Synthesis, characterization, sintering and dielectric properties of nanostructured perovskite-type oxide, Ba2GdSbO6

Abstract

Nanoparticles of barium gadolinium antimonate (Ba2GdSbO6), a complex perovskite-type oxide, has been synthesized using an auto ignition combustion process for the first time. The
nanoparticles thus obtained have been characterized by powder X-ray diffraction, thermogravimetric analysis, differential
thermal analysis, Fourier transform infrared spectroscopy and transmission electron microscopy. The XRD studies have shown
that the as-prepared powder is phase pure Ba2GdSbO6 and has a complex cubic perovskite (A2BB′O6) crystalline structure with lattice constant, a = 8·449 Å. The TEM image reveals that the particle size of the as-prepared nano powder was in the range 30–60 nm. The nanocrystals
of Ba2GdSbO6 synthesized by the combustion technique could be sintered to 96% of the theoretical density by heating at a temperature of
1560°C for a short duration of 3 h. The surface morphology of the sintered pellet has been studied by scanning electron microscope
(SEM). The dielectric constant (ɛ
r) was 20 and the loss factor (tanδ) was 0·03 at 3 MHz. By the present combustion technique a phase pure nanopowder of Ba2GdSbO6 could be obtained by a single step process without the need of any calcination step.

Full text

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Bull. Mater. Sci., Vol. 31, No. 5, October 2008, pp. 719–722. © Indian Academy of Sciences.

719
Synthesis, characterization, sintering and dielectric properties of
nanostructured perovskite-type oxide, Ba2GdSbO6
C VIJAYAKUMAR, H PADMA KUMAR†, SAM SOLOMON††, J K THOMAS†,
P R S WARRIAR* and J KOSHY†
Department of Physics, University College, Trivandrum 695 034, India
†Electronic Materials Research Laboratory, Department of Physics, Mar Ivanios College,
Trivandrum 695 015, India
††Department of Physics, St. Johns College, Anchal 691 306, India
MS received 24 November 2007; revised 16 April 2008
Abstract. Nanoparticles of barium gadolinium antimonate (Ba2GdSbO6), a complex perovskite-type oxide,
has been synthesized using an auto ignition combustion process for the first time. The nanoparticles thus obtained
have been characterized by powder X-ray diffraction, thermogravimetric analysis, differential thermal analysis,
Fourier transform infrared spectroscopy and transmission electron microscopy. The XRD studies have shown
that the as-prepared powder is phase pure Ba2GdSbO6 and has a complex cubic perovskite (A2BB′O6) crystalline
structure with lattice constant, a = 8⋅449 Å. The TEM image reveals that the particle size of the as-prepared nano
powder was in the range 30–60 nm. The nanocrystals of Ba2GdSbO6 synthesized by the combustion technique
could be sintered to 96% of the theoretical density by heating at a temperature of 1560°C for a short duration
of 3 h. The surface morphology of the sintered pellet has been studied by scanning electron microscope (SEM).
The dielectric constant (εr) was 20 and the loss factor (tanδ) was 0⋅03 at 3 MHz. By the present combustion
technique a phase pure nanopowder of Ba2GdSbO6 could be obtained by a single step process without the
need of any calcination step.

