Cu K-edge XANES of (La_1-xSr_x)₂CuO₄, YBa₂Cu₃O_y and related Cu oxides. valence, structure and final-state effects on 1s-4pπ and 1s-4pσ absorption

The Cu K-edge absorption spectra of high-T_c superconductors and related oxides, (La_1-xSr_x)₂CuO₄ (L(S)CO, x=0, 0.075 YBa₂Cu₃O_y (YBCO, y≈7-6), YBa₂ (Cu_1-xCo_x)₃O_y (Co substitution for Cu(1), x=0.33, y≈7) (y≈7, ≈6) are interpreted by comparing with those of some divalent Cu complex oxides, Sr₂CuO₃ and Ca₂CuO₃ with the 1D CuO chain structure and Nd₂CuO₄ and Gd₂CuO₄ with the 2D CuO sheet structure. The twin-peak structure observed in the main absorption band arises from the final-state (core-hole screening) effect through ligand-to-metal charge transfer and can be assigned to the 1s-4pσ₁ (3d¹⁰L^-1) and 4pσ₂ (3d⁹) transitions. In the 1s-4pπ energy region below the 1s-4pσ main absorptio the onset and shoulder structures A-F are observed; the structures C and F are assigned to 1s-4pπ₁ and 1s-4pπ₂ transitions in the 2D CuO sheets of pyramidally coordinated [Cu(2)O₅] in YBCO and octahedrally coordinated [CuO₆] in L(S)CO, the structures B and E to those in the 1D CuO chain at the Cu(1) site of oxygen-rich YBCO, and the structures A and D to those in the linearly coordinated [Cu(1)O₂] ions of oxygen-deficient YBCO. The twin-peak structure both in the 1s-4pπ and 1s-4pσ energy regions indicates that the valence of Cu in YBCO and LSCO always has both d⁹ and d¹⁰ components; that is, the oxygen atoms coordinated to Cu take an important part in the balance of holes in the systems and in the hole conductivity on the 2D CuO sheet.