Synthesis of Pb1-xRxZr0.65Ti 0.35O3 (x = 0.05, 0.10 and 0.15; R = Ce, Pr and Nd) powders by Gel Entrapment Technique and study on their thermo-physical and electrical properties

Article abstract of DOI:10.1016/j.jallcom.2010.08.041

Rare earth doped lead zirconate titanate of composition (Pb _1-xR_xZr_0.65Ti_0.35)O₃ where x = 0, 0.10 and 0.15 and R = Ce, Pr and Nd was prepared by Gel Entrapment Technique (GET). The gel was suitably c

Full text of DOI:10.1016/j.jallcom.2010.08.041

Journal of Alloys and Compounds 508 (2010) 184–190
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Synthesis of Pb1−xRxZr0.65Ti0.35O3 (x = 0.05, 0.10 and 0.15; R = Ce, Pr and Nd)
powders by Gel Entrapment Technique and study on their thermo-physical and
electrical properties
Rajesh V. Paia,∗, R. Mishrab, M.R. Paib, S.K. Mukerjeea, V. Venugopalc
a Fuel Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
b Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
c RC&I Group, Bhabha Atomic Research Centre, Mumbai 400 085, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 May 2010
Received in revised form 8 August 2010
Accepted 12 August 2010
Available online 19 August 2010
Rare earth doped lead zirconate titanate of composition (Pb1−xRxZr0.65Ti0.35)O3 where x = 0, 0.10 and 0.15
and R = Ce, Pr and Nd was prepared by Gel Entrapment Technique (GET). The gel was suitably calcined at
1073 K and the resultant product was characterized by XRD, SEM, BET surface area analysis and Dynamic
Light Scattering. The dielectric constants of PZT were found to decrease with Ce doping where as it
increased with Pr doping. The pyrochlore phase in Nd doped samples was found to be responsible for
lower conductivity and dielectric constant values. The redox behaviour of the compounds was studied
by TPR–TPO measurements.
Keywords:
Ferroelectrics
X-ray diffraction
Impedance
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
have been reported on ceramics [8–10]. Initially, it was important
to discern where the Ln dopants would most likely reside in the
Lead zirconate titanate (PZT) [1] ceramics have been used exten-
sively for various device applications because of their excellent
dielectric, pyroelectric, piezoelectric and electro optic properties
[2]. PZT is a perovskite ABO3 solid-state solution wherein the B-
site composition can be continuously varied from the titanium-free
PbZrO3 (PZ) at one extreme to pure PbTiO3 (PT) at the other. The
ratio of Zr to Ti dramatically affects the properties of the PZT mate-
rials and is often represented numerically in an abbreviation, such
as PZT (30/70), where the composite ratio is 30% Zr and 70% Ti.
Optimal excess PbO addition and rapid heating to the annealing
temperature are found to lead to improved electrical characteris-
tics [3,4]. The inclusion of acceptor or donor dopants (i.e. the charge
of the dopant being lower or higher than the metal cation for which
it substitutes) is often used to alter the properties of PZT by sup-
pressing or promoting oxygen or cation vacancies [5,6]. While there
have been substantial improvements in many of the properties of
PZT using acceptor and donor dopants, it is important to garner
greater understanding and control over the properties of ferroelec-
tric materials. Although a great deal of research has focused on
doping PZT with La3+ [5,7] and a number of the other individual
lanthanide (Ln3+) cations, few systematic Ln doping PZT studies
ABO3 structure (i.e., A- or B-site dopants). As a starting rationale,
the rules of Goldschmidt suggest that dopants will reside on a spe-
cific lattice site (either A or B) if a dopant ion’s radius is within 15%
of the replaced ion [11]. Additionally, it has been shown that the
Ln cations will occupy both the A and B sites [5,8–10,12,13–15]
but the preference for B-site occupancy increases as the radius
of the dopant decreases [8,9,14].The decrease in cation size from
˚
˚
1.17 A (La) to 1.00 A (Lu) [16] results in a transition from A-site
4+
4+
(rPb2+ = 1.2 A) to B-site (rTi = 0.68 A, rZr = 0.79 A) occupancy. For
intermediate ionic radii values, shared site occupancy (also termed
amphoteric or self-compensating) has been shown for trivalent
dopants in other perovskite systems, such as BaTiO3 [17] and
SrTiO3, [13] and has been predicted for PZT (95/5) [8,9]. Therefore,
in the PLnZT system, the Ln3+ ions may compensate for a pair of
missing Pb2+ and B4+ (Ti or Zr) cations. The studies conducted by
Sharma et al. [14] for the (Pb1−3x/2Lnx)(Zr0.65Ti0.35)O3 system indi-
cated a dominant A-site occupancy for the early Ln cations. The
˚
˚
˚
˚
Ce dopant in their study was assumed to be +3 (1.03 A) and thus
occupy the A site; however, other studies indicate that the Ce cation
˚
is in fact in the +4 (0.92 A) state, and thus, B-site occupancy in PZT
[12] and other perovskite systems [15] would be favored. A mixed
A/B site occupancy for Ln dopants in the PZT (95/5) system was
predicted by Troccaz et al. using tolerance factor calculations [9].
