Cu K-edge x-ray-absorption spectroscopic study on the octahedrally coordinated trivalent copper in the perovskite-related compounds La2Li0.5Cu0.5O4 and LaCuO3

Cu K-edge x-ray-absorption spectra for the chemically well-defined CuIII-containing oxides, La2Li0.5Cu0.5O4 and LaCuO3, have been carefully studied in order to correlate the complex spectral features with the copper valence, the local symmetry, and the bonding nature. According to the extended x-ray-absorption fine structure spectra, it was found that the (Cu-O) bond distances for both compounds agree well with the x-ray crystallographic data, but in x-ray absorption near-edge structure (XANES) spectra, a clear higher-energy shift of the Cu K-edge position was observed for both trivalent copper oxides compared to divalent ones of Nd2CuO4 and La2CuO4. When the site symmetry of trivalent copper is lowered from Oh to D4h as in La2Li0.5Cu0.5O4, the feature by the shakedown process was observed to be split to A and A', corresponding to transitions to the 1s13dn+1L-14pπ1 final state and -1s13dn+1L-14pσ1 one, respectively. Therefore, it is proposed that Cu K-edge XANES spectral fine features below the main edge should be interpreted with the molecular-orbital concept, mainly depending upon the copper valence and the crystal-field effect by the local symmetry on the copper site. This work also reveals that a spectral separation (∼2.7 eV) between CuII and CuIII compounds is smaller than the theoretically expected one (∼4 eV), and the XANES spectrum of metallic compound shows a tendency of a lower energy shift compared to that of insulating or semiconducting one. Thus these facts should be taken into account when determining the presence of trivalent copper from Cu K-edge XANES spectra of copper-based superconductors.