Geoscience Reference
In-Depth Information
complex solutions like the mixture of chlorides and fluorides, sodium wolframite
and sodium chloride and sodium nitrate are used
[41,42]
. Among the mixed wolfra-
mites, MM
2
1
(WO
4
)
2
(M
Li, Na, K; M
2
1
5
Cd, Mn, Fe) are the common ones
prepared by the hydrothermal method. The wolframites of trivalent metals form
two large groups: (i) simple trivalent metal wolframites and (ii) mixed trivalent
metal wolframites. Besides, there are several hydrous wolframites, Me
3
1
WO
4
(OH)
[43]
, oxychloride wolframite Me
3
1
WO
4
Cl, and oxywolframites nMe
2
O
3
a
5
mWO
3
.
The ratio of n:m varies in this case between 1:1 and 1:3
[44]
.
Most of these simple or mixed tungstates containing divalent and trivalent
metals exhibit magnetic properties. However, it is the mixed rare earth tungstates
which occupy a unique place in the tungstate family owing to their excellent opti-
cal, electrical, and magnetic properties. Among these high-temperature poly-
morphs of mixed rare earth tungstates, Mr(WO
4
)
2
(M
Li, Na, K, Rb, Cs;
5
R
rare earth elements) crystals have a common crystal structure which is similar
to that of tetragonal scheelite, (CaWO
4
) under atmospheric pressure; however, the
low-temperature polymorphs of these mixed rare earth tungstates belong to the
monoclinic system
[45]
. Klevstov and coworkers
[46
5
50]
have worked out in
detail the synthesis of simple and mixed rare earth tungstates and one of the
important systems is Pr
2
O
3
a
WO
3
a
a
H
2
O, with a concentration of LiCl-
20 wt%.
Figure 8.12
shows the TN-diagram of crystallization (N
LiCl
Pr
2
O
3
/WO
3
).
5
550
C, the field of crystallization of
monocationic tungstates, PrWO
4
OH and Pr
2
W
2
O
9
, occupy relatively lower
values of N
450
C and increase
With a decrease in T
,
.
RWO
4
(OH): hydroxyl wolframates containing almost the entire series
of rare earth elements (Pr
Lu) have been obtained by Klevstov and coworkers
[46]
. The lighter lanthanides crystallize in the aqueous solutions LiCl, as LiR
(WO
4
)
2
, and a similar picture is obtained for KCl, RbCl giving rise to KR(WO
4
)
2
or RbR(WO
4
)
2
.
These mixed alkali rare earth tungstates are most important among tungstates
in general. Although these tungstates were reported during the late 1950s and
1960s, not much has been studied with reference to their crystal growth and phase
relations in the respective systems, and there exists some lacunae in the characteri-
zation of alkali rare earth tungstates. The probable reason for this may be the non-
availability of good quality single crystals due to the high temperature of
synthesis involved. There are several reports on the synthesis and characterization
of alkali rare earth tungstates, but most of them deal with the flux growth and
solid-state reactions
[45,50
52]
. The crystals are fine grained and poor in quality
without a proper morphological development. Similarly, there are quite a few
reports on the hydrothermal growth of alkali rare earth tungstates, but the PT con-
ditions of synthesis were quite high (T
900
C, P
3 kbar)
[40,41]
.
Byrappa and Jain
[53]
have reported the hydrothermal growth of NaLa(WO
4
)
2
crystals under fairly low PT conditions (T
5
700
5
2
240
C, P
100 bar), using a
suitable mineralizer solution. Carobbi and Tancredi
[54]
have studied the system
Ce
2
(WO
4
)
3
a
5
5
80
H
2
Oat25
C and this work has formed some basis for the
work of Byrappa and Jain
[53]
to grow NaLn(WO4)
2
(where Ln
5
La, Ca, Nd)
crystals at moderately lower PT conditions.
Na
2
WO
4
a