Conventional method of preparing these ceramic materials is
by the solid-state reactions [18,19]. But the advantages of solu-
∗
Corresponding author. Tel.: +91 22 25504090.
E-mail address: pairajesh007@gmail.com (R.V. Pai).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.08.041

R.V. Pai et al. / Journal of Alloys and Compounds 508 (2010) 184–190
185
tion based routes lie in the formation of the product at much
lower temperatures with larger surface area and better microho-
mogeneity compared to solid-state route especially for present
multicomponent system. Lower synthesis temperatures prevent
volatilization of the component oxide facilitating fixing of required
stoichiometry. The present paper deals with synthesis of PZT pow-
ders containing 5, 10 and 15 mol% Ce, Pr and Nd by Gel Entrapment
Technique (GET) [20]. AC impedance spectroscopy has been uti-
lized to study the electrical properties such as dielectric properties
and electrical conductivity of the samples at different tempera-
tures. Depending on their dopant ion concentration the electrical
properties have been modified/deteriorated due to the different
site occupancy. The redox behaviour of these compounds have
also been investigated was studied by Temperature Programmed
Reduction–Oxidation (TPRO) studies to determine the stability
under reducing atmosphere.
(5%) + He at a gas flow rate of 20 ml min−1, in temperature range 25–1000 ◦C, at a
heating rate of 6◦ min−1
.
The powders calcined at 1073 K were cold pressed into pellets of 10 mm diam-
eter and 2 mm thickness using a uniaxial hydraulic press. Sintering of these pellets
was carried out at different temperatures from 1173 K to 1373 K. In order to pre-
vent PbO vaporization, an equilibrium PbO vapour pressure was established using
PbZrO3 as setter, and placing everything in the covered alumina crucible. The sin-
tered density of the pellets was measured by liquid displacement method.
For the dielectric property measurement, the sintered pellet samples were
coated with platinum paste in order to make the pellet conductive. The dielectric
constant of these pellets was determined using a Solartron AC Frequency Analyzer
(Model 1260) in the frequency range 10 MHz to 1 Hz. The experiments were carried
out at different temperatures from 473 K to 873 K in an interval of 25 K. The temper-
ature was controlled by a microprocessor and measured by a K-type thermocouple
placed very close to the sample. At each temperature, the samples were equilibrated
for 20 min before recording the spectra.
3. Results and discussion
The room temperature XRD analysis of the samples heated to
1073 K indicated that the materials were single phase with cubic
lattice structure except for Nd substituted samples in which some
pyrochlore phases were also observed. Pyrochlore phase is usually
formed as a transient phase [22] before the PZT formation. From
Fig. 2, it can be seen that the onset of formation of the pyrochlore
phase was evolved in the precursor stage along with some other
impurity phases, apart from the cubic phase. Even though these
impurity phases disappeared on further heating to 1023 K, the
pyrochlore phases remained in the Nd containing samples. The
pyrochlore phase in Nd containing samples found to increase with
the increase in Nd content. Phase pure cubic phases were obtained
for 5, 10 and 15 mol% Ce and Pr doped samples. The cell parame-
ters of the compounds obtained from least square fitting of the XRD
data were found to be in good agreement with the reported values.
