Geoscience Reference
In-Depth Information
25%), there
occurs the oxidation of the tellurium with the formation of crystals of telluric acid:
With further rise in the concentration of aqueous nitric acid solution (
.
Te
6HNO
3
!
H
2
TeO
4
ð
solid
Þ
1
6NO
2
1
2H
2
O
ð
9
:
8
Þ
1
The morphology of paratellurite crystals is very interesting. The crystals grown
in hydrochloric acid solutions exhibit more complex forms than the crystals obtained
in nitric acid solutions. The paratellurite monocrystals grow in the form of a tetrago-
nal prism (110) and tetragonal dipyramids (101) and (102). The growth forms of
tellurium oxychloride monocrystals are pinacoids (001) and (010), rhombic prisms
(110) and (130), and rhombic dipyramids (111), (112), (113), and (114).
Figure 9.19
shows the habits of (a) tellurium dioxide and (b) tellurium oxychloride crystals.
Popolitov and coworkers
[95]
have studied, in detail, the kinetics of crystalliza-
tion of paratellurite under hydrothermal conditions with reference to the mass trans-
port, temperature gradient, solution concentration, growth rate, and so on
[94]
.On
the basis of the kinetic study of growth conditions for
-TeO
2
crystals, it was found
that the growth rate in the homogeneous solution is limited by the diffusion mass
transport of the dissolved substance toward the growing faces, while in the binary
phase, the surface, processes are limited. A method for the optimal growth of the
uniform
α
-TeO
2
monocrystals was developed wherein the crystals are grown on
seeds and in 3.65 wt% water solution of HCl at 150
α
200
C.
Acoustic characteristics of
α
-TeO
2
monocrystals obtained by the hydrothermal
method are similar to those of Czochralski grown crystals. A reader can find more
information in Ref.
[96]
.
Iron Oxides
Iron oxides and oxyhydroxides attract researchers owing to their functional prop-
erties like magnetic and catalytic properties, pigments, coatings, and gas sensors.
Figure 9.19 Habits of (a) tellurium
dioxides and (b) tellurium oxychloride
crystals
[94]
.
x
(a)
(b)
102
101
z
110
001
114
113
112
111
110
130
010
y