Keywords. Samarium barium antimonite; nanoparticles; combustion synthesis; sintering.
1. Introduction
Materials with perovskite structure exhibit a wide range
of properties due to their diversity of crystal structures
(Kimura et al 2000; Litchenberg et al 2001). Ba2LnSbO6
(Ln = lanthanides) compounds with double perovskite
A2BB′O6 structure have been extensively studied in the past
(Blasse 1965; Garcia Cascado et al 1984; Alonso et al 1997;
Karunadasa et al 2003). Among these materials, Ba2GdSbO6
compound was of special interest because of its applica-
tions (Kurian et al 1995) as substrate for high tempera-
ture superconducting YBa2Cu3O7–δ films. The crystal
structure of Ba2GdSbO6 material was reported as double
cubic perovskite (Kurian et al 1995). In all the above
studies, conventional solid state synthesis route was used
for the preparation of their single phase compounds
which involved prolonged calcination at high tempera-
tures (> 1200°C for 24 h) with intermediate grindings,
yielding large coarse grained micron sized powders. The
coarse-grained powders synthesized using the conven-
tional solid state route have the disadvantages of larger
particle size, high temperature processing and relatively
lower phase purity (Hoffler and Averback 1990; McCan-
dlish et al 1992).
The synthesis of advanced ceramic materials as nano-
particles is currently gaining widespread interest in materials
research (Karagedov and Lyakhov 1999; Ravichandran et
al 1999; Gleiter 2000). The advantages of nanocrystalline
materials are superior phase homogeneity, sinterability
and microstructure leading to unique mechanical, electrical,
dielectric, magnetic, optical and catalytic properties. The
combustion synthesis is a processing technique currently
being used as a route for the production of engineering
ceramics and other advanced materials in their nano powder
form and this process provides energy and cost saving
advantages over the other processing routes (Saha and
Pramanik 1997; Patil et al 2001; James et al 2004). In
this paper, synthesis, characterization, sintering and di-
electric properties of phase pure Ba2GdSbO6 nanoparti-
cles using a single step modified combustion process are
reported for the first time.
2. Experimental
The basic requirement for the synthesis of an oxide mate-
rial by the chemical route is that it should be possible to

*Author for correspondence (prswariar@yahoo.com)

