Geoscience Reference
In-Depth Information
Figure 7.80 Photographs of NH
4
Zr
2
V
3
O
12
crystals obtained under hydrothermal
conditions
[320]
.
as these oxides in hydrated states have a tendency to adopt layered structures accom-
modating foreign cations in the interlayer regions. Layered phases containing alkali-
metal ions, the alkali-metal intercalates A
0.3
V
2
O
5
alkali ion) have been
successfully synthesized under hydrothermal conditions. These vanadates are
usually prepared hydrothermally at 250
C from aqueous VOSO
4
solutions under
autogenous pressure
[340
nH
2
O(A
5
342]
.
7.17 Hydrothermal Synthesis of Borates
Borates have been studied for the past 85 years. Over 85 phases have been estab-
lished with 58 known compounds belonging to over nine different structure types.
The study of borates in general was purely of academic interest and did not carry
any great significance until the late 1970s when mixed alkali borates were first used
in miniature laser applications
[343]
. Subsequently, lithium boracite was reported to
be exhibiting superionic properties
[344]
. However, these applications did not gener-
ate any special interest among materials scientists, but it is the
-barium borate,
which opened a new chapter for borates
[345]
. The recent discovery of piezoelectric
Li
2
B
4
O
7
and nonlinear optical LiB
3
O
5
has attracted a great deal of attention from
materials scientists, and today it is the alkali borates that are being studied exten-
sively worldwide
[346,347]
. The growth of alkali borates is being carried out mostly
by melt and flux techniques. There are some reports about the solid-state reactions,
but the work on borates, in general by hydrothermal technique, was almost
untouched until the past decade. Only a few alkali borates have been obtained by the
hydrothermal technique. The Li
2
O
β
B
2
O
3
system was first studied in 1958 by Sastry
and Hummel
[348]
by the flux method.
a
