3(s) was obtained by the method
adopted for Cr2TeO6(s). The individual partial pressures of
TeO2(g) and O2(g) are given in Table 5, and the corre-
sponding least squares fit between ‘‘log p’’ and 1/T for
TeO2 is shown in Fig. 6, and can be represented by the
relation
References
log p (kPa) 5 (212283.9/T 1 9.582)60.06 (979–1052 K)
[1] E.H.P. Cordfunke, R.J.M. Konings, J. Nucl. Mater. 152 (1988) 301.
[2] H. Kleykamp, J. Nucl. Mater. 132 (1985) 221.
[3] G. Chattopadhyay, S.R. Bhardwaj, Evaluated phase diagrams of
binary metal–tellurium systems of the D block transition elements,
BARC Report No. 1449, 1989.
[4] N. Sokolov, K.K. Samplavskaia, M.H. Karapetianz, Deposited
manuscript, VINITI No. 3795-7b, Dep., 1976.
[5] G.G. Gospodinov, K.M. Gjurova, Thermochim. Acta 210 (1992)
321.
(11)
The standard Gibbs free energy of the reaction shown in
Eq. (2) is given by the relation
1
]
DrG8 5 DfG8Fe2O3(s) 1 DfG8TeO2(g) 1 2 DfG8O2(g)
2 DfG8Fe2TeO6(s)
(12)
[6] K. Krishnan, G.A. Rama Rao, K.D. Singh Mudher, V. Venugopal, J.
Alloys Comp. 288 (1999) 96.
[7] R.A. Young, A. Sakthivel, T.S. Moss, C.O. Paivaa-Santos, Program
DBWS-9411, Georgia Institute of Technology, Atlanta, GA, 1994.
[8] G. Bayer, Ber. Dtsch. Keram. Ges. 39 (1962) 535, quoted in
JCPDS-ICDD, X-ray diffraction file (1995), Card Nos. (15-696) and
(15-686).
[9] M. Saes, N.P. Raju, J.E. Greedan, J. Solid State Chem. 140 (1998)
7.
[10] O. Kubachewski, C.B. Alcock (Eds.), Metallurgical Thermoch-
emistry, 5th Edition, Pergamon, New York, 1979.
[11] E.H.P. Cordfunke, R.J.M. Konings (Eds.), Thermochemical Data For
Reactor Materials and Fission Products, North-Holland, Amsterdam,
1990.
Fig. 6. Temperature dependence of the vapor pressure for TeO2(g) over
Fe2TeO6(s) and Fe2O3(s).