Thermodynamic stability of Sm2TeO6

Article abstract of DOI:10.1016/j.tca.2007.11.004

The vapour pressure of Sm₂TeO₆ was measured using a thermal analyser with a horizontal arm. This TG based transpiration technique, was validated by measuring the vapour pressure of pure TeO₂(s). The temperature dependence of the latter was measured to be log p (Pa) = {14.2 - 13321/T (K)} (±0.03) in the range 884-987 K. These data yielded a Δ H₂₉₈ ° value of 269.7 ± 0.6 kJ mol^-1 for the enthalpy of sublimation (third-law method) of TeO₂ which compared well with the data reported in the literature. The temperature dependence of the vapour pressure of TeO₂ over the mixture Sm₂TeO₆(s) + Sm₂O₃(s) generated by the incongruent vapourisation reaction,Sm₂TeO₆(s) → Sm₂O₃(s) + TeO₂(g) + 1/2O₂(g)could be expressed aslog p (Pa) = { 18.56 - 25469 / T (K) } ± 0.06 (1374 - 1533 K). The standard Gibbs energy of formation of Sm₂TeO₆(s) was derived from the above vapour pressure data in conjunction with auxiliary data for the other coexisting phases. The temperature dependence of the Gibbs energy of formation of Sm₂TeO₆ over the temperature range 1374-1533 K could be represented asΔ G_f ° (S m₂ Te O₆) (kJ mo l^{- 1}) = { - 2399.3 + 0.5714 T (K) } ± 5.8. The Gibbs energy of formation of Sm₂TeO₆(s) is being reported for the first time.

Full text of DOI:10.1016/j.tca.2007.11.004

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Thermochimica Acta 467 (2008) 80–85
Thermodynamic stability of Sm2TeO6
S. Balakrishnan, R. Pankajavalli, K. Ananthasivan, S. Anthonysamy∗
Materials Chemistry Division, Chemistry Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India
Received 3 August 2007; received in revised form 5 November 2007; accepted 6 November 2007
Available online 17 November 2007
Abstract
The vapour pressure of Sm2TeO6 was measured using a thermal analyser with a horizontal arm. This TG based transpiration technique, was vali-
dated by measuring the vapour pressure of pure TeO2(s). The temperature dependence of the latter was measured to be log p (Pa) = {14.2 − 13321/T
(K)} ( 0.03) in the range 884–987 K. These data yielded a ꢀH2◦98 value of 269.7 0.6 kJ mol−1 for the enthalpy of sublimation (third-law method)
of TeO2 which compared well with the data reported in the literature. The temperature dependence of the vapour pressure of TeO2 over the mixture
Sm2TeO6(s) + Sm2O3(s) generated by the incongruent vapourisation reaction,
Sm2TeO6(s) → Sm2O3(s) + TeO2(g) + 1/2O2(g)
could be expressed as
log p (Pa) = {18.56 − 25469/T (K)} 0.06 (1374 − 1533 K)
The standard Gibbs energy of formation of Sm2TeO6(s) was derived from the above vapour pressure data in conjunction with auxiliary data for
the other coexisting phases. The temperature dependence of the Gibbs energy of formation of Sm2TeO6 over the temperature range 1374–1533 K
could be represented as
ꢀG◦f (Sm2TeO6)(kJ mol−1) = {−2399.3 + 0.5714 T (K)} 5.8
The Gibbs energy of formation of Sm2TeO6(s) is being reported for the first time.
© 2007 Published by Elsevier B.V.
Keywords: Samarium tellurate (Sm2TeO6); TeO2; Transpiration technique; Gibbs energy of formation; Vapour pressure; TG
1. Introduction
systems with different lanthanide elements are being investi-
gated. This paper presents the results obtained with the system
Sm–Te–O. This ternary system comprises three compounds
Sm2TeO6, Sm2Te4O11 and Sm2Te6O15. The thermodynamic
stability and vapour pressures of these compounds are yet to be
established unambiguously. We measured the vapour pressure
of TeO2(g) over a mixture containing Sm2TeO6 and Sm2O3 and
derived the standard Gibbs energy of formation of Sm2TeO6
from the vapour pressure data.
The vapour pressure of pure TeO2 has been determined in
the past by many investigators [3–14]. The measurement of
the vapour pressure of pure TeO2 would be most appropriate
for validating the apparatus and procedure, in the context of
the lanthanide bearing ternary systems Ln–Te–O. In view of
the above, we re-determined the vapour pressure of pure TeO2
[15].
