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morphology varies from those obtained using carbonate solutions.
Figure 7.6
shows
the crystallization fields for the system Na
2
O
650
C,
a
R
2
O
3
a
SiO
2
a
H
2
O, T
5
450
P
2500 atm ((i) surplus R
2
O
3
; (ii) surplus SiO
2
)
[35]
. In NaF solution, this
compound crystallizes as fine crystalline mass. It can be concluded that under
hydrothermal experimental conditions, the presence of F, Cl, CO
3
, and SO
4
anions
in the solvents greatly influences the [SiO
4
] anions to form highly complex anions
like [Si
6
O
15
] and [Si
6
O
14
]. It is interesting to note that rare earth elements, includ-
ing the lighter rare earths (La
400
5
Nd), form chain silicates with discrete polyhedra.
Such discrete rare earth polyhedra were first observed among silicates, and, sub-
sequently, in germanates, phosphates, and so on.
[44
46]
Haile et al.
[37,47,48]
studied the hydrothermal synthesis of new alkali silicates,
potassium neodymium phases, and sodium phases, in great detail. The alkali sili-
cates are interesting, both scientifically and technologically, as fast ionic conduc-
tors because of their typical open framework structures and the unique possibilities
for crystal chemical tailoring. Their studies on the systems K
2
O
a
Nd
2
O
3
a
SiO
2
and Na
2
O
a
Nd
2
O
3
a
SiO
2
yielded: K
3
NdSi
3
O
8
(OH)
2
,K
3
NdSi
7
O
17
,K
5
NdSi
9
O
22
,
K
8
NdSi
12
O
32
OH, and
KNd
9
(SiO
4
)
6
O
2
, out of which the first four have no known isomorphs. Similarly, of
NaNdSi
6
O
14
,Na
3
NdSi
6
O
15
OH, K
3
NdSi
6
O
15
,K
3
NdSi
6
O
15
(P
bam
), K
2
Nd
Si
4
O
10
2.5H
2
O, NaNd
9
(SiO
4
)
6
O
2
,Na
3
YSi
6
O
15
,Na
3
YSi
7
O
17
,
Na
7
YSi
12
O
29
,Na
3
YSi
3
O
9
,Na
5
YSi
4
O
12
,Na
6
YSi
13
O
29
(OH)
3
, and NaYSi
6
O
14
, six
are new. The synthesis has been carried out in aqueous solutions of MOH, M
2
CO
3
,
MF, MHF
2
(where M
5
Na, K), K
2
B
4
O
7
as solvent, and either a glass of high silica
content, 4K
2
O(Na
2
O)
17SiO
2
, or a mixture of Nd
2
O
3
, vitreous silica and
K
2
CO
3
(Na
2
CO
3
) as the precursor material. Pressures, temperatures, and solution
molarities utilized ranged from 0.3 to 1.4 kbar, from 350
C to 600
C, and up to
10 M, respectively. The authors found that high-solution molarities, high tempera-
tures, and to some extent, high pressures and long reaction time favored the
Nd
2
O
3
N
1:1
Na
3
NdSi
6
O
15
·2H
2
O
1:4
NaNdSi
6
O
14
Nd
4²/
³
(SiO
4
)
3
O
C
NaOH
10
20
30
C
NaOH
90
80
70
Figure 7.5 Crystallization pattern of Nd silicate
[44]
.
