Geoscience Reference
In-Depth Information
Byrappa
[85]
has reviewed the hydrothermal synthesis of various titanates and
the reader can find more information in his work.
Besides alkali titanates, trivalent metal titanates, like bismuth titanate, have been
studied in great detail owing to their high dielectric permeability. The Bi
2
O
3
a
TiO
2
system has been studied widely, and several phases like Bi
2
Ti
2
O
7
,B
2
Ti
3
O
9
,
Bi
4
Ti
3
O
12
,Bi
8
TiO
14
,Bi
12
TiO
20
,andBi
2
Ti
4
O
11
are obtained under the varying Bi
2
O
3
/
TiO
2
ratios from 25:1 to 8:1. The solvents consisted of aqueous KF solutions while
the initial charge was a mixture of the oxides Bi
2
O
3
and TiO
2
. The experiments were
conducted in the range 450
600
C with a percentage filling factor of 0.5
0.8.
Among these titanates, the Ti sillenite, Bi
12
TiO
20
, was almost independent of the
composition of the initial charge. When the TiO
2
content in the charge was more
than 5%, Bi
4
Ti
3
O
12
(77.00% Bi
2
O
3
and 20.80% TiO
2
) began to crystallize together
with the Ti-selenite. As the TiO
2
content of the charge was increased, the amount of
Bi
4
Ti
3
O
12
enhanced and this compound became the first predominant and also the
only phase over the range 17
25% TiO
2
. Due to the temperature gradient in the
autoclave, the bismuth titanates crystallize in the upper cooler region. The minimum
recrystallization temperatures (500
C) were characteristic of the Ti-selenite. The
other bismuth titanates required higher temperatures, but sufficiently rapid transfer
of all the compounds synthesized occurred at 550
600
C.
Figure 8.22(a)
(c)
shows
the schematic diagrams of the bismuth titanate crystals
[86]
. Similarly, Barsukova
Figure 8.22 Schematic diagram of
the bismuth titanate crystals: (a)
crystal of Bi
4
Ti
3
O
12
, (b) plate-like
crystal of Bi
2
Ti
4
O
11
, and (c)
primatic crystal of Bi
2
Ti
4
O
11
[86]
.
001
011
101
01
-
10
-
(a)
130
01
1
110
21
-
010
-
21
-
150
100
110
010
-
011
(b)
(c)
