GaPO4 Single Crystals: Growth Condition by Hydrothermal Refluxing Method

Bulk GaPO4 is an advanced piezoelectric material operating under high temperatures according to the alpha-beta phase transition at 970 degrees C. This work presents the technological development of a hydrothermal refluxing method first applied for GaPO4 single crystal growth. Crystals of 10-20 g were grown in mixtures of aqueous solutions of low- and high-vapor-pressure acids (H3PO4/HCl) at 180-240 degrees C (10-20 bars). The principal feature of the refluxing method is a spatial separation of crystal growth and nutrient dissolution zones. This leads to a constant mass transportation of the dissolved nutrient, even for materials with retrograde solubility. Mass transport is carried out by dissolution of GaPO4 nutrient in a dropping flow of condensed low-vapor-pressure solvent. This method allows an exact saturation at temperature of equilibrium and avoids spontaneous crystallization as well loss of seeds. Grown crystals have a moderate OH- content and reasonable structural uniformity. Moreover, the hydrothermal refluxing method allows a fine defining of GaPO4 concentration in aqueous solutions of H3PO4, H2SO4, HCl and their mixtures at set T-P parameters (T < 250 degrees C, p = 10-30 bars). The proposed method is relatively simple to use, highly reproducible for crystal growth of GaPO4 and probably could applied to other compounds with retrograde solubility.