The main features of GaPO hydrothermal crystal growth under conditions of direct and retrograde solubility

Abstract
GaPO₄ recrystallisation was studied under hydrothermal conditions at temperatures from 140 to 310C (autogeneous pressures from 2 to 150 bar). The acidic solvents of pH 4 to 0 at ambient conditions were used as mineralizers. The retrograde solubility of GaPO₄ was observed under 260C (pressure up to 20 bar) and direct - above 280C (pressure above 50 bar). Low values of the initial solvent pH on GaPO₄ solubility at temperatures up to 260C have an influence to increase GaPO₄ solubility and in the same time decrease crystalline quality, for example, rise of OH content is observed. Some solvents with pH near to 4 tested as mineralizers exhibit low solubility of GaPO₄, but increasing value at temperatures above 280C and lead to an attractive crystal growth. So, a possible explication of this phenomenon is a formation of few stable phases from an initial solution which can participate or no in reactions with GaPO₄ at certain T-P conditions.

The study of crystalline quality and physical properties as well piezoelectric of GaPO₄ single crystals have shown a significant superiority of crystals obtained at temperatures above 280C (pressures above 50 bar).