Fig. 3 shows the room temperature XRD patterns of these com-
pounds calcined at 1073 K. The sharp and singlet pattern indicates
better phase purity and crystallinity. In this system Pb loss due to
uncontrolled heating during self-propagating ignition of dried gel
is a matter of concern. However, in the present study it was found
that there was no Pb loss as the maximum temperature attained by
2. Experimental
The PZT gel precursors containing varying amounts of rare earths were prepared
by Gel Entrapment Technique [21]. For this titanium (IV) nitrate solution was pre-
pared by precipitating titanium trichloride into titanium hydroxide by addition of
liquor ammonia and then redissolving hydroxide in concentrated nitric acid over an
ice-bath. The titanium nitrate solution was stored in the refrigerator. The rare earth
(III) nitrate solutions were prepared by dissolving the A.R. grade rare earth oxides
in concentrated HNO3. The residual solution was evaporated to dryness. The nitrate
crystals of rare earths were then redissolved in 1 M HNO3. Both the nitrate stock
solutions along with A.R. grade zirconium oxynitrate were analyzed for their metal
contents by gravimetric method. In all compositions, the Zr/Ti molar ratio was con-
stant and maintained at 1.86, while the ratio of Pb/R (R = Nd, Pr and Ce) was 5.667,
9 and 19 when the R was 0.15, 0.10 and 0. 05 respectively. Accordingly appropriate
volumes of the metal nitrate solutions were mixed to achieve the required compo-
sitions. To this mixture of nitrate solution, 3 M hexamethylenetetramine (HMTA)
solution was added gradually at room temperature with constant stirring till a hard
gel was obtained. The gel obtained in the process was then dried in an oven at 423 K
and heated to 473 K to obtain a fluffy mass of the precursor material. The precursors
were further heat treated at 1023 K in air for 4 h to get phase-pure compounds. These
precursor powders were further heated to 773 K to remove any leftover organic
matter and finally to 1073 K to get phase-pure compounds. The typical flow sheet
of powder preparation is given in Fig. 1. The XRD patterns were recorded on an
˚
X-ray powder diffractometer using Cu K␣ radiation (ꢀ = 1.5406 A) with a graphite
monochromator to identify the phases. The applied voltage was 45 kV and the cur-
rent was 30 mA. Characterization of the powder by XRD line-broadening showed a
particle size of ∼40 nm. Specific surface area of the sample was determined by the
BET method. The particle size distribution analysis was carried out by Dynamic Light
Scattering (DLS).
Temperature programmed Reduction/Oxidation (TPR/TPO) is a sensitive and
specific technique for monitoring the redox property of the species present in the
system under reducing oxidizing conditions. TPR run was recorded on a TPDRO-1100
analyzer (Thermo Quest, Italy) under the flow of (H2 (5%) + Ar) and alternatively O2
Fig. 2. Room temperature XRD patterns of PZT doped with 10 mol% Nd in the pre-
cursor stage and calcined stage. (@ = PZT; * = pyrochlore phase; # = impurity phase).
Fig. 1. Flowsheet used for the preparation PZT powders.

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R.V. Pai et al. / Journal of Alloys and Compounds 508 (2010) 184–190
Fig. 4. Particle size distribution of PZT doped with 10 mol% of Pr by DLS.
Fig. 3. Room temperature XRD patterns of PZT’s doped with Nd, Ce and Pr calcined
at 1073 K.
to be 94% of Theoretical Density (T.D.). More grain growth was
observed in Pr and Ce containing PZT pellets which can be seen from
Fig. 5. The density determined by water displacement method
showed maximum density for Ce containing samples (varied from
91% to 94% of T.D.) among the series and minimum density for Nd
containing samples. The density of Pr doped samples and Nd doped
samples determined by water displacement method varied from
the charge was less than 773 K. The particle size distribution studied
by Dynamic Light Scattering (DLS) is shown in Fig. 4 which shows
an average particle size of ∼280 nm. Fig. 5(a)–(d) shows the SEM
photograph of the PZT pellets containing rare earths sintered to
1173 K for 2 h. The microstructure shows spherical, uniform grains
of submicron size. The density of the sintered pellet was found
Fig. 5. SEM photographs of (a) PZT; (b) PZT containing 10 mol% Nd; (c) PZT containing 10 mol% Pr; (d) PZT containing 10 mol% Ce.

