Crystal Structure of (Lu1−xCax)Ba2Cu3O7−z (x = 0, 0.2) Superconducting Compounds Studied by Neutron Powder Diffraction

The crystalline structures of two polycrystalline samples of (Lu1−xCax)Ba2Cu3O7−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 Tc observed in the Ca-doped sample is mainly caused by the hole overdoping effect rather than by oxygen vacancies in the CuO2 planes.