Geoscience Reference
In-Depth Information
Figure 7.79 Dependence of
growth rate on the pH of the
growth media
[321]
.
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
−
0.4
−
0.3
−
0.2
−
0.1 0
pH of the medium
0.1
0.2
0.3
7.16.2 Growth of Mixed Valent Vanadates
The first hydrothermally grown mixed valent vanadate is NH
4
Zr
2
V
3
O
12
—a proton
conductor
[320]
. In comparison with flux and melt techniques, the hydrothermal
growth of vanadates has been carried out at fairly lower temperature
pressure con-
ditions (T
,
300
C and P
,
100 bar).
Although the method is not very popular for vanadates, it has helped in synthe-
sizing several new vanadates with unique structures. Oka and coworkers
[315]
have prepared a series of vanadates under mild hydrothermal conditions.
A series of experiments were conducted by hydrothermal method to obtain
NH
4
Zr
2
V
3
O
12
crystals, the initial experiments with a varying molar ratio of the
nutrient components yielded only a crystalline powder without any experimental
morphology, the authors were able to finally synthesize small needle-like crystals
of NH
4
Zr
2
V
3
O
12
under
250
C, P
200 atm;
NH
4
VO
3
5 gm, and HCOOH (2.0 M) 6 ml
[320]
. The experiments were carried out
for 8 days using Teflon liners in Morey-type autoclaves of capacity 50 ml. Since
crystallization occurred due to spontaneous nucleation, the furnace temperature was
slowly increased to control the nucleation rate (
the following conditions: T
5
5
5
C/h.). The main drawback of
this method is the small size of the crystals (
Figure 7.80
).
Here it is appropriate to discuss the growth of some selected vanadate crystals
like FeVO
4
and MV
3
O
7
(M
B
Cd, Ca, Sr).
Iron vanadate, FeVO
4
, exhibits four polymorphs (FeVO
4
-I, FeVO
4
-II, FeVO
4
-III,
FeVO
4
-IV) belonging to triclinic, orthorhombic, orthorhombic, and monoclinic,
respectively
[338,339]
. In the preparation of FeVO
4
-II, Yao et al.
[340]
used the
aqueous solutions of VOCl
2
and FeCl
3
with V/Fe molar ratios of 0.5
5
1.0, where the
concentration of FeCl
3
was fixed to 0.1 mol/l. The solutions were sealed in Pyrex
ampoules and treated hydrothermally at 280
C for 40 h. Similarly, Oka and cowor-
kers prepared several layered structures of hydrated vanadium oxides, and bronzes,