Geoscience Reference
In-Depth Information
Figure 7.82 Phases crystallizing in
the system Li
2
O
a
B
2
O
3
a
H
2
O under
hydrothermal conditions
[356]
.
H
2
O
A = Li
4
B
7
O
12
Cl
B = Li
2
B
4
O
7
C = Li
3
B
5
O
8
(OH)
2
D = LiH
2
B
5
O
9
E = Li
2
HBO
3
A
+
C
B
+
E
D
Li
2
O
B
2
O
3
7.17.1 Hydrothermal Growth of Selected Borates
It is practically impossible to describe the hydrothermal growth of each and every
type of borate crystals. Therefore, the authors have described the hydrothermal
growth of only some important borate materials and their overall growth
elucidation.
Li
2
B
4
O
7
. Lithium tetraborate (Li
2
B
4
O
7
) is an important piezoelectric and nonfer-
roelectric crystal which has attracted great attention in research and industry for its
application in SAW and bulk acoustic wave (BAW) devices. It has a lower temper-
ature coefficient of delay than lithium niobate and tantalate and a higher electrome-
chanical coupling constant
-quartz. Since berlinite has several major
problems in its growth as large single crystals, lithium tetraborate is becoming an
attractive prospect as a piezoelectric crystal
[357]
. The raw materials for the growth
of Li
2
B
4
O
7
in large quantities are abundant, and the ready availability of isotopi-
cally pure
6
Li,
10
B, and
11
B as the nutrient materials make Li
2
B
4
O
7
growth both
practical and economical.
Before Garret et al.
[346]
, only small crystals or grains of Li
2
B
4
O
7
were
obtained. Although an attempt was made to synthesize Li
2
B
4
O
7
by the hydrother-
mal method, the size of the crystal was too small (
Figure 7.83
). It always had some
divalent metal oxide like MnO or CuO
[350]
. Perhaps this is mainly due to the cor-
rosive nature of the Li
2
O
than
α
B
2
O
3
melt toward the platinum crucible. Hence, Garret
et al.
[346]
used the nickle crucibles. However, the method adopted by these
authors is quite cumbersome because of the complexity of the technique that has a
graphite susceptor and inert atmosphere. The only method advocated up to now by
various authors to grow Li
2
B
4
O
7
crystals is the melt technique, particularly the
Czochralski method
[346,358,359]
. More recently, a group of Chinese researchers
have successfully obtained large crystals (2 in. diameter and 4 in. length) of high
quality Li
2
B
4
O
7
for the first time by the Bridgman technique
[360]
. Almost at the
a
