Towards the application of ab-initio ELNES/XANES spectral calculation to grain boundaries and interfaces.

Recently, an efficient computational method for calculating the ELNES/XANES spectra of ceramic crystals has been developed [1]. The method is based on a supercell calculation of the electronic structure using the orthogonalized linear combination of atomic orbitals (OLCAO) method. Separate calculations of the initial state of with core electrons and the final state with an excited core-electron in the empty conduction bands are necessary. The spectral calculation includes the proper inclusion of the dipole matrix. Application of this method to Si-K, Si-L-2.3, Al-K, Al-L-2.3, Mg-K, Mg-L-2.3, O-K, and N-K edges in Si3N4, SiO2, Si2N2O, Al2O3, MgO, MgAl2O4, YAG and other crystals all yield superior agreement with the measured spectra. It is also shown that the traditional approach of using the local density of states from the ground state band structure calculation to interpret the ELNES/XANES spectra is grossly inadequate. Clearly, the exited states in a solid are a very complicated subject. The associated spectra cannot be simply interpreted in terms of the coordination and the species of the nearest neighbor atoms. A full ab-initio quantum mechanical treatment with the inclusion of the electron-hole interaction is only a first step towards full understanding. The prospect of applying this method to structurally complicated systems such as grain boundaries and interfaces, and for materials characterization in general is discussed.