GaPO(4) Single Crystals: Growth Condition by Hydrothermal Refluxing Method

Bulk GaPO(4) is an advanced piezoelectric material operating under high temperatures according to the α-β phase transition at 970 °C. This work presents the technological development of a hydrothermal refluxing method first applied for GaPO(4) single crystal growth. Crystals of 10–20 g were grown in mixtures of aqueous solutions of low- and high-vapor-pressure acids (H(3)PO(4)/HCl) at 180–240 °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 GaPO(4) 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 GaPO(4) concentration in aqueous solutions of H(3)PO(4), H(2)SO(4), HCl and their mixtures at set T–P parameters (T < 250 °C, p = 10–30 bars). The proposed method is relatively simple to use, highly reproducible for crystal growth of GaPO(4) and probably could applied to other compounds with retrograde solubility.