Dielectric properties, thermal decomposition and related aspects of BiAlO3

Article abstract of DOI:10.1016/j.ssc.2008.03.039

BiAlO₃, prepared under high-pressure and high-temperature conditions has a rhombohedral perovskite structure (R 3 c with a = 5.3747 (4)?A? and c = 13.391 (1)?A?) and decomposes around 550?^{{ring operator}}C to form the intermediate Bi₂Al₄O₉ phase along with the Bi_{26 - x} Al_x O_{40 - y} and Al₂O₃. Above 580?^{{ring operator}}C, the intermediate phase changes into the Bi_{26 - x} Al_x O_{40 - y}. Dielectric measurements show an anomaly at the decomposition temperature and hysteresis at room temperature with a maximum polarization of ～16?μC/cm². Raman scattering measurements suggest softening of optical phonon modes, and are consistent with the occurrence of thermal decomposition of the rhombohedral phase to form the thermodynamically stable phase.

Full text of DOI:10.1016/j.ssc.2008.03.039

Solid State Communications 146 (2008) 435–437
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Solid State Communications
journal homepage: www.elsevier.com/locate/ssc
Fast-track communication
Dielectric properties, thermal decomposition and related aspects of BiAlO3
a
a
b
b
a,∗
a
R.V.K. Mangalam , S.V. Bhat , A. Iyo , Y. Tanaka , A. Sundaresan , C.N.R. Rao
a
Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560 064, India
Superconducting Materials Group, Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba 305 8568, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
BiAlO , prepared under high-pressure and high-temperature conditions has a rhombohedral perovskite
3
Received 14 March 2008
Accepted 31 March 2008 by P. Sheng
Available online 8 April 2008
◦
structure (R3c with a = 5.3747(4) Å and c = 13.391(1) Å) and decomposes around 550 C to form the
◦
intermediate Bi2Al4O9 phase along with the Bi26−xAl
x
O40−y and Al2O3. Above 580 C, the intermediate
phase changes into the Bi26−xAl
x
O40−y. Dielectric measurements show an anomaly at the decomposition
2
temperature and hysteresis at room temperature with a maximum polarization of ∼16 µC/cm . Raman
scattering measurements suggest softening of optical phonon modes, and are consistent with the
occurrence of thermal decomposition of the rhombohedral phase to form the thermodynamically stable
phase.
PACS:
7
7
7
7
7.80.-e
7.84.-s
8.30.-j
7.84.Bw
©
2008 Elsevier Ltd. All rights reserved.
Keywords:
B. High-pressure synthesis
D. Dielectric properties
D. Ferroelectricity
E. Raman scattering
1
. Introduction
predict that BiAlO3 (BAO) and BiGaO3 (BGO) perovskites with
3
+
Bi ions at the A-site will have large ferroelectric polarization
and piezoelectricity [12]. We, therefore, considered important
to investigate these materials experimentally. Our attempt to
prepare both BAO and BGO at ambient-pressure resulted in
The ferroelectric material most widely used in devices is the
distorted perovskite, Lead Zirconium Titanate (PZT) [1,2]. However,
the toxicity of lead in PZT is a serious environmental concern.
Therefore, it is essential to look for non-toxic materials whose
properties are comparable or superior to PZT. In the recent
years, a series of materials with the general formula BiMO3(M
Bi26−x(Al/Ga) O40−y with the γ-Bi2O3 structure (space group I23)
x
rather than the predicted perovskite structure. These materials,
however, exhibited ferroelectric hysteresis at room temperature
with low polarization values [13]. Belik et al. [14] have reported
the structures of BAO and BGO prepared under high pressure.
BAO has a perovskite-like non-centrosymmetric trigonal (R3c)
structure while BGO crystallizes in a pyroxene-like structure with
a centrosymmetric orthorhombic symmetry (Pcca). Recently BAO
is reported to be a ferroelectric material by Zylberberg et al. [15].
Here, we report dielectric, thermal properties and the temperature
dependence of optical phonon modes of BAO prepared under high-
pressure and high temperature conditions.
