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Figure 9.22 Magnetite crystals with
distorted morphology (a) in the
presence of CuO; (b) in the absence
of CuO
[102]
.
Demianets et al.
[104]
have obtained high-quality single crystals of hematite under
hydrothermal conditions
(T
420
450
C; P
1200
1600 atm; T
25
50
C;
5
5
5
mineralizer
alkali hydroxide or carbonate solutions) with self-sealed closure-type
autoclaves provided with silver liners. The authors proposed two growth mechanisms
responsible for the growth morphologies in hematite single crystals: layer-by-layer
growth and island growth.
Oxides of rare earth metals receive special attention as solid-state laser host materi-
als because of their excellent thermal properties. However, these rare earth oxides are
difficult to grow as single crystals. Perhaps for the first time, Joseph Kolis and cowor-
kers
[105]
have succeeded in the hydrothermal growth of bulk single crystals of
Lu
2
O
3
doped with Er
3
1
and Yb
3
1
. These bulk single crystals were grown at tempera-
ture 600
5
650
C and pressure 2 Kbar in aqueous solutions of KOH as mineralizer.
Figure 9.23
shows hydrothermally grown Lu
2
O
3
and Er:Lu
2
O
3
single crystals
[105]
.
There are several other reports in the literature on hydrothermal synthesis of
simple oxides like Mn
3
O
4
, PbO, VO
2
, GeO
2
, SnO
2
,V
2
O
3
,R
2
O
3
,In
2
O
3
,Bi
2
O
3
,
CdO, HgO, and so on.
9.5 Hydrothermal Synthesis of Mixed Oxides
Mixed oxides form one of the most important modern materials. The group
includes aluminates, garnets, spinel, metal oxides with complex structures, and so
