Geoscience Reference
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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,
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