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

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Cu K-edge x-ray-absorption spectra for the chemically well-defined ${\mathrm{Cu}}^{\mathrm{III}}$-containing oxides, ${\mathrm{La}}_{2}$${\mathrm{Li}}_{0.5}$${\mathrm{Cu}}_{0.5}$${\mathrm{O}}_{4}$ and ${\mathrm{LaCuO}}_{3}$, have been carefully studied in order to correlate the complex spectral features with the copper valence, the local symmetry, and the bonding nature. According to the extended x-ray-absorption fine structure spectra, it was found that the (Cu-O) bond distances for both compounds agree well with the x-ray crystallographic data, but in x-ray absorption near-edge structure (XANES) spectra, a clear higher-energy shift of the Cu K-edge position was observed for both trivalent copper oxides compared to divalent ones of ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{4}$ and ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$. When the site symmetry of trivalent copper is lowered from ${\mathit{O}}_{\mathit{h}}$ to ${\mathit{D}}_{4\mathit{h}}$ as in ${\mathrm{La}}_{2}$${\mathrm{Li}}_{0.5}$${\mathrm{Cu}}_{0.5}$${\mathrm{O}}_{4}$, the feature by the shakedown process was observed to be split to A and A', corresponding to transitions to the \ensuremath{\Vert}1${\mathit{s}}^{1}$3${\mathit{d}}^{\mathit{n}+1}$${\mathit{L}}^{\mathrm{\ensuremath{-}}1}$4${\mathit{p}}_{\mathrm{\ensuremath{\pi}}}^{1}$〉 final state and \ensuremath{\Vert}1${\mathit{s}}^{1}$3${\mathit{d}}^{\mathit{n}+1}$${\mathit{L}}^{\mathrm{\ensuremath{-}}1}$4${\mathit{p}}_{\mathrm{\ensuremath{\sigma}}}^{1}$〉 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 (\ensuremath{\sim}2.7 eV) between ${\mathrm{Cu}}^{\mathrm{II}}$ and ${\mathrm{Cu}}^{\mathrm{III}}$ compounds is smaller than the theoretically expected one (\ensuremath{\sim}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.