Bi:SrTiO
3
: A quantum ferroelectric and a relaxor
Chen Ang,* Zhi Yu, P. M. Vilarinho, and J. L. Baptista
Department of Ceramics and Glass Engineering, University of Aveiro, 3810 Aveiro, Portugal
Received 29 December 1997
Quantum ferroelectric and ferroelectric relaxor behavior has been found in (Sr
1-1.5x
Bi
x
)TiO
3
. For x
0.0267, the quantum ferroelectric relationship T
m
( x -x
c
)
1/2
holds with x
c
0.0005, and obvious hysteresis
loops were observed. The polarization relaxation shows critical slowing down. At high Bi concentration, a
crossover from quantum ferroelectric to relaxor behavior occurred. The coexistence of ferroelectric and relaxor
behavior in Bi:SrTiO
3
was attributed to a ferroelectric domain state induced by random fields.
S0163-18299802314-5
Since the discovery of a family of materials, namely,
‘‘relaxors,’’
1–3
such as PbMg
1/3
Nb
2/3
O
3
PMN or
PbSc
1/2
Ta
1/2
O
3
PST, which show a rounded permittivity
peak and a low-frequency dispersion, the physical nature of
the relaxors has been a challenging subject. On the other
hand, similar relaxor behavior is also found in quantum
paraelectric KTaO
3
doped with Li KLTRefs. 4 and 5 or
SrTiO
3
doped with Ca SCT.
6,7
For example, both ferroelec-
tric and dipole glass behavior was observed in KLT where a
crossover from the dipole glass to a ferroelectric domain
state occurred as the Li content increased.
8,9
The two systems have been independently developed, and
recently the common features were pointed out, mainly in-
cluding 1 a large permittivity, 2 a low-frequency disper-
sion of permittivity and long relaxation time, 3 nonergod-
icity of permittivity,
10
4 nanometer scale polar clusters,
11
and 5 compliance with both Vogel-Fulcher type
12
and de
Almeida–Thouless type relationships.
10
It is recognized that
the polar clusters, dipolar interaction, and random electric
fields play a key role in the relaxors. However, different
explanations were proposed for these observations. For ex-
ample, for typical relaxor PMN, a ‘‘dipolar glass’’ model
was suggested by Viehland et al.,
12
while a random-electric-
field model was suggested also for PMN by Westphal et al.
13
Further studies are needed and a search for new systems is
meaningful. Toulouse et al. pointed out that one of the ob-
stacles in understanding the behavior of relaxors has been
their complex structure and suggested that KLT with simple
perovskite structure could be a model system for relaxors.
14
In this paper, the authors suggested that (Sr
1 -1.5x
Bi
x
)TiO
3
SBT could be a new model system for relaxors.
SBT exhibits a dielectric relaxation behavior, which was
first reported by Skanavi et al.,
15,16
who attributed it to the
mechanism of ‘‘hopping ion’’ polarization in the perovskite
lattice absent of ferroelectricity. On the contrary, on the basis
of slim electric hysteresis loops for SBT,
1,17
Smolenskii
et al.
1
classified it as a ferroelectric relaxor similar to PMN.
However, further studies either on the relaxor behavior or
ferroelectric mechanisms in this system were not reported.
In this paper, we show that quantum ferroelectric behav-
ior and the essential features of a ferroelectric relaxor were
found in SBT and that a crossover from ferroelectric to re-
laxor behavior occurred with increasing Bi concentration.
The ceramic samples of (Sr
1 -1.5x
Bi
x
)TiO
3
(0 x
0.167) were prepared by a solid-state reaction. The x-ray-
diffraction results indicate that all the samples are single cu-
bic phase, and energy dispersion analysis indicated that the
Bi concentration is in agreement with the nominal composi-
tion, within the experimental error, and that the distribution
of Bi atoms is uniform. Strontium vacancies V
Sr
are created
as Sr
1 -1.5x
Bi
x
(Vs
r
)
0.5x
TiO
3
, to balance the charge misfit
due to the substitution of divalent Sr
2+
ions by trivalent Bi
3+
ions.
16,18
It was also shown that the dielectric permittivity
was independent of the electrodes three kinds of metals,
PHYSICAL REVIEW B
CONDENSED MATTER AND MATERIALS PHYSICS
THIRD SERIES, VOLUME 57, NUMBER 13 1 APRIL 1998-I
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