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obtained. This compound belongs to the structure-type olivine. At lower pressures
and temperatures (P 5
650 C) in the NaOH rich region
3
20 atm and T 5
558
(
Eu, phases belong to the tetragonal modification and
the crystals are plate like, thick, 1
80 wt%) for groups Nd
.
2 mm in size, and twinning is common.
Na 2 RGeO 4 (OH) (where R
La
Nd) crystals form at GeO 2 /R 2 O 3 5
1:2 and
5
concentration of NaOH
6 wt%. Crystals have equidimensional habit. Output of
100% was obtained in solutions when H 2 O
.
10%. Na 2 R[GeO 4 ](OH) belongs to
,
the olivine-type structures. Thus, for La
Gd group of germanates of olivine-type
structures in NaOH solution, the following scheme with an increase in the concen-
tration of NaOH has been proposed:
NaR 3 ½
GeO 4 2 ð
OH
Þ 2 !
NaR
½
GeO 4 !
Na 2 R
½
GeO 4 ð
OH
Þ
ð
7
:
13
Þ
In spite of the rearrangement of Na 1 and rare earth cations, the common olivine-
type motif is retained over a wide interval not only on the concentration of NaOH
but also on the ratio of GeO 2 /R 2 O 3 . When GeO 2 /R 2 O 3 .
90 wt%, NaOH
solutions' complex germanates like Na 2 R 6 Ge 8 O 26 appear for Nd and Pr representa-
tives. However, the output is less than 10% and crystals are small (
3:1 in 20
,
1 mm).
Lu are used, a slightly different picture is seen. NaR 2 Ge 4 O 10 (OH),
NaRGeO 4 , and Na 4 R 2 Ge 4 O 13 are the characteristic phases. NaR 2 Ge 4 O 10 (OH) phase
crystallizes in less than 10 wt% NaOH with GeO 2 /R 2 O 3 5
When Dy
1:3.
NaRGeO 4 -orthogermanates crystallize in the R 2 O 3 rich region in the system
Na 2 O
1:2.5
H 2 O.
In the GeO 2 -rich region, for Dy
a
R 2 O 3 a
GeO 2 a
Lu groups, Na 4 R 2 Ge 4 O 13 is the characteristic
phase with 100% output in 30
60 wt% NaOH solutions at ratio GeO 2 /R 2 O 3 5
2.5:1
3:1. The crystals are up to 2 mm in size.
Similarly, the rare earth germanates have been synthesized using aqueous solu-
tions of KOH and KF [44] . Unfortunately, output of 100% has not been obtained
for several germanates. Quite often two or three phases crystallize simultaneously.
All the KR-germanates have been grouped into two structure types: K 2 R 4 [Ge 4 O 13 ]
(OH,F) 4 and K 4 R 2 [Ge 4 O 13 ](OH,F) 4 . Figure 7.47 shows the phase formation of rare
N (GeO 2 /R 2 O 3 )
K 4 Ge 9 O 20
1:1
K 2 R 4 [Ge 4 O 13 ](OH, F) 4
K 4 R 2 [Ge 8 O 20 ](OH, F) 2
1:2
K 2 RF 5
Pr
Sm
Gd
Dy
Er
Yb
La
Nd
Eu
Tb
Ho
Tu
Lu
Figure 7.47 Phase formation of rare earth germanates in the aqueous solution of KF [44] .
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