DIRECT ELEMENTARY REACTIONS OF BORON AND NITROGEN AT HIGH-PRESSURES AND TEMPERATURES

Highly exothermic direct reactions between elements boron and nitrogen at high pressures and temperatures yield technologically important fo rms of hexagonal and cubic boron nitride (BN). The crystal structures of the reaction products vary with pressure. Below 10 GPa, hexagonal B N is the product; cubic or wurzite BN form at higher pressures. Under nitrogen-rich conditions, another hexagonal allotrope occurs; this see ms to be a highly transparent, low density h'-BN. No direct reactions occur at ambient temperature even at pressures as high as 50 GPa, impl ying that a large activation barrier limits the kinetics of these exot hermic processes. Direct reactions between boron and oxygen are also d iscussed.