THE STRUCTURAL ASPECT OF HIGH-PRESSURE SUPERHARD PHASE SYNTHESIS

Structural aspects are considered for the direct phase transitions in carbon and boron nitride at high pressures from the viewpoint of marte nsite and diffusion transitions. The mechanism for the transitions of graphite and graphite-type BN into superhardphases is controlled prima rily by the crystalline perfection of the initial structures that show martensite transformations to metastable phases (lonsdaleite and BN w urtzite allotrope), while highly defective ones show diffusion transfo rmation to high-pressure stable phases (diamond and cubic boron nitrid e). The perfection in the initial structure has a very marked effect o n the transformation mechanism during shock compression, which is the main technique in the commercial production of superhard phases.