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mobile making this compound a good Na
1
ion conductor. The discrete polyhedra
of metalayers are mainly responsible for
ionic conductivity, as
in K
3
R
3
(Si
6
O
16
)
2
OH, where R
Gd, Ho
[69]
. Excess rare earth elements lead to the conden-
sation of their polyhedra which gives low conductivity (10
2
4
5
10
2
3
ohm
2
1
cm
2
1
at
300
C).
The discrete nature polyhedra in these compounds are responsible for these spe-
cial properties like photoluminescence.
Figure 7.19
shows the photoluminescence
spectra in NaY[SiO
4
] crystals
[86]
.
7.8 Sodium Zirconium Silicates
Most of the sodium zirconium silicates belong to the NASICON family
[87
89]
.
There are over 20 known sodium zirconium silicates, out of which only six are
minerals. The sodium zirconium silicates find extensive applications in modern
technology, especially after the discovery of unique properties in these compounds
as inorganic adsorbents used in industry and agriculture and for environmental
safety. Among the inorganic adsorbents, zeolites were most popular initially.
Unfortunately, aluminosilicate-based zeolites have one essential drawback, a low
chemical stability in acid and alkaline media, which imposes certain restrictions on
their application. Titanium silicates, such as Na
2
Ti
2
SiO
7
2H
2
O
[90,91]
, and
Na
2
TiSi
2
O
7
2H
2
O
[92]
, are among a large family of known titanium containing
minerals and synthetic compounds, and they exhibit most promising selective ion-
exchange properties suitable for highly alkaline nuclear waste remediation.
Clearfield and coworkers
[91,93,94]
have found that the reason for the unique
behavior of crystalline titanium silicates is connected with the correspondence of
the geometrical parameters of their adsorption sites (channels, cavities) to the size
of the selectively adsorbed Cs
1
or Sr
1
ions. By analogy, the other polyvalent metal
silicates—e.g., zirconium silicates—might be as fruitful in yielding the desired
exchangers as was the case for titanium silicates. Moreover, zirconium silicates
occur widely in nature, and their formation under hydrothermal conditions (in a
wide range of temperatures from 300
C to 550
C) has been given considerable
attention to solve some of the general geophysical and mineralogical problems. It
is reasonable to expect that some of these compounds, due to the presence of ion-
exchange functional groups, as well as the existence in their structure of well-
defined channels and cavities, could exhibit interesting ion-exchange properties,
namely, high selectivity to certain ions and molecules. Unfortunately, the ion-
exchange behavior of such zirconium silicates has not been studied in detail yet.
Na
2
O
H
2
O system has been studied over a wide range of the ini-
tial component ratios, ZrO
2
/GeO
2
5
a
ZrO
2
a
SiO
2
a
3:1
1:9 (mole); NaOH concentration 0
50 wt
600
C, pressure 40
%, temperature 300
98]
.
The phase composition of zirconium silicates formed in Na-containing media is
shown in
Figure 7.20
and the crystallization field is shown in
Figure 7.21
. The
analysis of the products of hydrothermal synthesis and X-ray data allows one to
300 MPa by several workers
[95
