Soft XAFS study on the 4d electronic structure of ruthenium in complex perovskite oxide

The bond covalency effect on the electronic structure of the perovskite oxides, La₂MRuO₆ (M=Zn, Mg, and Li) and Ba₂YRuO₆, has been investigated by the complementary methods of X-ray absorption spectroscopy and X-ray diffraction. Ru K-edge and Ru L-(III)edge XANES analyses clarify that the ruthenium ions with oxidation states of IV (d⁴) and V (d³) are stabilized in a nearly regular RuO₆ octahedron. Comparison of L-edge XANES spectra for the compounds with isovalent ruthenium ion has shown that the t(2g) level is mainly influenced by the A site cation, whereas the e(g) level is mainly affected by the neighboring B site cation, which can be understood on the basis of the bond competition scheme. The present Ru K-edge EXAFS analysis results reveal that the first neighbor coordination (Ru-O shell) is nearly isotropic even in monoclinic perovskites, which is consistent with the XRD and XANES results. The experimental EXAFS spectra in the range R≤~ 4.5 Å are well reproduced by ab initio calculation based on crystallographic data, which supports the long-range structure presented by Rietveld refinement. The EXAFS curve-fitting analyses allow us to distinguish the differences in (Ru-O) bond lengths for isovalent ruthenates, which is supported by the XANES spectral features and the chemical interpretation of bond covalency. (C) 2000 Elsevier Science Ltd.