Abstract
Generation of holes is facilitated in the Cu(Ba0.8Sr0.2)2 (Yb1−xCax)Cu2O6+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–(Yb1−xCax)–CuO2 block containing two identical superconductive CuO2 planes and the “charge-reservoir” block consisting of a single CuOz chain has been quantitatively investigated by means of O K-edge and Cu L2,3-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 CuOz 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 CuOz chain. Moreover, the value of Tc at a fixed CuO2-plane hole concentration is shown to be higher for Ca-substituted than for O-doped samples.
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