  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 {\deg}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.