=
transition metals) have been reported to exhibit not only
ferroelectricity but magnetism as well. Except BiFeO3, all other
3
+
perovskites with Bi at A site are prepared at high pressure and
high temperature conditions [3–6]. In these multiferroic materials,
the origin of ferroelectricity is in the A-site Bi ions with the
3
+
2
6
s
lone pair whereas magnetism is due to transition metal
2
ions [7,8]. A large ferroelectric polarization (∼90 µC/cm ) was
reported [9] in thin films of BiFeO3 and substantiated on the
basis of first-principles calculations [10]. An ordered perovskite
BaBi3 Bi Nb O3 with Bi
+
5+
5+
3+
ion at the B-site was reported
0
.5 0.2
0.3
recently to exhibit ferroelectricity at room temperature [11].
2. Experimental
These facts demonstrate that the Bi3 ion with the 6s2 lone pair
+
2
+
2
would be an alternate candidate to replace the toxic Pb :6s
BAO was prepared at high-pressure using a cubic anvil high
ion in ferroelectric materials. Recent theoretical calculations do
pressure apparatus. Since the commercially available bismuth
◦
oxide was found to include Bi2O2CO3, it was heated at 700
C
in oxygen atmosphere to make it pure Bi2O3 and kept in a dry
box. A 1:1 ratio of Bi2O3 and Al2O3 was mixed in the glove
box, pelletized and packed inside a gold capsule. The capsule
was placed in the high pressure apparatus and a pressure of
∗
Corresponding author. Tel.: +91 80 2208 2824; fax: +91 80 2208 2766.
E-mail addresses: sundaresan@jncasr.ac.in (A. Sundaresan), cnrrao@jncasr.ac.in
(
C.N.R. Rao).
0
038-1098/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2008.03.039

4
36
R.V.K. Mangalam et al. / Solid State Communications 146 (2008) 435–437
Fig. 1. Observed, calculated and difference X-ray diffraction patterns of BAO.
Vertical tick marks are symmetry-allowed reflections and the second row
corresponds to the Bi Al O secondary phase.
2
4
2
40
4
.5 GPa was applied and heated to ∼800 oC and kept at this
temperature for 2 h. After 2 h, the heater was switched off and
the sample was allowed to cool to room temperature and the
pressure was reduced slowly. The solid sample was removed from
the Au capsule. This preparation procedure is slightly different
from the one adopted in our previous work [13] and the present
method was necessary to obtain the perovskite BiAlO3 phase. Phase
purity was checked with a Rigaku X-ray Diffractometer and high
temperature XRD data were collected using Bruker D8 Discover
using Cu Kα radiation. Rietveld refinement was carried out on
the X-ray data using the program Fullprof [16]. Capacitors were
prepared by depositing gold on both sides of the pellets using
DC sputtering. Ferroelectric measurements were carried out using
Radiant Technologies Precision workstation. Capacitance and
dielectric measurements were carried out using Agilent Impedance
Fig. 2. (a) DTA curve of BiAlO showing endothermic and exothermic peaks at 550
3
◦
and 580 C, respectively. The endothermic peak corresponds to the decomposition
of the high-pressure rhombohedral structure while the exothermic peak indicates
the formation of γ-Bi2O3 structure. (b) Temperature-dependent of the real part of
the dielectric constant exhibiting a maximum at the decomposition temperature.
◦
at 570 and 600 C were characteristic of a mixture of Bi2Al4O9,
Bi26−xAl
x
O40−y and Al2O3 confirming that both the DTA peaks are
◦
irreversible. The material heated to 750 C, however, showed
only the latter phases, suggesting that the thermodynamically
stable phase is the Bi26−xAl O40−y, while Bi2Al4O9 may be an
x
analyzer from 100 Hz to 1 MHz in the temperature range of
intermediate one. From these results, we suggest the origin of the
endothermic peak to be the decomposition of the metastable high
pressure rhombohedral phase and the subsequent formation of the
intermediate Bi2Al4O9. The exothermic peak is associated with the
◦
3
0–500 C. Temperature-dependent Raman spectra of the BAO
were recorded using LabRAM HR800 Raman spectrometer with an
Ar laser (514.5 nm) as the excitation source. Thermogravimetric
and differential thermal analysis (TG-DTA) was carried out (Mettler
Toledo, TG-850) on BAO in the range between room temperature
formation of Bi26−xAl O40−y which has a large number of atoms per
x
unit cell than the orthorhombic Bi2Al4O9. It is noteworthy that we
do not observe a structural transformation of rhombohedral BAO
◦
◦
−1
and 700 C (heating rate = 10 C min ) in oxygen atmosphere.
