Geoscience Reference
In-Depth Information
7.11.3 Zincogermanates
The hydrothermal growth of zincogermanates, although not as popular as that of
zincosilicates, has been studied in detail. The appreciable solubility of sodium zinc
germanates in water solutions of caustic soda at elevated temperatures and pres-
sures allows its preparation in the form of coarse well-formed crystals, the sizes of
which amount to some cubic centimeters.
In the majority of the experiments, the synthesis of sodium zinc germanate
was carried out in the presence of Mn
2
1
ions in the solution with the aim of pre-
paring crystals activated by Mn
2
1
. It is well known that Mn
2
1
is widely used as
an activator for the preparation of various zinc silicate phosphors. Like in zincosi-
licates, zincogermanates in the presence of Mn
2
1
exhibit photo-, electro-, cath-
ode-, X-ray, and thermoluminescence. Among the zincogermanates, only the
Na
2
ZnGeO
4
phase is very popular and well studied. Similarly, Na
2
CoGeO
4
has
been studied in detail owing to its interesting piezoelectric and pyroelectric prop-
erties
[205
208]
.
In contrast to the Na
2
O
H
2
O system, the Cd system not only
does a change in the alkali concentration in solution leading to an exchange of the
crystallization phases, but the phase boundaries are also substantially displaced as
the Cd:Ge ratio alters. For surplus GeO
2
in pure water, only the rutile form of the
oxide crystallizes, together with a cadmium germanate with CdO:GeO
2
5
a
ZnO
a
GeO
2
a
1:2. As
the amount of CdO in the system increases, the amount of free GeO
2
in the crystal-
lization products diminishes. The cadmium germanate CdGe
2
O
5
remains as a
stable solid phase up to a ratio of CdO:GeO
2
5
1:1. Further increasing of the cad-
mium content leads to the formation of an olivine-like phase Cd
2
GeO
4
, and this
remains dominant in the phase diagram of the system
[205]
. The Na
2
O
a
CdO
a
GeO
2
a
H
2
O has yielded eight crystalline phases under hydrothermal conditions
[205,209,210]
. The other germanates of interest are lead germanates, alkali gal-
lium germanates, antimony germanates, neobium germanates, alkali alumino-ger-
manates, barium germanates, bismuth germanates, titano-germanates, and so on
[211
217]
.
Most of these germanates show interesting physical properties. Hence, a system-
atic understanding of germanate phases obtained from a wide variety of germanate
systems will be of great interest in solid-state science.
7.12 Properties of Germanates
Among germanates, rare earth germanates and LISICON [Li
14
Zn(GeO
4
)
4
] have
been studied extensively owing to their unusual physical properties. Most of the
germanates, especially rare earth germanates, have very high melting points, high-
density and refractive indices.
Table 7.21
gives the physicochemical properties of
some rare earth germanates
[44]
. Electrical and magnetic properties have been
studied for oxyorthogermanates, R
2
GeO
5
, and diorthogermanates, R
2
Ge
2
O
7
. These
studies confirm that R
2
GeO
5
and R
2
Ge
2
O
7
belong to a class of dielectrics
[218]
.
