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emphasized that from the point of view the size of the coordination number of rare
earth equals 8, this group is more logical than the coordination number 6.
The stability field of tetragonal modification of NaR[SiO
4
] for the lighter lantha-
nides ends at Pr (
Table 7.11
), but the La-representative crystallizes in a different
structure type “A” (orthorhombic I) and its boundary depends upon the concentra-
tion of NaOH. For lighter rare earths, the characteristic feature is the significant
contribution of f-orbit that changes the boundary corresponding to the participation
of f-orbits in the R
O bonding. With an increase in the NaOH concentration, the
coordination number for Pr and Nd decreases from 9 to 8, i.e., the reduction in the
participation of f-orbits in the bonding. During the transition of structure type A
(orthorhombic I) to type B (tetragonal), the basic nature of the rare earth motif, i.e.,
columns from rare earth polyhedra, is retained.
In the heavier lanthanide groups (Er
a
Lu), the change in the NaOH concentra-
tion does not speak on the coordination number, but the degree of association of
rare earth motif changes from discrete nature in the group Na
3
R[Si
2
O
7
]uptoa
highly associated nature in NaR[SiO
4
] (orthorhombic II). For both of the groups,
the characteristic feature is the octahedral surrounding for rare earth elements.
Here, the end members of rare earth series are well inscribed as [YO
6
] and, there-
fore, it is easy to obtain Na
3
Y[Si
2
O
4
] (analogous to Sc-diorthosilicate).
7.5.3 Phase Formation in the Rare Earth Silicate Systems
in the High Silica Region
It is well known that, in the region of surplus [Si
O], the formation of highly con-
densed silicates can be expected. Until now, the synthesis under such conditions
was carried out in pure water and obviously, the higher the amount of [Si
a
O]
taken, the higher its concentration and more complex silicates were obtained.
However, the synthesis did not lead to the differentiation of crystalline individuals
and, as a rule, resulted in the formation of coarse crystalline and fine crystalline
masses. The action of silicon in the aqueous solvents of salts and alkalies remains
unknown. From the works of Dimitrova
[35]
, it was found that in alkaline solutions
and solutions with surplus silica in the system, the heavier phases form.
In order to explain the possibility of obtaining silicates in the form of monocrys-
tals in high silica region, a series of experiments have been carried out by
Dimitrova
[35]
and the results are shown in
Figure 7.6
.
The region in which the crystallization was carried out lies in the stability field
of quartz. Therefore, quartz exists in all the experiments. But this occurs only in
cases when the NaOH concentration is less than 15%. As the concentration of
NaOH increases, quartz disappears and there appears Na silicates and similarly the
heavier phases—an amorphous glassy substance. Under these conditions, the forma-
tion of silicates takes place with the complexity of the anionic motif. For the lighter
rare earths, NaR[Si
6
O
14
] forms and their structural units consist of [Si
3
O
7
]
NN
(O/
Si
a
2.33). With a decrease in the concentration of Si, another structure type having
[Si
6
O
15
] radical, e.g., NaRSi
6
O
15
5
2H
2
O(O/Si
2.5) crystallizes. With an increase
5