◦
till it decomposes at 570 C, all our measurements having been
carried out in air or in an oxygen atmosphere. High temperature
3. Results and discussion
−5
XRD data recorded in vacuum (10
bar) indeed showed that
there was no structural transformation or decomposition up
Rietveld refinement of the XRD data of BAO gave a structure
with the rhombohedral symmetry (space group R3c) with the
◦
to 700 C.
In Fig. 2(b), we show the temperature-dependent real part (ε’)
lattice parameters a
=
5.3747(4) Å and c
=
13.391(1) Å,
of the dielectric response of BAO at various frequencies in the
in agreement with the literature report [14]. The XRD pattern
obtained from the final Rietveld refinement is shown in Fig. 1. A
◦
temperature range 27–600 C. The dielectric constant at low fre-
◦
◦
small amount (5%) of the ambient-pressure phase (Bi26−xAl
x
O40) is
quencies has a well-defined maximum around 550 C and in-
present as a secondary phase. Differential thermal analysis (DTA)
creases further with increase of temperature above 580 C. Fig. 2(a)
◦
carried out while heating in the temperature range 30–700
C
and (b) suggest that the dielectric maximum corresponds to the
decomposition temperature of the high-pressure rhombohedral
phase and not to a high-symmetry phase. Furthermore, the dielec-
tric constant exhibits frequency dispersion over the entire range
of temperature employed without any appreciable shift in the di-
is shown in Fig. 2(a). It can be seen from this figure that there
◦
are two peaks, an endothermic one at 550 C and an exothermic
◦
one at 580 C. The cooling curve did not show any anomaly,
suggesting that the process associated with the exothermic peak
is irreversible. One can easily miss the endothermic peak unless
the DTA curve is recorded carefully (slowly). In the earlier
electric maximum. It is known that Bi26−xAl
x
O40−y is ferroelectric
◦
at room temperature (TC = 400 C). Above the TC, the dielectric
study [14] only an irreversible exothermic peak was found around
constant increases with temperature due to dc conduction. The in-
◦
◦
5
80 C and was attributed to the decomposition of BAO into
crease in the dielectric constant of BAO observed above 570 C in
Bi2Al4O9, Bi26−xAl
x
O40−y and Al2O3 phases based on the XRD
the present study may have such an origin.
◦
patterns obtained after heating the sample above 580 C. Thermal
decomposition is an endothermic process while a phase formation
can be exothermic. In order to confirm that the endothermic peak
In Fig. 3, we show a polarization-electric field curve of BAO
recorded at room temperature on a 200 µm thin sample using Ag
electrode. The shape of the curve is typical of ferroelectric material
◦
2
was irreversible, a sample was heated up to 570 C and then
cooled to room temperature. The XRD patterns of BAO heated
with a maximum polarization value of 16 µC/cm and remanent
2
polarization of 11.5 µC/cm . A coercive field of 69 kV/cm was

R.V.K. Mangalam et al. / Solid State Communications 146 (2008) 435–437
437
Fig. 3. Ferroelectric hysteresis of BiAlO at room temperature.
3
obtained for an applied voltage of 2.25 kV with a frequency of
1
0 Hz. However, the observed polarization value for BAO at room
2
temperature is lower than that predicted (75.6 µC/cm ) [12]
2
and also calculated using structural data (32 µC/cm ) [14].
The literature value of polarization reported for bulk BaTiO3 is
2
2
6 µC/cm at room temperature which is considerably higher than
that observed in the BAO system [17]. The remanent polarization
of commercial PZT samples reported by PI Ceramics (PIC 151) is
Fig. 4. Temperature dependence of the frequency and the intensity of various
Raman bands in BiAlO .
3
2
0.36 C/m with a coercive field of 0.9 kV/mm [18]. However, the
polarization value obtained for BAO is in agreement with value
reported by Zylberberg et al. [15].
Acknowledgements
In order to obtain an insight into the behavior of soft-mode
phonons and thermal behavior of the high-pressure rhombohedral
phase and the subsequent appearance of the ambient pressure
phase, we have recorded Raman spectra of BAO at various
temperatures. Due to limitations in our instrument, we could
This work forms a part of the work under MOU between AIST,
Japan and JNCASR, Bangalore. AS and RVKM acknowledge AIST for
visiting fellowship.
