Theoretical Fingerprints of Transition Metal L_(2,3) XANES and ELNES for Lithium Transition Metal Oxides by ab Initio Multiplet Calculations

摘要

With the ab initio multiplet method, transition metal (M) L_(2,3)-edge X-ray absorption near-edge structures (XANES) and electron energy loss near-edge structures (ELNES) can be predicted in detail. In this study, theoretical fingerprints, their orientation dependences, and theoretical branching ratios of M L3-and L2-edges for LiMO2 and Li-extracted MO2 (M = Mn, Fe, Co, Ni) are obtained. The spectra are found to be strongly dependent on the oxidation state and spin state of M ions in all compounds, which proves that they can be unambiguously determined by matching the experimental spectra with the theoretical fingerprints. The variation of the spectra with the crystal structure is small inLiMO2, whereas it is sensibly large in Li-extracted MO2. This can be ascribed to the difference in covalency between M-3d and O-2p as represented by the fraction of the O-2p in the molecular orbitals that are mainly composed of M-3d atomic orbitals in M~(3+)/M~(4+) oxides. The L3/(L3 + L2) branching ratio is also computed systematically. It depends on the atomic numbers, oxidation states, and spin states of M ions but is insensitive to crystal structure. The effects of Jahn-Teller (JT) distortion on the spectra are examined for Mn~(3+) and Fe~(4+) (d~4 high-spin state). The presence and orientation of the distortion can be easily and clearly detected by measuring the orientation dependence of the spectra, although the orientationally averaged spectra do not change much with the distortion.