Geoscience Reference
In-Depth Information
earth germanates in the aqueous solution of KF. K
2
R
4
[Ge
4
O
13
](OH,F)
4
covers
Nd
Lu. When R
2
O
3
/GeO
2
ratio is less than 1:1, the resultant product always carries K
4
Ge
9
O
20
in the form of
hexagonal prism and plates. Surplus R
2
O
3
gives K
2
RF
5
(R
Tb rare earth cations, and the second group covers Er
Lu) and related
phases. The boundaries of the appearance of fluorides depend upon the concentra-
tion of K and the type of rare earth ion. The expansion of the boundaries of K
2
RF
5
and K
2
R[Ge
4
O
13
](OH,F)
4
slowly moves toward the La and KF solution concentra-
tion greater than 45%, and the crystallization of germanates practically ceases. The
crystal size decreases sharply toward the heavier rare earth elements, so that the Tb
representative cannot be obtained as good single crystals of sufficiently large size.
For Er
Tb
5
Lu groups, K
4
R
2
[Ge
8
O
20
](OH,F)
2
type is obtained. The refinement of
K
4
Yb
2
[Ge
8
O
20
](OH,F)
2
structure revealed [Ge
8
O
20
] radical. The rare earth motif is
represented by the infinite chain from rare earth octahedra shared at the vertices.
The scandium-bearing system has been studied in detail. Despite the fact that it
does not cover a wide range of germanates, the commonly observed phases are, for
example, KScGe
2
O
6
,K
2
Sc
2
Ge
2
O
7
(OH)
2
,Sc
2
Ge
2
O
7
,Na
4
Sc
2
Ge
4
O
13
,Na
2
ScGeO
4
(OH),
and NaScGe
2
O
6
in the K
2
O
H
2
O
systems, respectively
[35,44]
. Most of these phases crystallize independently
depending upon the GeO
2
/Sc
2
O
3
ratio and the concentration of the alkaline solu-
tion.
Figure 7.48
shows the NC diagram of the crystallization of various phases in
a
Sc
2
O
3
a
GeO
2
a
H
2
OandNa
2
O
a
Sc
2
O
3
a
GeO
2
a
N
Sc
2
GeO
5
(III)
(I)
KScGe
2
O
6
+ (III) + I
1:2
K
2
Sc
2
[Ge
2
O
7
](OH)
2
1:3
1:4
+K
2
Ge
4
O
9
1:5
10
20
30
40
C
KF
(Sec%)
Figure 7.48 NC diagram of crystallization of various phases in the K
2
O
a
Sc
2
O
3
a
GeO
2
a
H
2
O
system
[35]
.