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C Vijayakumar et al

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obtain all the ions involved in solution before initiating
further reactions. Therefore, for the preparation of
Ba2GdSbO6, it is necessary to have an aqueous solution
of Ba, Gd and Sb ions. To get a clear solution containing
the Gd, Ba and Sb ions, Gd2O3 was dissolved in dilute
nitric acid, Ba(NO3)2 in water, Sb2O3 in boiling tartaric
acid and they were mixed together. Aqueous solution
containing ions of Gd, Ba, Sb were prepared from high
purity Gd2O3 (99⋅9%, CDH, India), Ba(NO3)2 (99⋅9%, CDH,
India) and Sb2O3 (purified, Merck). To get the precursor
complex, citric acid was added to the solution containing
the metal ions, maintaining the citric acid to the cation
ratio at unity. The oxidant–fuel ratio of the system was
adjusted by using nitric acid and ammonium hydroxide,
and the ratio was kept at unity. The solution containing
the complex precursor mixture at a pH of ~ 7 was heated
using a hot plate at ~ 250°C in a ventilated fume hood.
The solution boils on heating and undergoes dehydration
and decomposition leading to a smooth deflation and
producing foam. The foam then ignites by itself on per-
sistent heating giving voluminous and fluffy product of
combustion. The combustion product was subsequently
characterized as single-phase nanocrystals of Ba2GdSbO6.
Structure of the as-prepared powder was examined by
powder X-ray diffraction (XRD) technique using a X-ray
Diffractometer (Model Bruker D-8) with Nickel filtered
CuKα radiation. The differential thermal analysis (DTA)
and thermogravimetric analyses (TGA) were carried out
using Perkin-Elmer TG/DT thermal analyser in the range
30–1100°C at a heating rate of 20°C/min in nitrogen at-
mosphere. The infrared (IR) spectra of the samples were
recorded in the range 400–4000 cm–1 on a Thermo-Nicolet
Avatar 370 Fourier transform infrared (FTIR) spectrometer
using KBr pellet method. Particulate properties of the
combustion product were examined using transmission
electron microscopy (TEM, Model-Hitachi H-600, Japan)
operating at 200 kV. The samples for transmission ele-
ctron microscope (TEM) were prepared by ultrasonically
dispersing the powder in methanol and allowing a drop of
this to dry on a carbon-coated copper grid. The selected
area diffraction (SAD) pattern is also recorded for a camera
length of 1⋅35 m. To study the sinterability of the nanopar-
ticles obtained by the present combustion method, the as-
prepared Ba2GdSbO6 nanoparticles were mixed with 5%
polyvinyl alcohol and pressed in the form of cylindrical
pellet of 14 mm diameter and ~ 2 mm thickness at a pre-
ssure of about 350 MPa using a hydraulic press. The pel-
let was then sintered at 1560°C for 3 h. The theoretical
density of the Ba2GdSbO6 was calculated from the lattice
constants and sintered density was determined using
Archimedes method. The surface morphology and energy
dispersive X-ray analysis of the sintered samples were
examined using scanning electron microscopy (SEM,
Model-Hitachi S 2400 Japan). For low frequency dielectric
studies the pellets were made in the form of a disc capaci-
tor with the specimen as the dielectric medium. Both flat
surfaces of the sintered pellet were polished and then
electroded by applying silver paste. The capacitance of
the samples were measured using an LCR meter (Hioki-
3532-50) for the frequency range 100 Hz–3 MHz.
3. Results and discussion
Figure 1 shows the XRD pattern of the as-prepared powder
obtained by the present combustion synthesis. The broad
peak in figure 1(a) at d = 2⋅98726 Å corresponds to the
major (220) reflections of Ba2GdSbO6. All the peaks are
indexed for a complex cubic (Fm3m) perovskite structure
and the lattice constant of Ba2GdSbO6 determined from
XRD pattern is a = 8⋅449 Å, which agrees very well with
the reported values (Kurian et al 1995). The crystallite size
calculated from the full width at half maximum (FWHM)
using Scherrer formula for the major (220) peak in figure
1(a) is ~ 23 nm. The as-prepared powder was heated in
the range 600°C for 1 h. The XRD patterns of the sample
heated at 600°C is shown in figure 1(b). No structural
changes were observed in the heated samples as com-
pared to as-prepared powder, which indicates that the
formation of Ba2GdSbO6 phase was complete during the
combustion process itself without a need for any calcina-
tion step. It may be noted that single phase Ba2GdSbO6
powder can be obtained via the solid state reaction route,
only after calcining the reaction mixture at 1200°C for 24 h
with three intermediate grindings (Kurian et al 1995).
The present method is the successful one for the preparation
of single phase Ba2GdSbO6 without the need for any fur-
ther calcination or high temperature annealing procedure.
In other combustion processes reported for the prepara-
tion of nano crystals of some ceramic oxides, polyvinyl
alcohol and urea were used as chelating agents and fuel,
respectively and in all these cases phase pure powder was
obtained only after high temperature annealing (Saha and



Figure 1. XRD pattern of a. as-prepared Ba2GdSbO6
nanocrystals and b. heated at 600°C.

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Synthesis, characterization, sintering and dielectric properties of Ba2GdSbO6

721
Pramanik 1997; Patil et al 2001). But in the present
modified combustion method, citric acid was used as the che-
lating agent instead of polyvinyl alcohol and urea was
replaced with ammonia. By changing the complexing
agent and oxidant fuel system it was possible to get a single
phase Ba2GdSbO6 as nanocrystals in a single step com-
bustion.
A typical DTA and TGA curves of the as-prepared
Ba2GdSbO6 powder up to 1100°C are shown in figure 2.
The TGA curve shows a weight loss of < 3% which can be
caused by the liberation of adsorbed moisture in the sample
and other than this, there is no weight or enthalpy change
occurring in the sample at high temperatures up to 1100°C,
which implies that the combustion is complete and no
organic matter is present in the sample. There is no evi-
dence of any phase transition taking place in the sample
up to a temperature of 1100°C.
Figure 3 shows FT–IR spectrum of the typical as-prepared
Ba2GdSbO6 nanopowder prepared through combustion
synthesis. All the peaks in figure 3 are characteristics of


Figure 2. DTA/TGA trace of as-prepared Ba2GdSbO6 nano-
crystals.


