Geoscience Reference
In-Depth Information
7.5.1 Phase Formation in Surplus R
2
O
3
During the synthesis of rare earth silicates with surplus rare earth elements, the
structures of the phases forming and their stability mainly depend upon the charac-
teristics of the rare earth motif, which is closely related to the ionic radii of the rare
earth elements. The ionic radii of rare earth elements vary from 1.061
˚
(La) to
0.848
˚
(Lu). The coordination number also varies from 6 to 8. Similarly, the vari-
ation in the ionic radii of the rare earth elements in the structure is compensated by
the distortion of the 3d rare earth motif, so that the structure type is retained for the
entire rare earth series.
7.5.2 Silicates
In
Figure 7.6
,
the fields of crystallization of various phases in the system
Na
2
O
H
2
O are shown. In solutions with lower concentrations of
NaOH, the representative phase is R
4
2/3 [SiO
4
]
3
O. Moreover, their stability
(
Figure 7.16
) extends toward the lower rare earth elements. The possible reason for
this is the search of their structural elucidation.
a
R
2
O
3
a
SiO
2
a
Figure 7.15 Framework
[Si
2
O
5
] in the structure of K
2
Ce
[Si
6
O
15
]
[73]
.
Figure 7.16 Rare earth
elements
[35]
.
C
NaOH
(wt%)
30
10
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Y
Er Tu Yb Lu
