First-principles calculations of electron-energy-loss near-edge structure and near-edge x-ray-absorption fine structure of BN polytypes using model clusters

Abstract

Effects of a core hole in the unoccupied density of states are systematically studied using model clusters of $h\ensuremath{-}\mathrm{BN},$ $c\ensuremath{-}\mathrm{BN},$ and $w\ensuremath{-}\mathrm{BN},$ in order to reproduce and interpret experimental electron-energy-loss near-edge structure /near-edge x-ray absorption fine structure at both B- and N-K edges. Wave functions are found to localize significantly near the core hole, thereby changing their energies as well as spatial distribution. They are very different from the Bloch states assumed in a band-structure calculation on the basis of a structural unit cell. When the presence of the core hole is ignored, in other words at the ground state, small clusters such as $h\ensuremath{-}({\mathrm{B}}_{7}{\mathrm{N}}_{12})$ exhibit better agreement with the experiment as compared with large clusters such as $h\ensuremath{-}({\mathrm{B}}_{91}{\mathrm{N}}_{108})$ because wave functions are made to localize in the small clusters. Features appeared in the experimental spectra are interpreted in terms of chemical bondings among atomic orbitals using overlap population diagrams. Absolute transition energies by Slater's transition state method agree with experimental values within an error of 1%.