286
S.C. Parida et al. / Journal of Alloys and Compounds 347 (2002) 285–287
present study using a DIANO X-ray diffractometer with
Cu Ka radiation, are in good agreement with those
reported in JCPDS-file [ 36-924. Gravimetric analyses of
Ba and Te content in Ba3Te2O9(s) were carried out to
confirm the stoichiometry. The observed mass fraction
content of Ba and Te for the compound were
(0.50860.002) and (0.31460.002) compared with the
calculated values of 0.508 and 0.315, respectively.
was thus inferred from the above observation and the
incongruent vaporization reaction could be established as:
Ba3Te2O9(s) 5 3BaO(s) 1 2TeO2(g) 1 O2(g)
(1)
The mass of the condensate, TeO2(s), per dm3 of the
carrier gas at each experimental temperature is given in
Table 1. The apparent equilibrium vapor pressure of
TeO2(g) at each experimental temperature is calculated
from the equation:
2.2. Transpiration technique
p(TeO2, g) 5 hn(TeO2, g)/[n(TeO2, g) 1 n(O2, g)
1 n(carrier gas)]j ? P
(2)
The apparatus and procedure used for the equilibrium
vapor pressure measurement is described in our earlier
publication [8]. It consisted of a 30 mm i.d. quartz
reaction-tube with ‘O’-ring-sealed end couplings. The end
couplings had ports for carrier gas entry and evacuation
and for insertion of a thermo-well and a condenser tube.
The condenser was made of quartz with an 0.5-mm
opening. A Kanthal wire-wound horizontal furnace with a
flat temperature zone (within 60.5 K) of 75 mm at the
center was used for heating. The temperature was mea-
sured by a calibrated chromel-to-alumel thermocouple. The
thermocouple was calibrated against ITS-90. The sample
(Ba3Te2O9) was loaded in an alumina boat and placed
inside the reaction tube. The system was repeatedly
evacuated and flushed with pure oxygen gas up to T5300
K and the temperature was rapidly raised at a rate of 15 K
min21 to the desired value. Thermodynamic equilibrium
was obtained by determining the flow rate plateau which
was found to be between 1.5 and 2.4 dm3 per hour.
Measurements were carried out in this flow rate region.
The volume of carrier gas was measured by a wet-test
meter. The mass of the condensate, TeO2(s), was de-
termined gravimetrically.
where, n(TeO2, g), n(O2, g) and n(carrier gas) are the
respective number of moles. The values of n(O2, g) were
calculated from those of n(TeO2, g) using the stoichiome-
try of reaction (1). The values of p(TeO2, g) thus obtained
are given in Table 1.
The equilibrium constant for the incongruent vapor-
ization reaction (1) at each temperature (Fig. 1) has been
calculated from the vapor pressure data and are fitted to an
expression:
ln K (61.0) 5 11.32–55518.6 ? (K/T)
(3)
The Gibbs energy of reaction (1) can be written as:
DrG8 5 2 R ? T ? ln K
5 DfG8m(BaO, s, T) 1 DfG8m(TeO2, g, T)
2 DfG8m(Ba3Te2O9, s, T)
(4)
The values of DfG8m(Ba3Te2O9, s, T) have been calcu-
lated by inserting the values of ln K obtained above in Eq.
(4) and taking the values of DfG8m(T) for TeO2(g) and
BaO(s) from Refs. [9] and [10], respectively. The corre-
sponding equation is given by:
hDfG8m (Ba3Te2O9, s, T)63.0j/(kJ mol21) 5 2 2296.8
3. Results and discussion
1 0.4302 ? (T/K)
(5)
Needle like crystals were observed in the condenser tube
of the transpiration apparatus. An X-ray diffraction pattern
of the condensate matched that of TeO2(s). The mass of
TeO2(s) was determined gravimetrically. In theory, the
residue material left in the alumina boat after the experi-
ment should have contained both BaO(s) and Ba3Te2O9(s).
However, the X-ray diffraction pattern of the residue did
not show the presence of BaO(s). Hence, an alternative
procedure was followed to detect the presence of BaO(s) in
the residue material. The residue material left in the
alumina boat following the transpiration experiment was
put into distilled water, filtered using Whatman 542 filter
paper and a few drops of dilute H2SO4 were added to the
filtrate. A white precipitate was observed which confirmed
the presence of BaO(s) in the residue material as
Ba3Te2O9(s) is insoluble in water. The coexistence of
BaO(s) with Ba3Te2O9(s) during the vaporization reaction
The slope and intercept of Eq. (5) correspond to the
standard molar enthalpy and entropy of formation of
Ba3Te2O9(s) from the elements at an average experimental
temperature, Tav. 51198 K.
Table 1
Vapor pressure data p, mass of TeO2(s) m and equilibrium constant K for
the incongruent reaction: Ba3Te2O9(s)53BaO(s)12TeO2(g)1O2(g)
T (K)
mhTeO2(s)j
(mg dm23
p(TeO2, g)
(kPa)
ln K
)
1119
1149
1174
1206
1239
1258
1280
2.225?1022
3.691?1022
5.345?1022
8.206?1022
9.024?1022
1.367?1022
1.795?1021
3.444?1024
5.694?1024
8.274?1024
1.292?1023
1.812?1023
2.124?1023
2.924?1023
238.469
236.961
235.840
234.504
233.488
233.012
232.053