First-principles calculations of ELNES and XANES of selected wide-gap materials: Dependence on crystal structure and orientation

Theoretical calculations of electron energy-loss near-edge structure (ELNES) and x-ray absorption near-edge structure (XANES) of selected wide-gap materials including TiO₂, AlN, GaN, InN, ZnO, and their polymorphs are performed using a first-principles method. Calculations of 39 K and L ₃(L_2,3) edges are made using large supercells containing 72 to 128 atoms. A core hole is included in the final state, and the matrix elements of the electric dipole transition between the ground state and the final state are computed. Structures of some metastable crystals are optimized by a plane-wave basis pseudopotential method. Spectral differences in ELNES and XANES among polymorphs are quantitatively reproduced in this way. The origin of the spectral differences is pursued from the viewpoint of chemical bondings. Crystallographic orientation dependence of ELNES and XANES is also examined both by experiment and theory. The dependence is found to be much larger in K edges than that in L₃(L_2,3) edges.