Structural evolution in boron nitrides during the hexagonal-cubic phase transition under high pressure at high temperature.

Structural evolution during the phase transition from h (hexagonal)- to c (cubic)-boron nitrides (BN) under high pressure (6.5-7.7 GPa) at high temperature (1,700-2,150 degrees C) was examined by using high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS). At the initial stage of the evolution, some starting h-BN plates were strongly folded, while others were slightly bent. As a result, a strong texture was formed. HRTEM revealed that the interplanar distance between sp2 sheets became slightly shortened and they were slightly sheared to each other during the folding and bending. As a result, m (monoclinic)-BN was formed near the folding plane with lattice parameters; a = 0.433 nm, b = 0.250 nm, c = 0.32-0.33 nm, and beta = 90-92 degrees. In a succeeding stage, the value of beta increased to 92-95 degrees. c-BN grains appeared with nano-scale twins and sometimes partly included wurtzite-type BN. They started to grow with secondary twins at higher temperature. EELS analysis revealed that the band structure of sp2 sheets changed during the transition from h-BN to m-BN; the density of state for the pi* bond became prominently high in m-BN as compared to that in h-BN.