Abstract
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.
Highlights
The development of modern radio communication devices and different kinds of electronic measurement technologies puts increasing demands on the requirements of piezoelectric inventory materials
The procedure of the crystal reproducing of crystal growth at different temperatures
The GaPO4 crystals grown by the hydrothermal refluxing method at temperatures close to 240 ◦ C
Summary
The development of modern radio communication devices and different kinds of electronic measurement technologies puts increasing demands on the requirements of piezoelectric inventory materials. This means the values of the electromechanical constants and its temperature dependencies. The most widely used quartz crystals have an α–β phase transformation at 573 ◦ C [1]. There have been many studies investigating new alpha-quartz-like structure piezoelectric materials with homogeneous compositions, low dielectric losses even at high temperatures and significant piezoelectric properties in a wide temperature range exceeding 350 ◦ C [3]. The best piezoelectric coefficients of α-quartz-like structure materials have been found for α-GeO2
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