Crystal Structure of (Lu_1−xCa_x)Ba₂Cu₃O_7−z (x = 0, 0.2) Superconducting Compounds Studied by Neutron Powder Diffraction

The crystalline structures of two polycrystalline samples of (Lu_1−xCa_x)Ba₂Cu₃O_7−z with x = 0 (pure sample) and x = 0.2 (Ca-doped sample) have been analyzed by using neutron powder diffraction techniques to address the structural mechanisms associated with changes in superconducting behavior induced by Ca doping. The pure and Ca-doped samples exhibit superconductivity around 87 and 32 K, respectively. The room-temperature thermoelectric power measurements indicate that both the x = 0 and 0.2 samples are in overdoped state. The Rietveld refinements of the neutron diffraction data collected at room temperature for the pure and Ca-doped samples show that the higher level of oxygen vacancies is located on both the CuO₂ planes and the apical oxygen sites for the pure sample compared to those of the Ca-doped sample. The present results suggest that the decrease in T_c observed in the Ca-doped sample is mainly caused by the hole overdoping effect rather than by oxygen vacancies in the CuO₂ planes.