ELNES in Hexagonal and Cubic Boron Nitrides Evaluated with DV-Xα Method

Abstract

The hexagonal boron nitride (h-BN) exhibits a phase transformation to the cubic one (c-BN), the latter of which is well known as the second hardest material so far, under high pressure. This transformation is accompanied not only by the geometrical change in the crystal structure, e.g. stacking sequence of the close-packed planes, but also change in the chemical bonding from sp2 to sp3 characters. Electron energy-loss spectroscopy (EELS) is a hopeful way to characterize the state of bonding around the phase boundaries etc., and will provide rich information on the transformation mechanism. However, the energy-loss near-edge structure (ELNES) is rather sensitive to the orientation of specimens, if the structure is anisotropic as in the case of h-BN. This is sometimes obstructive to evaluations of ELNES, whereas a detailed investigation on this peculiar effect is needed. in the present work, the angular dependence of ELNES in h-BN was extensively studied by both calculations with DV-Xα (Discrete Variational X α cluster) method and experiments, beside investigations of the c-BN.