HYDROTHERMAL GROWTH AND TWINNING OF GALLIUM PHOSPHATE CRYSTALS. O.V.Zvereva and L.N.Demianets, Institute of Crystallography, RAN, Moscow, Russia

Modified hydrothermal method was developed for gallium orthophosphate single crystal growing. The concentrated mixture of phosphonc acids with the boiling temperature of 225deg.C was used as a growth media. Crystals were grown in the field of positive temperature coefficient of solubility (T>300deg.C) by the direct temperature gradient technique under the pressure of inert gas pumped into an autoclave to avoid the solvent boiling. The main features of the solubility and growth kinetics are presented.

Twinntag laws and twin types were studied for gallium phosphate grown by modified hydrothermal method. The twins were found to be formed in accordance with Brazil, Douphine and Leydolt's laws; such twins are characteristic for 32 class symmetry.

Brazil twins are typical for GaPO₄ single crystals. Douphine and Leydolt's twins occur very rear. The main specific feature of Brazil twins is an appearance of two planes of twinning right and left individuals; one plane [[pi]](10.2), similar to quartz, berlinite and GaPO₄ crystals grown hydrothermaly at T<300deg.C. The second plane is the other rhombohedron [[pi]]'(01.2). Earlier such twinning was not found in crystals grown at low temperature.

The quantity of twins depends very much on crystallographic orientation of initial seeds. To eliminate the twin quantity, the specific orientation, shape and size of seeds were found on the base of crystallographic and kinetic data for gallium phosphate single crystal growth.