GaPO₄ Single Crystals: Growth Condition by Hydrothermal Refluxing Method

Bulk GaPO₄ 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 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 °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 °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.