Geoscience Reference
In-Depth Information
growth method is from flux, or melt, or gas transport, whereas for more compli-
cated fluorine-bearing compounds, hydrothermal synthesis is the most convenient
method. The hydrothermal method helps in developing new crystal structures
which are difficult to obtain by other conventional methods. Among fluorides,
those containing rare earth elements, particularly the active elements like Nd
3
1
,
and Eu
3
1
are the most popular because of their excellent luminescent properties.
The fluorides bearing Fe, Mn, and other transitional metals show good magnetic
properties; however, it is the rare earth elements bearing fluorides that carry greater
importance as laser host materials. Kaminskii et al.
[1]
have reviewed in detail new
CW-laser (Continuous Wave-laser) crystals with semiconductor laser pumping on
the anisotropic fluorides with Nd
3
1
ions obtained by the hydrothermal method.
Similarly, Demianets
[2]
reviewed the hydrothermal growth of alkali, rare earth
fluorides and alkali zirconium fluorides and discussed their luminescent and ionic
conductivity properties. Bridenbangh et al.
[3]
discussed the hydrothermal growth
of alkali fluoride crystals and studied the phase equilibrium of the KF
a
GdF
2
a
H
2
O
system in detail under hydrothermal conditions.
8.2.1 Hydrothermal Synthesis of Rare Earth Fluorides
Rare earth fluorides have attracted materials scientists for more than 40 years, owing
to their unusual crystal structures and physical properties, but even today the pro-
blems concerning the polymorphism of double fluorides and the nature of morpho-
tropic transitions in these compounds have not yet been understood. The physical
properties of compounds formed in the AF
LnF
3
systems (A is the single-charged
cation) have not been studied, which may be explained by the absence of sufficiently
simple methods of synthesis of double fluorides of alkali and rare earth elements
single crystals. The use of hydrothermal solutions provides the study of the behavior
of the whole series of rare earth elements in the presence of F-ions under the same
conditions. Under hydrothermal conditions, it is easy to synthesize fluorides of com-
positions ALnF
4
,A
3
LnF
6
,A
2
LnF
5
(A
a
Li, Na, K, Rb, Cs), amongst others. These
compounds form the morphotropic series (K
2
LnF
5
) embracing most of the rare earth
elements except La, Ce, Pr, and Sc. Rare earth fluorides are being studied exten-
sively owing to their unusual spectroscopic characteristics, but these crystals are
grown as small nanosize crystals rather than the bulk single crystals.
The hydrothermal phase equilibria of the Ln
2
O
3
a
5
H
2
O system has been
studied in detail
[2,3]
. The stability fields of several compounds, such as KLnF
4
,
K
2
LnF
5
,K
3
LnF
6
,K
2
LnF
5
, and KLn
2
F
7
, have been fixed.
Figure 8.1
shows the
composition diagram of the system KF
KF
a
H
2
O at 450
C and 10 kpsi. The
region A in
Figure 8.1
is close to the limit of KGdF
4
solubility, relative to KF con-
centration, as estimated from the solubility measurements. The single-phase field
for KGd
2
F
7
should not be considered to extend to the H
2
O
a
GdF
3
a
a
GdF
3
join. The only
information on the right side of this field was obtained at very high H
2
O contents
(98 mol%); this was deemed insufficient to allow projection of a phase boundary;
however, the single-phase stability of KGd
2
F
7
likely does not persist much farther
to the right than the individual single-phase results shown. An additional two-phase
