Geoscience Reference
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are closely related to tungstates in their formation, crystal chemistry, and also physi-
cal properties. Several molybdates have been synthesized artificially by various
techniques, like melt, solid state, and flux. Their synthesis is closely related to the
synthesis of tungstates and hence, several isostructural compounds between tung-
states have been obtained. These molybdates show interesting optical, electrical, and
magnetic properties. Only a small number of molybdates have been obtained under
hydrothermal conditions and that too during 1960s and 1980s, especially from the
erstwhile Soviet Union. However, during 1990s, an extensive study on the hydro-
thermal/solvothermal chemistry and the structural chemistry of a large variety of
molybdates was initiated. Klevstov and coworkers
[46
50][44,46]
have worked out
the growth technology for molybdates along with tungstates. Crystals of CdMoO
4
have been obtained in 5
15 wt% NH
4
Cl, the nutrient materials are CdO and MoO
3
in stoichiometric ratio
[55]
. The crystallization was carried out at 350
500
C,
25
C, in titanium-lined autoclaves.
The hydrothermal synthesis of molybdates is usually carried out at a high
temperature,
Δ
T
10
5
500
C. The most well-studied
greater
than
system is
R
2
O
3
a
H
2
O
[56]
.
Figure 8.17
shows the TN-diagram of
crystallization in the system R
2
O
3
a
MoO
3
a
K2MoO4
a
H
2
O. Four major phases crystal-
lize in this system: RMoO
4
(OH), R
2
MoO
6
,KR(MoO
4
)
2
,andK
5
R(MoO
4
)
4
.
Hydroxyl molybdates, RMoO
4
(OH). This has been obtained for La
K
2
MoO
4
a
Eu at tem-
550
C and pressures of 1200
peratures of 450
1400 atm
[56]
. Rare earth oxides
and molybdenum in the mol ratios 1:2, 1:3, 1:4, and 1:5 were taken as the nutrient.
Good quality crystals have been obtained in aqueous solutions of LiCl and KCl
(20 wt%) with an addition of NH
4
Cl (5 wt%). With a rise in the temperature, the
crystal output decreases up to 50% at 525
C, and the tabular and needle-like crys-
tals of polyoxides of molybdenum increase.
Oxymolybdates, (R
2
MoO
6
). These are obtained in the ratio R
2
O
3
/MoO
3
5
1:1, and
oxymolybdates with rare earth elements from Pr to Er have been obtained under
hydrothermal conditions
[57]
. The synthesis is carried out at 500
600
Cinaqueous
solutions of K
2
MoO
4
with concentrations of less than 35 wt% and the starting R
2
O
3
/
MoO
3
ratios
1:1 to 1:2. The oxymolybdates under these conditions crystallize in the
low-temperature monoclinic modification isostructural to Nd
2
WO
6
[57]
.Kodamaand
Izumi
[58]
have obtained Bi
2
WO
6
crystals in pure water at T
5
500
Cand
300
5
P
800 atm. The nutrients Bi
2
O
3
and MoO
3
in the ratio 1:1 have been used.
Similarly, these authors have obtained Bi
2
MoO
6
in Li, Na, K, NH
4
(T
500
5
400
C,
200
5
300
C, P
P
175
400 atm) chlorides, in KF solution (T
100
50
300 atm) and
5
5
5
400
C, P
NaF solution (T
200
100
420 atm). Several compounds exist in the
5
5
Bi
2
O
3
a
MoO
3
system because they are efficient catalysts (for instance, for oxidation
of unsaturated hydrocarbons). Bi
2
MoO
6
is one of the compounds in the system and is
found in natural deposits as the mineral koechlinite, which is one of the polymorphs
of Bi
2
MoO
6
. This compound has three polymorphic forms. The
γ
-form transforms
quickly and irreversibly into the high-temperature form when it is heated above
600
C. Therefore, the hydrothermal method is very useful for preparing the
-form.
Under hydrothermal conditions, the presence of water vapor enabled reactions
between Bi
2
O
3
and MoO
3
to occur at much lower temperatures than those required
γ
