Detailed analyses of Cu K-edge x-ray-absorption near-edge structure (XANES) of electron-doped and hole-doped Cu-based oxides with several Ce/Sr substitution rates, ${\mathrm{Nd}}_{2\mathrm{-}\mathrm{x}}$${\mathrm{Ce}}_{\mathrm{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{y}}$ and ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_4$ have revealed that each Ce/Sr substitution has an effect on ∼4–5 Cu sites and that an additional electron or hole supplied with the substitution is distributed mainly to ∼4–5 [${\mathrm{CuO}}_2$] units, yielding ‘‘impurity (localized) states.’’ When x=0.15 the ‘‘impurity states’’ amount to ∼60–75 % of the whole [${\mathrm{CuO}}_2$] units; the interaction and superposition among the localized states are caused to produce bands like Fermi-liquid states. The substitution causes substantial and a little change in the XANES spectra of Nd-Ce-Cu-O and La-Sr-Cu-O, respectively. This means that the electron doping takes place mainly at the Cu site of [${\mathrm{CuO}}_2$] in Nd-Ce-Cu-O and that the hole doping in La-Sr-Cu-O takes place mainly at the oxygen site. In the ‘‘unreduced’’ Nd-Ce-Cu-O samples no monovalent species but rather divalent species with relatively large 3$d^{10}$ contributions are found. In the ‘‘reduced’’ and superconducting Nd-Ce-Cu-O sample (x=0.15), ${\mathrm{Cu}}_2$O-like monovalent species are found but probably correspond to segregated impurity phases. The essential effect of the reduction to induce the superconductivity in Nd-Ce-Cu-O (x∼0.15) has not yet been revealed.

• Received 19 June 1989

DOI:https://doi.org/10.1103/PhysRevB.41.131