THERMODYNAMIC STABILITY OF CdMoO4
Results and discussion
In order to ensure the quick equilibrium between
sample and its vapor products, the mixture of
CdMoO4(s)+MoO2(s) was well spread in the crucible
keeping the ratio of the sample-surface to the orifice
is high (about 40). This ratio as such was an order of
magnitude higher than that involved in the measure-
ment of the Clausing factor of the cell orifice using
silver standard in globular form. The nearly unit value
of the Clausing factor was indicative of the fact that
Ag vapor over the globule could attain saturation un-
der the effusive loss. Considering that the average
particle size in the mixture was 10 micron, the actual
evaporating surface was 105 times higher than the
projected surface of the sample mixture. The surface
augmentation using fine particles was made to tackle
any kinetic impedance from the heterogeneous reac-
tion (1). The vapor pressures were measured both in
ascending and descending mode of temperatures to
ensure the absence of kinetic impediments of the va-
por generation inside the cell.
Thermogravimetric and differential thermal analysis
(TG-DTA) plot (Fig. 2) for CdMoO4(s) recorded un-
der flowing-argon showed that the compound started
decomposing at about 1440 K after melting at
1400 K. From the available thermodynamic informa-
tion [12] on MoO2 solid and its vapor, it can be seen
that MoO2 will have negligible vapor pressure in the
temperature range of measurements and under the
background oxygen partial pressure of 10–8 bar
prevailing during the measurement.
A Pt, Pt – 13% Rh thermocouple protected by a
thin alumina sheath was used for measuring the sam-
ple temperature. The tip of the thermocouple was lo-
cated about 1 mm away from the sample but well
within the isothermal zone of the reaction tube. The
whole system was closed in a vacuum-tight, re-
crystallized alumina tube of diameter 30 mm. The
system as well as the balance was attached to a
high-vacuum system. The effusion experiments were
done under dynamic vacuum of 10–8 bar.
The mass calibration of the microbalance was
done using standard masses at room temperature. The
temperature calibration for the sample was done by
the drop method [12], using high purity metal stan-
dards: Sn, Sb, Ag and Au. Temperature of the sample
was varied using a microprocessor based temperature
programmer cum controller with an accuracy of
±0.5 K using a Pt, Pt – 13% Rh thermocouple.
The mass loss due to the vaporization of
Cd+MoO3 from the Knudsen cell was measured at dif-
ferent temperatures. The mass loss with time plotted in
isothermal runs was used to calculate the vapor pres-
sure. Mass loss measurements were taken at different
temperatures in increasing as well as decreasing orders
of successive isotherms. The observed reproducibility
in the mass loss rate in each isothermal run confirms
the absence of kinetic hindrance in the evaporative
loss. Several measurements were carried out in the
temperature range of 987 to 1111 K. The equilibrium
vapor pressures of Cd, MoO3 bearing species derived
from mass loss data were used to calculate the thermo-
dynamic stability CdMoO4(s). After the experiment the
residue in the silica crucible was analyzed by XRD
method and also by chemical means to confirm the
presence of the coexisting phases.
Fig. 2 TG-DTA plot of CdMoO4 recorded under argon atmo-
sphere, heating rate=10 K min–1
The vapor pressure of MoO2(s) at 1000 K for ex-
ample, calculated using the literature data [13] is
found to be of the order of 10–18 bar. The mass loss
corresponding to the above pressure from the
Knudsen cell will be negligible. However, an intimate
mixture of CdMoO4(s) and MoO2(s) under similar ex-
perimental conditions started losing mass at about
1000 K, which was much lower temperature than the
decomposition of CdMoO4(s). It can also be seen that
the contribution of MoO3(g) to the total pressure from
oxidation reaction, MoO2(s)+O2(g)=MoO3(g), at the
background oxygen pressure at 1000 K [12] is negli-
gible. The XRD and chemical analyses of the conden-
sates collected at the cooler regions of the Knudsen
assembly shows the presence of Cd(s) and MoO3(s).
Analysis of the residue from the Knudsen cell after
the effusion studies revealed that CdMoO4(s) and
MoO2(s) were present in the same molar ratio (1:1) as
before the reaction. Figure 3 gives the XRD pattern of
the residual mixture from effusion studies along with
those of the pure CdMoO4(s) and MoO2(s).
From the mass spectrometric data [9, 10] it can
be shown that the molybdenum bearing gaseous spe-
cies effusing out from the Knudsen cell under the ex-
perimental conditions, consist predominantly of
(MoO3)3, (MoO3)4 and (MoO3)5 in addition to negligi-
ble amounts of (MoO3)2. Considering this along with
the present observations, the reaction between
J. Therm. Anal. Cal., 86, 2006
549