Monoclinic phase of boron nitride appearing during the hexagonal cubic phase transition at high pressure and high temperature

Fine structures appearing on the phase transition from h (hexagonal) to c (cubic) boron nitride under high pressure (7.7 GPa) and high temperature (1800-2150 degrees C) are examined by high-resolution transmission electron microscopy. A prominent contraction of the interplanar spacing between sp(2) sheets from 3.33 to 3.10 Angstrom in so-called ''compressed h-BN'' is attributable to a monoclinic lattice distortion of the residual h-BN, which originates from the difference in the compressibility as well as the thermal expansion between adjoining h- and c-BN grains. The parameters of the monoclinic unit cell are a(m) = 4.33, b(m) = 2.50, c(m) = 3.1-3.3 Angstrom, and beta = 92-95 degrees. Thin plates of h-BN are often folded and the folding also causes the monoclinic structure. The sheet sequence of I (rhombohedral)-BN locally appears when the strong volume shrinkage occurs due to the formation of a c-BN grain. Nanoscale twins appear in resulting c-BN grains, as long as they are small, and tv(wurzite)-BN is sometimes included in them. (C) 1996 American Institute of Physics.