Geoscience Reference
In-Depth Information
7.11.1 Rare Earth Germanates
There are some known representatives of rare earth germanates: Ln
4.67
[GeO
4
]
3
O,
Ln
4
[GeO
4
]O
4
,Ln
13
[GeO
4
]
6
[O, (OH, F)
2
]
6
,Ln
2
Ge
2
O
7
(two structure types),
Ln
2
GeO
5
(three structure types)
[44]
. The experiments in the system Ln
2
O
3
a
GeO
2
a
H
2
O resulted in obtaining new types of germanates (
Table 7.17
). Two
of them correspond to the silicates structures, six germanates belong to new struc-
ture types. The crystallization of germanates was carried out at rather high AOH
(AF) concentration, but alkali cations took part only in intermediate stages of the
synthesis processes (solution, transport) as mineralizers. In the final composition of
rare earth germanates, the alkali cations were absent. The common features of all
germanates obtained was the prevalence of rare earth component in comparison
with germanium, 1
R
a
4(
Table 7.17
). For most of the new structure types,
the presence of morphotropic rows is characteristic (
Table 7.18
). The compound
Ln
4
Ge
3
O
9
(OH)
6
has been found in the systems with 10 rare earth elements. The
structural type Ln
3
GeO
5
(OH)
3
is characteristic for eight rare earth elements.
Structure formation of most rare earth germanates demands some additional anionic
groups (OH, F), which usually enter into the coordination polyhedra of rare earth
cation. Crystal chemical analysis of new rare earth germanates revealed some inter-
esting features of their structures.
The phase formation in the system M
1
2
O
#
Ln/Ge
#
a
R
2
O
3
a
GeO
2
a
H
2
O (where M
Li,
5
Na, K, Rb, Cs; R
rare earth elements) has been studied extensively. Like in sili-
cate systems, even in the germanate system, the phase formation is determined by
the type of rare earth cation and the ratio of R
2
O
3
/GeO
2
in the nutrient and the con-
centration of the solvent. In the system Na
2
O
5
H
2
O, the following
phases have been identified: La and Pr form Na
2
RGeO
4
(OH); Nd and Sm form
NaR
3
[GeO
4
]
2
(OH)
2
, and the rest of the elements (Eu
a
R
2
O
3
a
GeO
2
a
Lu, Y) form NaRGeO
4
crys-
tallizing in the rhombohedral modification (
Figures 7.44 and 7.45
). For all three
phases, the structure
genetic relation is an olivine-like ribbon, which reflects the
corresponding rare earth element motif, phase transitions: Na
2
RGeO
4
(OH)
!
NaR
3
[GeO
4
]
2
(OH)
2
!
40%) is accompa-
nied by the rearrangement of only cationic motif and expressed in the redistribution
of Na and R in olivine-like ribbon (
Figure 7.46
).
Thus, the structural elucidation of the rare earth silicates and germanates in the
R
2
O
3
-rich region of synthesis shows that the structure determining factors are the
3d arrangement of rare earth polyhedra and the stability of which depends upon the
NaOH solution. For lighter rare earth elements, the 3d arrangement remains
stable even at higher concentration of NaOH. However, the 3d arrangement loosens
at the expense of the enrichment of Na-polyhedra, e.g., structures of NaNdSiO
4
and
Na
2
PrSiO
4
(OH).
For heavier rare earth elements, in which the amphoteric character is sufficiently
strong, the decondensation of the cationic motif occurs from 3d bonding britolite
up to 2d network [R
2
O
8
]
NN
and at
NaRGeO
4
from Lu to La (NaOH
5
20
octahedra in
Na
3
RSi
2
O
7
. The same picture appears even for germanates. However, there is some
variation from silicate systems, which are absent in germanates, wherein the rare
the end discrete polyhedra