◦
record the spectra only up to 520 C. Group theoretical analysis of
References
the perovskite with R3c symmetry predicts 13 Raman active modes
[
[
1] E. Cross, Nature 432 (2004) 24.
(
Γ = 4A1 + 9E). We observe all the 13 modes at room temperature
2] Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T. Nagaya,
M. Nakamura, Nature 432 (2004) 84.
in agreement with spectrum reported in the literature [14]. With
increase in temperature, frequencies of all the A1 modes decrease
and the peaks become broader. The temperature dependence of
frequency and intensity of some of the Raman modes are shown in
Fig. 4. The figure reveals that both the frequency and the intensity
of the Raman bands decrease with increasing temperature up
[3] S. Niitaka, M. Azuma, M. Takano, E. Nishibori, M. Takata, M. Sakata, Solid State
Ionics 172 (2004) 557.
[
[
4] F. Sugawara, S. Iiida, Y. Syono, S. Akimoto, J. Phys. Soc. Japan. 25 (1968) 1553.
5] A.A. Belik, S. Iikubo, K. Kodama, N. Igawa, S.-I. Shamoto, S. Niitaka, M. Azuma,
Y. Shimakawa, M. Takano, F. Izumi, E.T. Muromachi, Chem. Mater. 18 (2006)
798.
◦
[6] S. Ishiwata, M. Azuma, M. Takano, E. Nishibori, M. Takata, M. Sakatac, K. Kato,
J. Mater. Chem. 12 (2002) 3733.
to 520 C. The room temperature Raman spectrum of a sample
◦
heated to 600 C showed a completely different Raman spectrum
[
7] R. Seshadri, N.A. Hill, Chem. Mater. 13 (2001) 2892.
with 22 modes. This finding supports the suggestion that the
[8] S.W. Cheong, M. Mostovoy, Nat. Mater. 6 (2007) 13.
◦
[9] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu, D. Viehland,
V. Vaithyanathan, D.G. Schlom, U.V. Waghmare, N.A. Spaldin, K.M. Rabe,
M. Wuttig, R. Ramesh, Science 299 (2003) 1719.
rhombohedral structure decomposes above 550 C.
[
[
10] J.B. Neaton, C. Ederer, U.V. Waghmare, N.A. Spaldin, K.M. Rabe, Phys. Rev. B 71
4. Conclusions
(
2005) 014113.
11] R.V.K. Mangalam, P. Mandal, E. Suard, A. Sundaresan, Chem. Mater. 19 (2007)
4114.
We have demonstrated that BiAlO3 undergoes an endothermic
◦
[12] P. Baettig, C.F. Schelle, R. LeSar, U.V. Waghmare, N.A. Spaldin, Chem. Mater. 17
transition at 550 C due to the decomposition of the metastable
(
2005) 1376.
high pressure rhombohedral phase, and an exothermic transition
[
[
[
13] R.V.K. Mangalam, R. Ranjith, A. Iyo, A. Sundaresan, S.B. Krupanidhi, C.N.R. Rao,
Solid State Commun. 140 (2006) 42.
◦
at 580 C due to the formation of Bi26−xAl
x
O40−y and Al2O3.
14] A.A. Belik, T. Wuernisha, T. Kamiyama, K. Mori, M. Maie, T. Nagai, Y. Matsui,
E.T. Muromachi, Chem. Mater. 18 (2006) 133.
The exothermic peak is accompanied by a dielectric constant
maximum. The material is ferroelectric at room-temperature with
15] J. Zylberberg, A.A. Belik, E.T. Muromachi, Z.-G. Ye, Chem. Mater. 19 (2007) 6385.
2
a maximum polarization of ∼16 µC/cm and the Raman bands
[16] R. Carvajal, Physica B 192 (1993) 55.
o
show decrease in frequency and intensity up to 520 C.
[17] B.W. Wessels, Annu. Rev. Mater. Sci. 25 (1995) 525.
[
18] T. Granzow, N. Balke, D.C. Lupascu, J. Ródel, Appl. Phys. Lett. 87 (2005) 212901.