Geoscience Reference
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10
7
(b) Oxidized
Ea = 0.26
eV
ρ
RT
= 1 ×10
4
ohm
cm
10
6
10
5
(c) Reduced
(a) As prepared
Ea = 1.85
eV
ρ
RT
=1.5 ×10
4
ohm
cm
10
4
10
3
3
4
5
6
7
1
× 10
3
K
°
Figure 8.19 Resistivity data for Cu
4
Mo
5
O
17
[64]
.
Source: Courtesy of the Academic Press, Orlando, FL.
part is its concentration in only a few mineral structures, like ilmenite, sphene,
rutile, and a few others. In the second part, it is an isomorphous substituting cation
in a majority of silicates and other complexes in spite of the heterovalency, large
difference in the ionic radii, and so on. The presence of Ti
4
1
and Zr
4
1
in the sili-
cates usually leads to the formation of mixed framework structures.
A lot of chemical elements can form the octahedral coordination in crystal
structures. Among these compounds, it is possible to enlist a rather numerous group
of oxides of Ti, Ta, Nb, Zr, and of transitional elements (Cu, Mn, Fe, Cr, Al,
amongst others). Among these, the titanates are the most important materials, not
only because of their structural diversity, but also their increased technological
applications, like electrooptic, laser, superionic, and high-temperature superconduc-
tors.. In contrast to silicates and their analogs, the titanates have octahedral coordina-
tion and have layer or framework configurations, but not isolated rings, islands,
chains, and so on.
8.9.1 Crystal Chemistry of Titanates
In titanates, formally in each [TiO
6
] unit, each oxygen atom gets about 4/6
0.67
unit from the central cation. Therefore, it is quite certain that during the crystalliza-
tion of titanates (Ti
4
1
), the oxygen anions should be bonded with additional cations
5