Telluriumisoneofthefissionproductsformedinmixedoxide
fuel used in fast breeder reactors (FBRs). It influences the chem-
istry of the fuel and causes corrosion of the cladding [1,2]. The
thermodynamic data on the compounds comprising tellurium
and other fission product elements are useful in understanding
the fuel-clad chemical interaction in FBR fuel pins. An experi-
mental programme is currently being pursued in our laboratory
with an objective to generate thermodynamic data on multicom-
ponent oxide systems involving tellurium and a fission product
element. As a part of this study the systematics in the ternary
∗
Corresponding author. Tel.: +91 44 27480098; fax: +91 44 27480065.
E-mail address: sas@igcar.gov.in (S. Anthonysamy).
0040-6031/$ – see front matter © 2007 Published by Elsevier B.V.
doi:10.1016/j.tca.2007.11.004

S. Balakrishnan et al. / Thermochimica Acta 467 (2008) 80–85
81
2. Experimental
carrier gas (He/O2). The apparent gain in mass upon heating was
found to be about 10–20 ␮g per 100 K, over the entire range of
flow. A similar drift was observed in the isothermal experiments
as well. The “mass loss” values obtained in the isothermal exper-
iments were corrected for this drift. A detailed account on the
equipment and methodology could be found elsewhere [17–19].
2.1. Materials
Reagent grade TeO2 of purity better than 99.9% was procured
from Aldrich Chemicals, USA and Sm2O3 of purity better than
99.99% was supplied by Indian Rare Earths Ltd., India. High
purity O2 (H2O < 2 ppm) and He (O2 and H2O < 2 ppm) were
obtained from M/s. Indian Oxygen Limited, Chennai, India.
2.4. Vapourisation reaction and intermediates
The heat effects and mass loss steps pertaining to the
vapourisation of Sm2TeO6 were identified using a simultane-
ous TG-DTA experiment. In a typical experiment, about 85 mg
of Sm2TeO6 was taken in a Pt crucible and heated in a flowing
stream of pure oxygen (flow rate 8 dm3 h−1) at a linear heat-
ing rate of 0.17 K s−1. In order to identify the intermediates
the TG-DTA experiment was interrupted and the specimen was
characterized by using XRD.
2.2. Preparation of solid solutions
The ternary oxide Sm2TeO6 was prepared by heating an
equimolar mixture of Sm2O3 and TeO2. The temperature suit-
able for preparing the ternary compound was identified by
carrying out a trial experiment in which the above mixture was
heated at 10 K min−1 in static air in the thermal analyser (Model
Seiko 320, Japan). This experiment revealed an exotherm at
1058 K with an attendant weight gain that continued up to
1223 K. However, no further gain in weight was observed at
higher temperatures. The solid mixture was homogenized using
an agate mortar and pestle and then compacted into pellets with
about10 mmdiameterand2–3 mmthickness. Thesepelletswere
heated at 923 K for 24 h followed by a soak at 1223 K for 48 h.
The purity of this ternary oxide Sm2TeO6 was ascertained by
using X-ray diffraction (XRD) analysis. Thus it could be con-
cluded that the amount of impurities in this compound is less
than the detection limits of XRD, viz., 2–5 mass%. The heat
treated samples were found to contain Sm2TeO6 (JCPDS XRD
file No.40-0332), neither Sm2O3 nor TeO2 was present in them.
3. Results and discussion
The apparent pressure of TeO2(g) in equilibrium with either
pure TeO2(s) or with a mixture of Sm2TeO6(s) and Sm2O3(s)
was calculated from the mass loss of the sample per unit volume
of the carrier gas flown over it, using the relation
WRTc
Papp
=
(1)
MVc
W is the mass loss of the sample, Vc is the total volume of
the carrier gas (saturated with the vapour species) and Tc is
temperature of the carrier gas and M is molecular weight of the
vapour species. In an earlier study [18] the temperature of the
specimen was used in place of Tc. It is more appropriate to use
the latter since the volume measurement is carried out only at
Tc Hence, the data presented in ref. [18] should be corrected
accordingly.