R.V. Pai et al. / Journal of Alloys and Compounds 508 (2010) 184–190
187
89% to 93% of T.D. and 88–90% of T.D. respectively. Even though the
prominent oxidation state for the lanthanides is +3, Ce and Pr can
exist in the +4 oxidation as well. Any presence of higher valency
is known to incorporate cationic vacancies in the matrix, which is
responsible for the enhanced cation diffusivity. The higher densifi-
cation of Ce and Pr doped samples may be due to this existence of
their multi-valent states. The lower value density observed in Nd
samples may be due to the presence of pyrochlore phase in these
samples.
In integrating the oxide-ferroelectric capacitors into a standard
LSI device fabrication process, an annealing step at 350–550 ◦C in
a H2-containing reducing atmosphere is essential. So in order to
monitor the redox behaviour and evaluate the compositional sta-
bility of these oxides, these samples were subjected to TPR/TPO
studies. Fig. 6 shows the TPR profiles of Ce containing PZT (PCeZT),
Pr containing PZT (PPrZT) and the base material PZT. Each TPR cycles
was followed by TPO cycle. Such four TPR cycles are presented
in Fig. 6. The graph shows two different reducing species for the
Fig. 6. Temperature Programmed Reduction (TPR) profiles of: (a) PCeZT (10 mol%); (b) PCeZT (15 mol%); (c) PPrZT (10 mol%); (d) PPrZT (15 mol%); (e) PZT.

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R.V. Pai et al. / Journal of Alloys and Compounds 508 (2010) 184–190
compositions PCeZT and PPrZT where as the undoped PZT sample
showed only one reduction step. Even though the prominent oxi-
dation state for the lanthanides is +3, Ce and Pr can exist in the +4
oxidation as well. The first reduction occurred in the case of both
PCeZT and PPrZT is attributed to the reduction of this higher oxi-
dation state to +3 oxidation state at around 530 ◦C. The reduction
of lanthanides from higher oxidation state to lower oxidation state
could be prominently seen in the first TPR cycle. The subsequent
TPR cycles did not clearly show reduction peaks for PPrZT samples
whereas the PCeZT samples show reduction peaks due to reduction
of Ce4+ to Ce3+. This can be clearly seen for the sample containing
15 mol% of Ce (b in Fig. 5). This shows that in the case of Ce contain-
ing samples even after multiple reductions steps Ce can occupy Ce4+
in the perovskite lattice. The 2nd reduction peak appears at around
660 ◦C in the case of PPrZT and PCeZT and at around 705 ◦C for
undoped sample (PZT) was attributed to the reduction of Pb2+ to Pb
metal. This shows that the stability of PZT in reducing atmosphere
is deteriorating with dopant incorporation in the lattice. Also the
stability with respect to temperature of reduction was observed to
decrease as we move on from first to fourth TPR cycle which shows
the reduction of Pb in the compound to Pb metal at lower temper-
atures as we move from Ist to IVth TPR cycle. The XRD patterns of
residues of TPO (after 3rd TPO cycle) and TPR cycles (after 4th TPR
cycle) were recorded and compared with the parent compound.
Fig. 7 shows the XRD pattern of TPR/TPO residues of PCeZT sam-
ples which were compared with the XRD pattern of initial PCeZT
sample. The TPR residues formed showed the presence of Pb metal
which formed due to the reduction of Pb2+ as well as lanthanide
oxide and solid solution of ZrO2 and TiO2. The XRD pattern of TPO
residue of PCeZT showed could be compared with that of original
PCeZT sample. The XRD pattern of TPO residue of PZT sample (after
4th cycle) could not be compared to the initial PZT. The TPR cycles
seemed to degrade the material to such an extent that the oxidation
could not restore the original phase. In this sample the major phase
was PbO and ZrO2 containing TiO2. Where as in the case of PCeZT,
Ce present in the lattice stabilized the original phase and helped in
the restoration of the phase whose TPO sample after 4 TPR cycles
could be compared with the initial phase.
