Inorganic Chemistry
Article
To get rid of the decomposition products and excess carbon, the
powders obtained after autoignition were calcined at 600 °C for 2 h.
To reduce Ce4+ to Ce3+ in these compositions, the powders were
pelletized, wrapped in tantalum foil, introduced in a quartz tube in the
presence of zirconium sponge, and then vacuum sealed online (10−6
mbar). Zirconium sponge acts as oxygen-getter. The tube was heated
at 1050 °C for 24 h. X-ray diffraction (XRD) has been recorded on all
the powders using monochromatized Cu−Kα radiation on a
PANalytical Xpert Pro. The standard molar enthalpies of formation
of different forms of the compound, that is, Gd1.2Ce0.8Zr1.2Al0.8O7
(nano), Gd1.2Ce0.8Zr1.2Al0.8O6.6 (heated under reduced conditions),
Gd1.2Ce0.8Zr1.2Al0.8O7 (heated in air at 1200 °C) were determined by
measuring the enthalpy of dissolution of different aforementioned
forms of Gd1.2Ce0.8Zr1.2Al0.8O7 compound and its corresponding
constituent oxides such as Gd2O3(s), CeO2(s), ZrO2(s), and Al2O3(s)
in liquid Na2O + MoO3 solvent (3:4 molar ratio) at 713 °C employing
a high temperature Calvet calorimeter (Setaram, Model HT-1000).
The calorimeter has an isothermal alumina block which contains two
identical one-end closed alumina cells surrounded by a series of
thermopiles. The temperature of the isothermal block was measured
using a Pt−Pt 10% Rh thermocouple ( 0.1 °C). The details of the
experimental measurements have been described elsewhere.5 The heat
calibration was carried out using a synthetic sapphire [NIST SRM-
720].
RESULT AND DISCUSSION
■
The gel-combustion synthesis is a well-known method which
has been successfully employed to synthesize nanocrystalline
materials.6−9 The gel-combustion process is basically a redox
reaction between metal nitrate (oxidant) and a fuel like glycine,
citric acid, and so forth.9,10 The main motivation for using this
preparative technique was to achieve atomistically blended
constituents in the solution that reduces the possibility of side
reaction, if any. Respective metal nitrates in stoichiometric
amounts were dissolved in a minimum amount of distilled
water. Glycine was added to the solution in 60% fuel-deficient
ratio. This solution on thermal dehydration formed a gel. On
further heating, the gel autoignited with large amount of
gaseous evolution giving a highly porous powder. The obtained
powders were calcined at 600 °C for 2 h to get rid of
carbonaceous impurity, if any.
The XRD patterns of the as-prepared samples after
calcination are shown in Figure 1. The nominal composition
The solvent Na2O and MoO3 (3:4 molar mixture) was initially
prepared by heating a mixture of perfectly dried Na2CO3 (BDH,
reagent grade) and MoO3 (BDH, reagent grade) in appropriate molar
ratio and slowly heated in a platinum disk inside the furnace up to 713
°C. Na2CO3 decomposes to Na2O forming a eutectic mixture. The
melt was maintained at 713 °C for 6 h for homogenization. The mass
loss of the product was monitored to ensure the correct stoichiometry
of the melt. The entire substance was removed from the platinum disk
by slowly scratching the melt and grinding it again to make a uniform
powder. The solvent powder was characterized by chemical analysis
and XRD techniques. The atom % of Mo, Na, and O obtained from
the chemical analysis of the solvent were found to be 16.2 0.2, 23.4
0.3, and 60.4 0.4, respectively. No other chemical impurity could
be detected. Powder from the same lot was used in all reaction
enthalpy measurements.
About 3 g of Na2O + MoO3 (3:4 molar) solvent was taken in each
of the two identical platinum tubes which act as a protective lining and
having outer diameter (OD) matching exactly with the alumina
reaction cell for proper thermal contact. The reaction cell assembly
was slowly lowered into the calorimeter, and the calorimeter was
programmed up to 713 °C at a heating rate of 0.5 °C/min and
Figure 1. XRD pattern of as-prepared Gd2−xCexZr2−xAlxO7
compositions calcined at 700 °C ($, Gd-rich phase; @, Ce-rich phase).
of the products obtained after the combustion process can be
generalized as Gd2−xCexZr2−xAlxO7 (0.0 ≤ x ≤ 2.0) considering
the aerial oxidation of Ce3+ ions to Ce4+.11,12 The XRD
investigation shows the presence of two single-phasic regions
having fluorite-type compounds toward both the end members,
with a narrow biphasic region, depending upon the x content in
the samples. One of the phases can be assigned as Gd-rich
fluorite-type structure with the value of x varying from 0.0 to
1.0. The other phase is Ce-rich fluorite-type structure with x
varying from 1.6 to 2.0. From the asymmetry in the shape of
the XRD peaks it was observed that the compositions with
value of x = 1.2 (Figure 1) and 1.4 (not shown in Figure 1)
show the presence of both the fluorite structures. Even though
the samples were prepared under identical conditions, the
FWHM of the peak was found to decrease initially in the
composition with x = 0.2−1.0 and then increase from the value
of x = 1.2−2.0. Initial decrease in FWHM can be attributed to
the nature of the sample. On the other hand, increase in the
fwhm from x = 1.2−2.0 can be attributed to the strain due to
incorporation of the substituted cations in the lattice. Similar
observation of increased FWHM as a function of aliovalent ion
substitution was reported for the Ce1−xNdxO2−x/2 system in
which all the samples were prepared under identical
condition.13The variation in cell parameter as a function of
substitution is shown in Figure 2. The well-defined trend of
maintained at 713
0.05 °C during the whole experiment. The
reaction tubes were equilibrated inside the chamber for sufficiently
long time till a steady baseline for heat flux signal was achieved. The
slope of the baseline of the differential heat flow signal was nearly zero
since the heat effect due to any small loss of the volatile components
will get nullified as the same effect was present in the sample and
reference cells. Small pellets containing a few milligrams of the samples
were dropped from room temperature to the reaction cell containing
liquid solvent maintained at 713 °C, and the corresponding enthalpy
change was determined by integrating the heat flow signal with respect
to time. The time required for the completion of the reaction was
determined by recording the heat flow signal (J/g) for different time
intervals. The reaction time was established when a steady baseline was
achieved, and the values of reaction enthalpy obtained as a function of
time converged into a constant value. For each dissolution experiment
the reaction time was determined, and the heat flow signals were
recorded for the same time period for all experiments. Similarly, for
each experiment fresh solvent was used so that the similar dilution
condition was maintained. The amount of the reactant dropped into
solvent was chosen in such a manner that the concentration of Gd, Ce,
Zr, and Al remained well below 1 atom %. The infinite dilution
condition was established by repeating the dissolution experiments on
a same lot of the solvent. The consistency in the values of the reaction
per unit mass of the reactants is indicative of the fact that the infinite
dilution condition was maintained during the measurements.
B
dx.doi.org/10.1021/ic401041e | Inorg. Chem. XXXX, XXX, XXX−XXX
Shukla, Rakesh
