A neutron diffraction study of Ca doped and oxygen deficient YBa₂Cu₃O_7-δ

A detailed study of the structure of Ca doped YBa₂Cu₃O_7-δ samples with different oxygen contents has been carried out. Neutron powder diffraction data were collected for nine samples with varying Ca and oxygen contents and the structural changes were determined by Rietveld refinement. The samples were prepared by annealing at different oxygen partial pressures and for the undoped samples δ values of 0.03, 0.38, and 0.95 were obtained. A decrease in the oxygen content with Ca doping is observed for the orthorhombic samples. T_c decreases due to overdoping in the fully oxidized samples and increases in the partially oxidized samples. The bond lengths change in a similar manner for both the fully and partially oxidized samples. The structural changes are related to the increasing oxygen deficiency in the CuO chains and to the change in average charge and ion size at the Y site. In the fully reduced samples, only the oxygen ions at the CuO₂ planes are significantly affected by substitution.