3.1. Electrical property measurements by impedance
spectroscopy
For impedance measurements, the sintered pellets were pol-
ished and coated with a thin layer of Ti paste and heated to 1073 K
for 2 h to improve contact with the electrode. The electrical proper-
ties of the samples were measured using a Solartron AC Frequency
Analyzer (Model 1260) in the frequency range 10 MHz to 1 Hz. Fig. 8
gives the Nyquist plot of the compound Pb0.95Ce0.05Zr0.65Ti0.35O3
at various temperatures. The relaxation frequency (fo) of the mate-
rial, independently of the geometrical parameter of the sample, was
found at the apex of the Nyquist semicircle fulfilling the condition
2ꢁfoRbCb = 1
From this relation, the bulk capacitance of the material (Cb), also
called the geometric capacitance, can be calculated and the bulk
dielectric constant εb can be determined using Cb = εbεoA/l; where
εo is the vacuum permittivity.
The semicircular pattern in the impedance spectrum is repre-
sentative of the electrical processes taking place in the material
which can be expressed as an equivalent electrical circuit compris-
ing of a parallel combination of resistive and capacitive elements.
The intercept of the semicircle on the real axis gives the resis-
tance of the corresponding component contributing towards the
impedance of the sample. The graph shows that at lower tem-
perature the resistance offered for conduction is very high. As
temperature increases, the conduction becomes easier. This shows
typical semiconductor behaviour in these samples. At lower tem-
perature only conduction through grains prevails where as at
higher temperature grain conduction as well as grain boundary
conduction is seen. The Nyquist plot for PZT undoped and doped
with 5 mol% of Pr, Ce and Nd is presented in Fig. 9. The impedance
of the sample decreased with Pr doping where as the impedance
showed increase with doping of Ce and Nd. The large increase of
impedance value in the case of Nd containing samples may be due
to the pyrochlore phase present in them.
The frequency (1 Hz to 10 MHz) dependent dielectric constants
(εr) of all the compounds were determined from the capacitance
values. Fig. 10 shows the variation of εr of PbZr0.65Ti0.35O3 as well
as Pb0.9R0.1Zr0.65Ti0.35O3 where R = Ce, Pr and Nd as a function of
temperature measured at 1 kHz as well as 10 kHz. It is clear that
the εr of all the compounds decrease with increase in frequency as
the capacitance value of these compounds decreases with increase
in frequency. The dielectric constant of these samples increased
with increase in temperature as the polarizability of the oxide sys-
Fig. 8. Nyquist plot of Pb0.95Ce0.05Zr0.65Ti0.35O3 at various temperatures.
Fig. 7. XRD patterns of TPO–TPR residues compared with original compound.

R.V. Pai et al. / Journal of Alloys and Compounds 508 (2010) 184–190
189
Fig. 9. Nyquist plot of PZT containing 5 mol% of rare earths at 600 ◦C.
Fig. 11. Variation of DC conductivity for PZT and Pb0.85R0.15Zr0.65Ti0.35O3 where
R = Ce, Pr and Nd with 1/T.
tem increases with temperature. Also, it can be seen from the figure
that the εr increases with Pr doping in the parent PZT sample where
as the εr value remarkably decreases with Ce as well as Nd doping.
The sample Pb0.9Pr0.1Zr0.65Ti0.35O3 showed εr of ∼22,489 at 710 ◦C
measured at 1 kHz. The undoped samples (PZT) showed a εr of
∼19,600 at this temperature whereas Ce and Nd containing samples
showed εr values of 5232 and 3002 respectively. Further the dielec-
tric constants of Ce and Nd doped samples were found to decrease
with increase in dopant concentration. Shao et al. [23] also reported
that the dielectric constant of PZT thin films decreased when the
film was doped with Ce. The authors attributed this behaviour to
the presence of pyrochlore phase in Ce containing thin films. But
in the present study pyrochlore phase was not observed in the Ce
containing samples even for 15 mol% of Ce. Ce has almost equal
probability to either replace Pb2+ at A site or Zr4+/Ti4+ ions at B
sites of ABO3 perovskite lattice. This is feasible, as Ce can exist both
tively. It is reported in the literature [24] that tendency of Ce to
occupy A site (as Ce3+) is more if the Ce ion concentration is less
than 1 mol% and beyond 1 mol%, both A as well as B site can be occu-
pied by Ce equally (Ce3+ in A site and Ce4+ in B site). They further
reported that the samples containing Ce in the A sites (3+ oxidation
state) improve the electrical properties. Similarly, in the present
study 5–15 mol% of Ce containing samples may be occupying a large
proportion of B sites in the ABO3 system which might be responsi-
ble for the decreased εr value in the Ce containing PZT. Even though
Pr also exhibits tri- and tetra-oxidation states, tri-oxidation state is
more stable than tetra-oxidation states whereas in the case of Ce
ion, extra stability would be attained by Ce4+ ion because of noble
gas electronic configuration by removal of 4 electrons. Hence Pr
occupancy in B sites in place of Zr4+/Ti4+is small and high value
of εr was observed in the present study. A site substitution reduces
oxygen vacancies by forming A site vacancy–oxygen vacancy defect
dipoles causing an increase in electric displacements which sub-
sequently results in increase in the dielectric constant. The donor
dopants have been reported to improve the dielectric and ferroelec-
tric properties of PZT thin films [25,26]. Considering the ionic radius
in tri- and tetra-valence states and can replace Pb2+ (rCe3+ = 1.43 A
˚
4+
and rPb2+ = 1.63 A) or Zr4+ (rCe4+ = 0.86 A and rZr = 0.73 A), respec-
˚
˚
˚
of Nd3+ (1.0 A) the B site occupancy by Nd should be the minimum.