2.3. Transpiration set-up
A horizontal thermal analyser (Model Seiko 320, Japan) was
employed as a transpiration apparatus for the vapour pressure
investigations. The horizontal disposition of a narrow tubular
furnace chamber and the central location of the horizontal dual
arm help minimize the errors due to buoyancy, convection and
thermomolecular effects. Both the sample temperature and the
differentialtemperatureweremeasuredwithin 0.5 Kbyusinga
Pt–13% Rh/Pt (Type-R) thermocouple. The accuracy in the tem-
perature measurement was testified by determining the freezing
points of pure metals viz., Sn, Pb, Sb, Al, Ag and Au. This cal-
ibration was found to conform to the ITS-90 [16] scale. Precise
flow calibration of the carrier gases viz., He (for the experiments
withpureTeO2)andO2 (fortheexperimentswithSm2TeO6)was
done using a capillary glass flow meter, which in-turn was cali-
brated by a soap bubble flow meter. Even though the precision in
the flow as measured by the glass capillary flow meter was found
to be within 0.5%, the overall precision in the integral volume
was of the order of 1%. Saturation of the carrier gas with the
vapour emanating from the sample was ensured by using a finely
divided powder of the specimen and by spreading it evenly in
a shallow Pt crucible kept in a narrow reaction chamber (fur-
nace tube). Prior to the measurements, blank runs were carried
out, in the temperature range 800–1500 K, with empty pans, by
maintaining an optimum flow (3–18 dm3 h−1) of an appropriate
3.1. Vapour pressure of pure solid TeO2
The vapour pressure of solid TeO2 was measured using He
as the carrier gas. It was presumed that the vapour comprised
only the monomeric TeO2(g) species. The contribution of all
other tellurium-bearing species [(TeO2)2, TeO, (TeO)2, Te2] to
the total pressure was estimated to be less than 5% from the mass
spectrometric studies of Muenow et al. [14].
In the isothermal experiment at 925 K, the mass lost per unit
volume of the carrier gas (helium) flown over TeO2 was found to
remain constant when the flow was between 8 and 12 dm3 h−1
.
This corresponds to the flow regime (Fig. 1) in which the carrier
gas is saturated with vapour species. Data obtained in the isother-
mal experiments carried out in this flow regime, are presented
along with the values of the vapour pressures pTeO derived from
2
them in Table 1.
TeO2(s) → TeO2(g)
(2)
A linear least square regression analysis of the experimen-
tal data obtained during the congruent vapourisation of TeO2(s)

82
S. Balakrishnan et al. / Thermochimica Acta 467 (2008) 80–85
Fig. 2. Temperature dependence of equilibrium vapour pressure of solid TeO2.
Fig. 1. Apparent vapour pressure versus flow rate of TeO2 at 925 K.
ical tables compiled by Knacke et al. [20]. The value of the
standard enthalpy of sublimation at 298 K was derived using
Eq. (4). The values of ꢀHs◦ub,298 K derived from the experimen-
tal data are listed in Table 2. Their temperature dependence is
depicted in Fig. 3. The mean of these values was found to be
shown in Eq. (2) yielded an expression (3) for the tempera-
ture dependence of the vapour pressure. These data are also
graphically depicted in Fig. 2.
ꢀ
ꢁ
13321
log p (Pa) = 14.2 −
0.03 (884 − 987 K)
(3)
(269.7 0.6) kJ mol−1
.
T (K)
ꢀHs◦ub,298 K = ꢀG◦vap − T ꢀFEF
(4)
The enthalpy of sublimation derived from the slope of the
temperature dependence of the vapour pressure at the mean
temperature of the measurement (936 K) was found to be
It is evident from Fig. 3 that the values of ꢀHs◦ub,298 K are
randomly distributed about the mean. This indicates that the
present measurements are devoid of any significant temperature
dependent systematic errors.
255 0.1 kJ mol−1
.
