Geoscience Reference
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ii. For heavier lanthanides:
R
42
=
3
½
SiO
4
3
O
!
NaR
½
SiO
4
!
Na
3
R
½
Si
2
O
7
ð
7
:
10
Þ
0
:
64
0
:
5
0
:
5
S
5
60%) and corresponding NaOH concen-
tration, the lighter rare earths show the following sequence:
For more concentrated solutions, Si (
.
Na
3
R
½
Si
6
O
15
2H
2
O
!
NaR
½
SiO
4
!
NaR
½
SiO
4
ð
OH
Þ
ð
7
:
11
Þ
S
3
0
:
5
0
:
33
5
All these phase transformations show that without depending upon the concen-
tration of Si, the degree of silification of phases decreases with an increase in the
NaOH concentration in the system.
It can be assumed that rare earth elements, having high basicity (La
Nd) in the
process of phase formation, form more associated rare earth motif with a character-
istic 3d bond
[35]
. Only at higher concentrations of Si, and in the presence of car-
bonate ions, less associated chains [R
3
O
12
]
N
as in NaRSi
6
O
14
, or discrete
polyhedra as in Na
3
RSi
6
O
15
2H
2
O, are formed.
7.7 Properties of Rare Earth Silicates
Rare earth silicates exhibit very interesting electrical and optical properties, which
make these compounds unique in several respects. For example, the discovery of
high ionic conduction in rare earth ring silicates has triggered an intensive search
for technically promising superionic conductors
[41,77
79]
. Because the popular
β
-alumina had several disadvantages—like its intrinsically high anisotropy, current
and stress inhomogeneities—which degrade the mechanical properties. In this
respect, the rare earth silicates containing alkali metals offer lower sintering tem-
peratures (1100
C compared to 1600
C for
-alumina), a wide chemical flexibility,
and ionic conductivities possibly surpassing, at 300
C, that of
β
-alumina. Beyeler
et al.
[79]
have grown single crystals of Na
5
YSi
4
O
12
using the hydrothermal
method and studied its directional ionic conductivity in the frequency range
between 1 kHz and 50 MHz.
Figure 7.18
shows the Arrhenius plot of Na
5
YSi
4
O
12
single crystals. Subsequently, several others have reported the high ionic conductiv-
ity in a large variety of rare earth silicates and also proposed the conduction mecha-
nism as there is a close relationship existing between the crystal structure and the
properties
[48,80
β
84]
. Several authors have reported ionic conductivity in various
alkali rare earth silicates
[48,82,85]
.
The ionic conductivity is closely related to the mixed framework structure. The
most important compound among rare earth silicates is Na
5
RSi
4
O
12
, wherein,
[SiO
4
] tetrahedra are linked to form [Si
12
O
36
] rings parallel to the basal plane of
the hexagonal cell. Maksimov et al.
[43]
could locate only 48/90 of the Na
1
atoms
