Hydrothermal Growth of Some Selected Crystals - Hydrothermal Technology - page 216

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