Characterization of boron nitride phase transformations in the Li–B–N system under high pressure and high temperature

Abstract The possible phase transformations of boron nitride in the Li–B–N system have been discussed by the chemical reactions of Li 3 N, hBN and cBN at the conditions of 5.0 GPa and 1300–1500 °C. The results of the reaction between Li 3 N and hBN shows that certain Li–B–N eutectic compound(s) which were produced in the thermodynamical stable region of cBN have no catalytic effect for cBN growth. It indicates that a certain irreversible BN precipitation/dissolution process takes place in Li 3 N melt. However, the reaction between Li 3 N and hBN is preferential when Li 3 N, hBN and cBN coexist in Li–B–N system, and the regrowth of cBN is observed, namely the morphology of cBN changing from irregular to fine shape crystals with well-facetted (1 1 1) surface. Furthermore, the estimated sizes of regrown cBN are almost same as those of the raw cBN. This indicates a simultaneous cBN dissolution and precipitation process in Li 3 N + hBN/cBN system under high pressure and high temperature.