Geoscience Reference
In-Depth Information
Figure 8.18 Photograph of an assembly of
γ
-Bi
2
MoO
6
single crystals obtained from
NH
4
F solution
[58]
.
like pressure, temperature, and solvent, are the same except for the type and ratio
of the starting components which determine the crystallization of other phases.
In recent years, there have been several new molybdates synthesized under
hydrothermal conditions exhibiting good ionic conductivity and some zeolitic struc-
tures. Among them, the copper molybdates are very interesting superionic conductors.
The Cu
1
6
Mo
6
5
O
18
and Cu
1
4
Mo
6
5
O
17
have been synthesized by the hydrothermal
technique using stoichiometric quantities of cuprous oxide and molybdenum trioxide
taken in a gold capsule which is filled with a known quantity of distilled water.
The capsule was sealed and held at 3 kb pressure and 500
C temperature for 12 h, fol-
lowed by a slow cooling (10
C/h) to room temperature. Under the microscope, very
thin crystals appeared and were reddish-brown in transmitted light
[63,64]
.
Figure 8.19
shows the resistivity data for Cu
4
Mo
5
O
17
crystals. The resistivity measurements were
made on single crystals using indium contacts soldered on the crystals in a nitrogen
atmosphere to avoid oxidation.
Mecarron III and Calabrese
[136]
reported many reduced molybdenum phos-
phates with octahedral
tetrahedral framework. Following this, there has also been
much interest in molybdates and molybdenum oxides due to their catalytic activity
and their capability of forming crystal structures which allow rich intercalation
chemistry using secondary and tertiary amines
[65
67]
. These compounds yield
microporous structures. The hydrothermal method helps to synthesize molybdates
in an acidic medium by using various templates as structure-directing units. The
structures of (Mo
x
O
y
)
n
2
can be “tailored” to a certain degree by altering the size,
conformation, and properties of templates and the acidity of the reaction system.
This results in a series of chain and layer-type polymeric molybdenum oxides with
intercalating cations. Thus, these new molybdates are the most perspective materi-
als which can only be obtained by the hydrothermal technique.
8.9 Hydrothermal Synthesis of Titanates
Titanium is one of the most abundant elements in the earth's crust and takes the
ninth place in the order of abundance. It can be considered in two parts. The first
