Geoscience Reference
In-Depth Information
7.00 mm
4.00 mm
Lu
2
O
3
Er:Lu
2
O
3
Figure 9.23 Hydrothermally grown (a) Lu
2
O
3
and (b) Er:Lu
2
O
3
single crystals
[105]
.
on. Especially, from 2005 the number of these complex oxides is increasing. Here,
we discuss the hydrothermal synthesis of only some important complex oxides.
9.5.1 Hydrothermal Synthesis of Aluminates
Aluminates show special properties like ceramics, electronic, catalytic, and even
when doped with active rare earth metal show luminescent properties. Several
growth methods like conventional ceramic processing routes, sol
gel, co-precipita-
tion, calcinations, spray pyrolysis, flame synthesis, and alumina impregnating
method are used. But the hydrothermal method provides good-quality crystals with
minimum possible defects. The majority of the aluminates are formed in pure water
at elevated temperatures (
500
C) and pressures (
1000 atm). The hydrothermal
reaction involving divalent metals takes place through MeO
.
.
MeAl
2
O
4
.
Since pure water is used in the synthesis of these oxides, their solubility is very
low, and to obtain 100% output, it is necessary to maintain the ratio of MeO/Al
2
O
3
in the nutrient at 1:1—i.e., to the near stoichiometry. Aluminates crystallize as small
crystals of
Al
2
O
3
!
1
0.1 mm. The aqueous solutions of NaOH are preferable in the synthe-
sis; however, crystallization of aluminates of transitional metals (Fe, Co, and Ni)
takes place with the formation of corresponding oxides. Litvin and Popolitov
[37]
have carried out the synthesis of aluminates of Ca, Be, Zn, Fe, Co, and Ni under
hydrothermal conditions.
For alkaline earth metals (MeO/Al
2
O
3
.
,
2:1), the characteristic feature is the
formation of hydrogarnets of type Me
3
Al
2
O
6
6H
2
O. Crowley
[106]
has carried out
the synthesis of Me
3
Al
2
O
6
6H
2
O in the system CaO
a
Al
2
O
3
a
H
2
O, at tempera-
tures less than 250
C and partial pressure of water 500
800 atm
[107]
. In pure
water, 100% output is achieved at MeO/Al
2
O
3
ratio in the nutrient equal to 3:1.
The stoichiometry of the nutrient may be strongly inclined toward the surplus MeO
while synthesizing in the aqueous solutions of NaOH. Moreover, in the synthesis of
strontium hydrogarnet, it is shown that the monophase synthesis sharply increases
in the highly concentrated solutions as shown in
Figure 9.24
for the system
SrO
H
2
O at 500
C
[108]
. The crystals are either cubic or rhombic
a
Al
2
O
3
a
NaOH
a
dodecahedral
in habit. Synthesis takes place at CaO/Al
2
O
3
.
5:1 within the
