Theoretical site- and symmetry-resolved density of states and experimental EELS near-edge spectra of ${\mathrm{AlB}}_2$ and ${\mathrm{TiB}}_2$

We report the orientation dependent B K-edge electron-loss near-edge spectroscopy spectra of the isostructural compounds ${\mathrm{AlB}}_2$ and ${\mathrm{TiB}}_2.$ The observed differences between the experimental spectra of the borides are discussed in terms of the variation in bonding between the two materials. The orientation dependence is rationalized in terms of the anisotropic nature of the common crystal structure. For a detailed comparison with experiment, we have employed a first-principles method for the calculation of the partial density of states. This method is based on band calculations using the full-potential linearized augmented plane-wave method within density-functional theory. We ascribe the origin of the discrepancies between the broadened ground-state density of states and experiment to the presence of a core hole in the final state. The core-hole effect is self-consistently accounted for via the use of ab initio supercell calculations, leading to improved agreement with experiment particular in certain crystallographic directions, where screening of the core hole is expected to be much reduced. Additionally a detailed analysis of the valence-band structure, charge-transfer effects, and covalency in the diborides has been undertaken.