The temperature dependent systematic errors in the measured
values of the vapour pressure were examined by using the third-
law analysis Eq. (4). The free energy functions (FEF), required
for this analysis were derived using the values of S2◦98 and Cp0
of TeO2 (both solid and gas) obtained from the thermochem-
Mills [3] and Cordfunke and Konings [4] have extensively
reviewed the thermochemical data on TeO2 which include
calorimetric measurements, vapour pressure values and EMF
data. The vapour pressure of TeO2 was a subject of study from
1941 till recent years. Various experimental techniques such
as Knudsen cell mass loss method [5–8], mass spectrometry
[9–11,14] and transpiration [12,13] have been used to measure
the vapour pressures. In order to validate the experimental tech-
nique used in the present study and to testify the accuracy of the
vapour pressure data it would be instructive to compare all the
measured values of vapour pressures with that obtained in this
Table 1
Vapour pressure of TeO2(s)
Experiment T (K)
Mass loss ((g)
Vc (dm3)
Tc (K)
Pressure (mPa)
number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
884
889
895
899
905
909
915
925
925
925
926
935
940
941
946
951
960
971
976
982
987
120
120
140
180
220
232
334
315
350
400
404
660
820
760
890
12.414
10.909
10.909
10.909
10.909
10.909
10.909
8
295
295
297
295
295
295
297
297
297
297
297
295
295
295
295
295
297
297
297
297
297
148.7
169.2
198.7
253.8
310.2
327.1
474.1
609.9
602.3
9
10.909
10.909
10.909
10.909
10.909
10.909
10.909
9
9
9
9
9
567.9
573.6
929.5
1154.8
1071.8
1255.1
1464.6
2168.4
3407.4
3717.2
4543.2
4956.2
1040
1260
1980
2160
2640
2880
Initial weight of the sample: 85 mg, T (K): temperature corresponding to each
experimental measurement, Vc (dm3): volume of the helium carrier gas, Tc (K):
temperature of the helium carrier gas.
Fig. 3. Third-law plot of enthalpy of sublimation of TeO2; the horizontal line is
the mean value of ꢀHs◦ub,298 K
.

S. Balakrishnan et al. / Thermochimica Acta 467 (2008) 80–85
83
Table 2
Comparison of vapour pressure data of solid TeO2 with the literature
B
T (K)
Log p (Pa) = A −
T Range (K)
p at 900 K (Pa)
ꢀHs◦ub(mean T)
ꢀHs◦ub,298 K (Third-law)
(kJ mol−1
Method
Ref.
(kJ mol−1) (K)
)
A
B
13.70
13.00
14.38
14.54
13.78
14.66
14.65
15.87
14.24
15.02
14.20
12570
12000
13169
13292
12650
14666
13432
14529
13534
13988
13321
870–930
846–1006
730–981
873–1006
913–993
782–903
778–906
926–970
750–950
805–905
884–987
0.54
0.54
0.55
0.59
0.53
0.023
0.53
0.53
0.16
0.3
240.7 (900)
229.8 (926)
252.1 (856)
254.5 (940)
242.2 (953)
280.8 (843)
257.2 (842)
278.2 (948)
259.1 (850)
267.8 (855)
255.0 (936)
266.2 0.7
261.0 2.0
266.0
265.3
266.8
289.8
266.9
264.4
264.0
268.6
269.7
264.0 8.4
266.2 0.6
KEML
T
KEML
KEML
T
KCMS
KCMS
KEML
KCMS
KCMS
T
[5]
[12]
[6]
[7]
[13]
[14]
[9]
[8]
[10]
[11]
This work
[3]
[4]
0.25
Review
Review
KEML: Knudsen effusion mass loss; T: transpiration; KCMS: Knudsen cell mass spectrometry, ꢀHs◦ub (mean T): enthalpy of sublimation at the mean temperature
of the present measurements, ꢀHs◦ub,298 K: enthalpy of sublimation at 298 K by third-law, p: pressure (p) at 900 K, T range: temperature range.
study. Such a comparison (presented in Table 2) reveals that,
by and large, there is a reasonable agreement among the cited
values of p (Pa) at 900 K and those obtained in the present study.
Since the temperature range over which each of these investi-
gations was carried out differ considerably, a comparison of the
enthalpy of sublimation at the mean temperature of these mea-
surements is not useful. However, a meaningful comparison of
the vapour pressure data (presented in Table 2) could be made
with the help of the “third-law” analyses. Mills◦[3] and Cord-
funke and Konings [4] have reviewed the ꢀHsub,298 K values
using the mass spectrometric data of Piacente et al. [9]. The
value of ꢀHs◦ub,298 K of TeO2 (269.7 0.6) kJ mol−1 derived
from the present study agrees very well with that reported by
the other investigators [5–11,13] as well as with those recom-
mended by the reveiwers [3,4] within 4 kJ mol−1. In view of the
above it could be concluded that the vapour pressures of TeO2
obtained in this study are quite reliable. Thus, the present study
validates the reliability of this TG based transpiration technique
for measuring the vapour pressures.
gravimetric experiments and is depicted in Fig. 4. This “chair”
shaped curve revealed a plateau between the flow values of 7 and
9.5 dm3 h−1, over which the mass loss was independent of the
flow. In this flow regime the carrier gas was saturated with TeO2
vapour. All the isothermal mass loss measurements were carried
out by employing a flow pertaining to this plateau region.
