XANES Study on the Generation and Distribution of Holes via Ca Substitution and O Doping in Cu(Ba0.8Sr0.2)2(Yb1−xCax)Cu2O6+z

Generation of holes is facilitated in the Cu(Ba_0.8Sr_0.2)₂ (Yb_1−xCa_x)Cu₂O_6+z (Cu-1212) system by two independent ways, i.e., by Ca substitution (0≤x≤0.35) and O doping (0<z<1). The distribution of holes between the CuO₂-(Yb_1−xCa_x)-CuO₂ block containing two identical superconductive CuO₂ planes and the “charge-reservoir” block consisting of a single CuO_z chain has been quantitatively investigated by means of O K-edge and Cu L_2,3-edge X-ray absorption near-edge structure (XANES) spectroscopy. The resultant values for the CuO₂-plane hole concentration are compared with those calculated employing the bond-valence-sum (BVS) method from the neutron powder diffraction (NPD) data previously reported for the same samples. The results of the two methods are in good agreement. The two independent hole-doping ways are found to result in different distributions of holes over the crystal, i.e., different ratios of hole numbers at the CuO₂ plane and the CuO_z chain. With Ca substitution holes are directed efficiently into the CuO₂ plane, while for O doping holes are more homogeneously distributed between the CuO₂ plane and the CuO_z chain. Moreover, the value of T_c at a fixed CuO₂-plane hole concentration is shown to be higher for Ca-substituted than for O-doped samples.