Effects of crystal structure on Co-L-2,L-3 x-ray absorption near-edge structure and electron-energy-loss near-edge structure of trivalent cobalt oxides

摘要

Co-L2, 3 x-ray-absorption near-edge structures (XANES) and electron-energy-loss near-edge structures (ELNES) are calculated from first principles, taking into account the configuration interactions among molecular orbitals for model clusters obtained using fully-relativistic one-electron calculations. The spectra for LaCoO3 at low temperature and for LiCoO2, both of which have low-spin Co3+ ions in similar local environments, are well reproduced. To elucidate the cause of the spectral shape, the effects of the crystal structure and coordination distance are investigated using idealized crystal structures of layered rock salt, perovskite, rock salt, and spinel. The calculated spectra for these crystals with the same coordination distance are found to show clear differences despite identical 3d electronic configurations and local environments of the first ligand shell. The crystal structure dependence can be explained by the difference in the Madelung potential acting on the 3d orbitals. The results of the calculations for various coordination distances indicate that not only the crystal field splitting parameter, namely, 10Dq, but also the coordination distance is a key factor in determining the spectral shape. This is attributed to the change in the spatial distribution of the 3d orbitals.