Semispiral structure of turbostratic boron nitride formed under high pressure and high temperature

Fine structures appearing in a hexagonal boron nitride (h-BN) under high pressure (7.7 GPa) and high temperature (1800 degrees C) are examined by high-resolution transmission electron microscopy. At the edge of plate-like h-BN crystal sp(2) sheets are strongly bent to form a semispiral structure of t (turbostratic)-BN with an average interplaner spacing of 3.5 Angstrom; edges of h-BN plates are covered by the curved sp(2) sheets so that dangling bonds are minimized. This occurs when the edges of a group of h-BN plates comes up to the planes of another group. The similar structure occurs also inside the crystal near the plate edge. The origin of the semispiral t-BN is discussed from the viewpoint of chemical bonding.