Geoscience Reference
In-Depth Information
berlinite crystals and the same experimental setup can be used for the growth of
gallium orthophosphate crystals. The growth of gallium orthophosphate can be
carried out at 180
C for 7
10 days. By this arrangement, it was expected that the
dissolved nutrient would be efficiently used to grow single crystals on the seed with-
out the occurrence of spontaneous nucleation. As the lattice parameters of both berli-
nite and gallium phosphate are very close, some authors have used even berlinite
crystals with a definite orientation/cut as seeds and obtained GaPO
4
epitaxy on
AlPO
4
seeds using 6 M H
2
SO
4
solvent, at a temperature greater than 200
C
[126]
.
The experimental temperature is critical in the case of gallium phosphate because
the crystals obtained at a temperature less than 200
C show the presence of water,
which deteriorates the piezoelectric characteristics. Hence, it is advisable to use acid
mineralizers with a boiling temperature of 225
240
C. Practically, there is no water
in such a solvent, and it consists of orthophosphoric acid (90%) and pyro-phosphoric
acid (10%). The solubility is presaturated with GaPO
4
to avoid the dissolution of the
seeds. To keep the solution from boiling during the crystal growth, inert gas is
pumped into an autoclave up to 50 bar
[131]
.
Cochez et al. (1994)
[126]
have proposed the following scheme for the growth
of
α
-GaPO
4
as the best method:
Thin exitaxy of GaPO
4
on berlinte seeds by
VTG in 6M H
2
SO
4
,
T
c
> 200°C
Flat crystals
Crystal growth by SHT and composite gradient
Methods in 15M H
3
PO
4
, 150 <
T
c
<180
°
C
Cut in X seeds
Crystal growth of X GaPO
4
seeds by VTG
in 6M H
2
SO
4
,
T
c
< 200°C
Crystals with low (OH) content
The first stage of GaPO
4
crystal growth process is the epitaxy of GaPO
4
on
berlinite seeds. The second stage consists of growing two-dimensional crystals
through the slow heating method, or the horizontal gradient method, or the compos-
ite method (i.e., the combination of the first two). In the third stage, the X seeds are
grown which helps in reducing the (OH)
2
content in the crystals.
Popolitov and Yaroslavskii
(1990)
[129]
have studied the crystallization
processes of
-GaPO
4
under hydrothermal conditions. They have studied the rate
of crystallization, (i.e., the amount of GaPO
4
crystals formed per unit time), action
of solvent, its concentration, and the temperature regime. Gallium phosphate glass
α