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Crystal growth of hexagonal boron nitride (hBN) from Mg-B-N solvent system under high pressure

Transparent and colorless hexagonal boron nitride (hBN) single crystals were grown from the Mg-B-N system using high-pressure high-temperature cubic anvil technique. By varying the synthesis conditions we could determine the sequence of phase transformations occurring in the Mg-B-N system, construct the pressure-temperature (P-T) phase diagram and discuss the possible growth mechanism. The largest plate-like-shaped hBN crystals with sizes up to 2.5 mm in length and up to 10 (micro)m in thickness were grown at 30 kbar and 1900-2100 °C. The hBN crystals exhibited strong, narrow diffraction peaks typical of well-ordered stacking crystal planes, with the c-axis perpendicular to the crystal face. A characteristic Raman peak observed at 1367 cm-1 with a full width at half maximum of 8 cm-1 corresponds to the E2g vibration mode and indicates the high purity and order of hBN crystals grown by this method. From the practical point of view this work can stimulate further explorations of the Mg-B-N solvent system to obtain isotopically-enriched h10BN crystals, which can act as a key element in solid-state neutron detector devices.