The vapour pressure of TeO2(g) over samarium tellurate was
calculated at different temperatures from the observed mass loss
of the sample, mtotal per unit flow volume of the O2(g) at a
pressure of 1 bar. Both O2(g) and TeO2(g) contribute to the total
weight loss mtotal. The loss of weight due to the loss of TeO2(g)
alone was computed from this value using Eq. (6),
ꢂ
ꢃ
M
TeO2
mTeO = mtotal
(6)
2
MTeO + 0.5MO
2
2
where M is the molecular weight and m is the mass loss of the
respective compounds. The vapour pressures, pTeO computed
2
from these experiments are enlisted along with the values of
3.2. Non-isothermal TG of Sm2TeO6
The simultaneous TG-DTA pertaining to the incongruent
vapourisation of Sm2TeO6(s) revealed that this compound
begins to lose mass from 1373 K in pure O2. The X-ray diffrac-
tion of a specimen from an interrupted TG-DTA experiment
confirmed the presence of the compounds Sm2TeO6(s) and
Sm2O3(s). Thus the vapourisation of the ternary compound
could be represented by Eq. (5).
1
2
Sm2TeO6(s) → Sm2O3(s) + TeO2(g) + O2(g)
(5)
3.3. Vapourisation behaviour of Sm2TeO6 by the
transpiration technique
The dependence of the mass loss on the flow of the carrier
gas (pure oxygen) at 1422 K was obtained from the thermo-
Fig. 4. Apparent vapour pressure versus flow rate of Sm2TeO6 at 1422 K.

84
S. Balakrishnan et al. / Thermochimica Acta 467 (2008) 80–85
Table 3
Table 3 Vapour pressure data and calculation of ꢀG◦f of Sm2TeO6
(a)
TeO
T (K)
Mass loss (mg)
Vc (dm3)
p
(Pa)
p0TeO (Pa)
ꢀG◦f , (Sm2TeO6)(b) (kJ mol−1
)
ꢀG◦f , (Sm2TeO6)(c) (kJ mol−1
)
2
2
1374
1380
1392
1403
1417
1422
1422
1422
1427
1438
1448
1458
1478
1514
1533
0.4363
0.5817
0.8544
1.3633
2.3086
2.7266
2.7812
2.3086
2.9448
3.1265
5.4533
8.0163
12.361
26.7483
41.9357
7.059
7.059
8.571
8.571
8.571
8.571
9.474
7.059
8.571
8.571
8.571
8.571
7.059
8.571
8.571
0.964
1.285
1.554
2.479
4.199
4.957
4.575
5.096
5.354
5.685
9.915
14.575
27.290
48.633
76.246
7719
8345
9884
−1614.0
−1610.8
−1603.9
−1597.7
−1589.5
−1586.9
−1586.9
−1586.9
−1584.1
−1577.9
−1571.8
−1566.2
−1554.9
−1534.5
−1523.1
−1609.5
−1606.3
−1599.4
−1593.2
−1585.0
−1582.5
−1582.5
−1582.5
−1579.6
−1573.4
−1567.3
−1561.7
−1550.4
−1530.0
−1518.7
11484
13933
14795
14795
14795
15808
18212
20909
23687
30323
46567
58635
Initial weight of the sample: 68 mg, T (K): temperature corresponding to each experimental measurement, Vc (dm3): volume of the oxygen carrier gas, Tc (K):
temperature of the oxygen carrier gas (Tc = 299 K), p(a) : calculated by using Eq. (1), Tc = 299 K, p0 : extrapolated values from Eq. (15), ꢀG◦(Sm2TeO6)(b)
:
TeO
TeO
f
2
2
standard Gibbs energy of formation of Sm2TeO6 using Eq. (13), ꢀG◦f (Sm2TeO6)(c): standard Gibbs energy of formation of Sm2TeO6 using Eq. (16).
