Page 491 - Book Hosokawa Nanoparticle Technology Handbook

10 DEVELOPMENT OF NOVEL FERROELECTRIC MATERIALS APPLICATIONS
One of the origins of the larger P of the
s
BiT–BBTi crystals is the Bi substitution at the Ba site.
-6
log (current density/A · cm -2 ) -7 BiT-BBTi crystals the Sr site occurs, and the charge difference 2
For strontium bismuth tantalate with Sr-deficient
BiT crystals
and Bi-excess composition, the Bi substitution at
2
3
is compensated through the
and Sr
between Bi
formation of Sr vacancies [14]. This enhances Ps
2
(SrBi Ta O ) to 29 C/cm
from 19 C/cm
9
2
2
-8
Ta O ), and raises T from 295 to 410 C
Bi
(Sr
2.13
2
0.81
9
C
[14]. It has been reported for BBTi ceramics with
random orientation that the Bi substitution at the Ba
BBTi crystals
2
site enhances P from 14 C/cm (BaBi Ti O ) to
-9
4
r
4
15
2
18 C/cm (Ba Bi 4.07 Ti O ), and leads to an increase
15
0.9
4
0 20 40 60 80 in T from 415 to 440 C. The compositional analysis
C
of the BiT–BBTi crystals shows the Ba-deficient and
E // a axis (kV/cm)
Bi-excess composition (Ba 0.75 Bi 8.32 Ti O ), indicat-
27
7
ing that excess Bi is substituted at the Ba site.
Figure 10.4 Indeed, higher T was observed for the BiT–BBTi
C
Leakage current density as a function of electric field crystals (540 C) than for the stoichiometric ceramics
applied along the a axis (25 C).
(490 C). This is direct evidence of the Bi substitu-
tion at the Ba site. However, the effects of the Bi
substitution alone cannot explain the large P of the
s
BiT–BBTi crystals.
75 BiT-BBTi crystals
It is considered that the lattice distortion induced by
the alternate stacking of m 3 and m 4 layers plays
Polarization ( C/cm 2 ) -25 BBTi BBTi contains Ba ions with large ionic radius at the A
50
a dominant role in enhancing P of BiT–BBTi. Since
s
BiT
25
crystals
site, parameters a and b are larger than those of BiT.
0
The lattice mismatch between the BiT and the BBTi
reaches 0.42% along the a axis and 0.88% along the
-50
averages of those of values for the BiT and the BBTi.
These results imply that tensile and compressive
-75 crystals b axis. The parameters a and b of BiT–BBTi were the
1 Hz
stresses built up in the m 3 and m 4 layers of the
superlattice structure, respectively. These stresses
-300 -200 -100 0 100 200 300
are suggested to be concentrated on the Bi O layers that
E // a axis (kV/cm) 2 2
interleave the two kinds of perovskite block. This
crystallographic environment induces a local symme-
Figure 10.5 try breaking of the Bi O layers. All Bi ions of the
Polarization hysteresis loops along the a axis (25 C). 2 2
Bi O layers in BiT and BBTi are identical from the
2
2
crystallographic point of view. In contrast, the Bi ions
of the Bi O layers in the BiT–BBTi are divided into
2
2
two cations: one is connected to the perovskite blocks
and the coercive field (E ) of BiT–BBTi were of the m 3 layer, and the other is adjacent to that of
c
2
52 C/cm and 120 kV/cm, respectively. Note that the m 4 layer. The symmetry breaking leads to an
2
this P value is larger than those of BiT (46 C/cm ) unusual ferroelectric displacement of the Bi ions of
s
2
and BBTi (16 C/cm ) crystals. It has been widely the Bi O layers in the BiT–BBTi. It has been
2
2
known that BiT has the largest P among the BLSFs, reported that the Bi ions of the Bi O layers in
2
2
s
and that the P of BBTi is much smaller [13]. If the BiT–SrBi Ti O 15 are displaced along the a axis
4
s
4
intrinsic ferroelectric distortions in BiT and BBTi (the polar direction) by 2% of the parameter a from
were maintained in the superlattice structure, the P s the corresponding positions of the high-temperature
of BiT–BBTi would be the average of those for BiT tetragonal structure. Similar displacements of the Bi
2
and BBTi (approximately 30 C/cm ). However, the ions are expected to the BiT–BBTi, which enhance P .
s
P observed for the BiT–BBTi crystals is much In addition to the lattice distortions of the Bi O layers,
2
s
2
larger than the average P . The enhanced P of the the alternate stacking of the m 3 and m 4 layers is
s
s
BiT–BBTi crystals implies that the lattice strain considered to promote the ferroelectric distortions in
induced by the alternate stacking of m 3 and m 4 the perovskite blocks in the BiT–BBTi.
layers with different cell size promotes ferroelectric Superlattice-structured BiT–BBTi single crystals
distortion. were grown by a self-flux method, and the structure
463