˚
But in our study the addition of Nd in PZT reduces the dielectric
constant. This may be due to the presence of pyrochlore phase in
the Nd containing samples. During sintering, pyrochlore phase is
usually formed as a transient phase before the PZT formation. The
pyrochlore phase is oxygen deficient and so it is a metastable tran-
sient phase [22]. Thus, in the non-crystallite PZT’s, the presence
of impurity phases such as pyrochlore play an important role in
deciding the dielectric constant.
The DC conductivity of the samples was calculated using the
relation, ꢂ = 1/R*(l/A), where R is the electrical resistance, l the
thickness of the sample and A the area of cross section of the sam-
ple. The activation energy, Ea, for all the samples were calculated
from the plot of ln(ꢂT) versus 1/T using the conductivity relation
ꢂT = ꢂ0 exp(−Ea/(KBT)), where ꢂ0, is the pre-exponential factor, Ea
the activation energy and KB the Boltzmann constant. The DC con-
ductivity increases with the increase in temperature. At higher
temperature, PZT as well as rare earth doped PZT samples behaves
as a semiconductor and in semiconducting ceramics, free carriers
interact with the charged grain boundaries giving rise to increase
in the ionic conductivity. During the course of motion through the
solid, the available limited number of mobile carriers gets trapped
in relatively stable potential wells. Due to a rise in temperature,
Fig. 10. Variation of εr of PbZr0.65Ti0.35O3 as well as Pb0.9R0.1Zr0.65Ti0.35O3 where
R = Ce, Pr and Nd as a function of temperature measured at 1 kHz as well as 10 kHz.

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R.V. Pai et al. / Journal of Alloys and Compounds 508 (2010) 184–190
the donor cations take a major part in the conduction process
and the conductivity increases. At 10 mol% and 15 mol% dopant
concentration, PCeZT showed maximum conductivity among the
same class of samples. It was also observed that in PCeZT samples
the conductivity increases with increase in dopant concentration
whereas for both the Pr and Nd containing samples the conductiv-
ity decreases as we increase the dopant concentration. This may be
due to the fact that at higher Ce ion concentration, more cationic
vacancies are created at A site which contributes more towards the
conductivity values. The variation of DC conductivity for PZT and
Pb0.85R0.15Zr0.65Ti0.35O3 where R = Ce, Pr and Nd with 1/T is given
in Fig. 11. The conductivity in Nd doped samples showed a mini-
mum value among all the samples and decreased with dopant ion
concentration. The amount of pyrochlore phase in the Nd contain-
ing samples increased with increase in Nd concentration leading to
reduction in the conductivity of these samples.
agement. We also express our gratitude to Shri Sunil Kumar PIED
for his support in carrying out the SEM analysis.
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of Ce and Pr were prepared by a novel solution based technique.
But Nd containing samples showed the presence of pyrochlore
phases in the calcined powders. The redox behaviour studies con-
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The authors wish to express their sincere thanks to Dr. S.K.
Aggarwal, Head, Fuel Chemistry Division, for his constant encour-