where activities of solids (aSm O and aSm TeO ) are equal to one
2
3
2
6
and pO = 1 bar. Hence,
2
ꢀG◦r (5) = −RT ln p(TeO )
ꢀG◦r (5) (kJ mol−1) = {487.7 − 0.2595 T (K)} 0.1
(10)
(11)
2
ThestandardGibbsenergyofformationofSm2TeO6 couldbe
expressed in terms of the equilibrium partial pressure of TeO2(g)
and the stabilities of Sm2TeO6 and Sm2O3 as follows
ꢀG◦f (Sm2TeO6, s) = ꢀGf◦(Sm2O3) + ꢀGf◦(TeO2, g)
+RT ln p(TeO )
(12)
2
Combining the expression (11) along with the key data for
ꢀG◦f (Sm2O3, s) and ꢀGf◦(TeO2, g) from the literature [20],
ꢀG◦f (Sm2TeO6, s) was calculated to be
Fig. 5. Temperature dependence of equilibrium vapour pressure of Sm2TeO6.
ꢀG◦f (Sm2TeO6)(kJ mol−1) = {−2399.3 + 0.5714 T (K)}
5.8 (1374 − 1533 K)
(13)
mass loss, temperature and cumulative volume of the carrier gas
Vc, in Table 3.
The temperature dependence of the vapour pressure is shown
in Fig. 5. The linear least squares regression analysis of these
data yielded expression (7).
ꢀG◦ (Sm2TeO6, s) with liquid TeO2 as the reference state is
presfented in Eq. (14).
ꢀG◦f (Sm2TeO6, s) = ꢀGf◦(Sm2O3, s) + ꢀGf◦(TeO2, l)
p
TeO2
+ RT ln
(14)
log p (Pa) = {18.56 − 25469/T (K)}( 0.06)(1374 − 1533 K)
0
p
TeO2
(7)
The temperature dependence of vapour pressure of TeO2(g)
(p0TeO ) over pure liquid TeO2 was measured in our laboratory
using2the same TG based transpiration technique and was found
to be [15],
The standard Gibbs energy change, ꢀG◦r(5) for the reaction (5)
could be expressed as
ꢀG◦r (5) = −RT ln K (where K = equilibrium constant) (8)
log p0TeO (Pa) = 12.38 − 11672/T (K)(1006 − 1070 K) (15)
2
ꢄ
ꢅ
1/2
a Sm2O3xp(TeO2)xpO2
a Sm2TeO6
= −RT ln
(9)
The standard Gibbs energy of formation of Sm2TeO6 was
obtained with the help of Eq. (14) using auxiliary data on the

S. Balakrishnan et al. / Thermochimica Acta 467 (2008) 80–85
85
ꢀG◦f (TeO2, l) and ꢀG◦f (Sm2O3, s) from the literature [20].
Acknowledgement
This is shown in Eq. (16)
The authors express their sincere gratitude to Shri. R. Mad-
havan for recording XRD patterns.
ꢀG◦f (Sm2TeO6)(kJ mol−1) = {−2394.4 + 0.5711 T (K)}
5.8(1374 − 1533 K)
(16)
References
[1] H. Kleykamp, J. Nucl. Mater. 131 (1985) 221.
The slope and intercept of the Eqs. (13) and (16) are related
to the standard molar enthalpy and entropy of the formation
of Sm2TeO6(s), respectively (valid over the temperature range
1374–1533 K). The Gibbs energy of formation of Sm2TeO6(s)
is being reported for the first time. Since no information is avail-
able on the heat capacity of this compound, the second law
and third-law analyses of the vapour pressure data could not
be carried out. Hence it was not possible to evaluate and ver-
ify the values of the ꢀHf◦ Sm2TeO6(s) at 298 K. However, the
average enthalpy of formation of Sm2TeO6 obtained from the
present work by using the method of slopes, was found to be
−2396.9 5.8 kJ mol−1 at the mean temperature of the present
measurements.
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The applicability of a TG based transpiration technique for
measuring vapour pressures was verified by measuring the
vapour pressure of pure TeO2 over the range 884–987 K yield-
ing a value of 269.7 0.6 kJ mol−1 for the third-law enthalpy
of sublimation which compares well with the literature data.
The ternary compound, Sm2TeO6(s) was prepared by the solid
state reaction of an equimolar mixture of Sm2O3, and TeO2 and
was characterized by X-ray and TG-DTA. The vapour pressure
of TeO2(g) over Sm2TeO6(s) was derived from these vapour
pressure measurements. The values of ꢀG◦ Sm2TeO6(s) were
derived from these vapour pressure measurefments and are being
reported for the first time. The average enthalpy of formation
of Sm2TeO6 obtained from the present work was found to be
−2396.9 5.8 kJ mol−1
.