The Cu K-edge absorption spectra of high-Tc superconductors and related oxides, (La1-xSrx)2CuO4 (L(S)CO, x=0, 0.075 YBa2Cu3Oy (YBCO, y≈7-6), YBa2 (Cu1-xCox)3Oy (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.
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