High-resolution analytical electron microscopy of boron nitrides laser heated at high pressure

High-resolution transmission electron microscopy and electron energy loss spectroscopy have been carried out for cubic and hexagonal boron nitrides (BN) laser heated in argon or nitrogen media at pressures of 5-11 GPa in a diamond anvil cell. In particular, recrystallized products of irradiation from a fluid phase in the form of tiny flakes have been investigated The observations revealed perfect crystallinity (either of cubic or hexagonal BN) in flakes recrystallized from the fluid and traces of melting in the bulk. Multi-shelled circular and polygonal BN nanotubes, which did not contain any additional inclusions, were found after laser heating of cubic and hexagonal BN in nitrogen. The nanotubes typically exhibited 3-10 shells, a characteristic inner dimension in cross-section of 2-6 nm and stoichiometry of B/N similar to 1. They were found to have grown either from a cubic BN matrix or from a mixture of amorphous + turbostratic + hexagonal BN, which had recrystallized on the specimens' surface from the fluid phase.