Figure 3. FT–IR spectrum of Ba2GdSbO6 powder obtained
through combustion synthesis.
the material except the one at 3415 cm–1 which is due to
the presence of adsorbed moisture. This corroborates the
XRD and thermal analysis result that the combustion is
complete and no organic matter is present in the sample.
The transmission electron microscopic (TEM) studies
on the powder morphology of the as-prepared Ba2GdSbO6
nanopowder obtained by the combustion synthesis showed
that the nanoparticles are of submicron size 30–60 nm
with a mean size of 40 nm as shown in figure 4 with regular


Figure 4. TEM micrographs of as prepared Ba2GdSbO6 nano
powder in bright field and corresponding electron diffraction
pattern.


Figure 5. SEM micrograph of the Ba2GdSbO6 sintered pellet.

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C Vijayakumar et al

722
hexagonal shape. Figure 4 is a typical TEM bright field
image of the as prepared powder and the inset is the cor-
responding selected area electron diffraction pattern (SAD).
The particles are of regular shape with sharp grain bounda-
ries. The ring nature of the electron diffraction pattern is
indicative of the polycrystalline nature of the crystallites,
but the spotty nature of the SAD pattern can be due to the
fact that the finer crystallites having related orientations
are agglomerated together resulting in a limited set of
orientations.
The sintering behaviour of the nanocrystals of Ba2GdSbO6
powders synthesized through the present combustion
route was studied. The relative green density of the speci-
men used for the sintering study was 55% and sintered
density was > 96% of the theoretical value. Compared to
the material prepared through the conventional route, the
sintering duration was found to be reduced considerably
from 12 h to 3 h. Surface morphology of the sintered
Ba2GdSbO6 specimen was examined using SEM and is
shown in figure 5. It is clear from the micrograph that
densification was achieved without significant microstruc-
tural coarsening. Average grain size determined from the
SEM micrograph is ~ 500 nm.
Figure 6 shows the variation of dielectric constant (εr)
and loss factor (tanδ) of the sintered pellet measured in the
frequency range 100 Hz–3 MHz. The study revealed that
the dielectric constant (εr) does not show significant
variation with frequency and it falls from 26–20 in the
measured frequency range of 100 Hz–3 MHz. The loss
factor (tanδ) also gets decreased when the frequency is
increased from 100 Hz–3 MHz and has a value of 0⋅03 at
a frequency of 3 MHz at room temperature. The low values
of dielectric constant (εr) and loss factor (tanδ) fulfils one
of the most important requirements of a material to be
used as a substrate for the fabrication of high Tc super-
conducting films.


Figure 6. Variation of dielectric constant (εr) and loss factor
(tanδ) with frequency.
4. Conclusions
Ba2GdSbO6 has been synthesized as single phase nano
crystals (30–60 nm) via an exothermic chemical process
for the first time. The solid combustion product obtained
was characterized by X-ray diffraction (XRD), electron
diffraction, differential thermal analysis (DTA), thermo-
gravimetric analysis (TGA), infrared spectroscopy (FT–IR)
and transmission electron microscope (TEM). X-ray di-
ffraction and electron diffraction have shown that the as-
prepared powder is phase pure Ba2GdSbO6 and has a
complex cubic perovskite A2BB′O6 structure with lattice
constant, a = 8⋅449 Å. DTA and TGA studies have shown
that there is no phase transition taking place in the as-
prepared powder.
The transmission electron microscopy image of the as-
prepared powder reveals that the particle size is in the
range 30–60 nm. The nanopowder could be sintered to a
density of 96% of the theoretical density at 1560°C for
3 h. The SEM image of the sintered material indicates
high densification of the material with average grain size
of 500 nm. The room temperature dielectric constant (εr)
and loss factor (tanδ) of the sintered pellet at 3 MHz was
20 and 0⋅03, respectively which indicates that the mate-
rial is suitable for microwave applications. The combu-
stion synthesis has an advantage that the phase pure
Ba2GdSbO6 could be obtained by a single step process
without the need of any calcination step.
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