Synthesis of cubic and hexagonal boron nitrides by using Ni solvent under high pressure

The reaction boundary between boron nitride (BN) and nickel (Ni) metal was studied at high pressures (HP) of 4.5-6 GPa in the high temperature (HT) 1300-1900 degrees C region. It can be seen that Ni dissolves BN followed by the precipitation of BN crystals under HP and HT. Full conversion from hexagonal BN (hBN) to cubic BN (cBN) took place above 6 GPa and 1700 degrees C while cBN started to crystallize from 5.5 GPa and 1500 degrees C. Near the reaction boundary in the cBN formed region, hBN also spontaneously recrystallized. Although crystallized cBN exhibited irregularity in shape, the recrystallized hBN exhibited crystalline facets and intrinsic optical characteristics. It was found that high quality hBN crystals can be synthesized in the Ni-BN system under HP and HT.