O K-edge and Cu L₂₃-edge XANES study on the concentration and distribution of holes in the (Pb_2/3Cu_1/3)₃Sr₂(Y, Ca)Cu₂O_8+z superconductive phase

By means of high-resolution O K-edge and Cu L₂₃-edge x-ray absorption near-edge-structure spectroscopy continuous increase of the CuO₂-plane hole concentration with increasing Casubstitution level has been established for the superconductive, oxygen-depleted (z≈0) (Pb_2/3Cu_1/3)₃Sr₂(Y_1-xCa_x)Cu₂O_8+z [(Pb_2/3Cu_1/3)-3212] phase with a three-layer PbO-Cu-PbO charge-reservoir block. For the O K-edge absorption, a pre-edge peak at ∼528.3 eV is seen, originating from the excitation of the O 1s electron to the O 2p hole state located in the CuO₂ plane. With increasing Ca-substitution level, the intensity of this peak continuously increases within the substitution range studied, i.e., 0<~x<~0.5. Consistently, with increasing x, the shoulder on the high-energy side of the main absorption peak at ∼932.0 eV in the Cu L₂₃-edge spectra, i.e., a feature typically assigned to formally trivalent copper, enhances. From the Cu L₂₃-edge spectra it was furthermore confirmed that the charge-reservoir copper remains in the monovalent state, indicating that the holes created through Ca substitution are directed solely into the superconductive CuO₂ plane. In terms of increasing the CuO₂-plane hole concentration, Ca substitution was found to work more efficiently in (Pb_2/3Cu_1/3)-3212 as compared to, e.g., the related Bi-2212 phase.