A NEUTRON-DIFFRACTION STUDY OF CA DOPED AND OXYGEN-DEFICIENT YBA2CU3O7-DELTA

A detailed study of the structure of Ca doped YBa2Cu3O7-delta samples with different oxygen contents has been carried out, Neutron powder di ffraction data were collected for nine samples with varying Ca and oxy gen contents and the structural changes were determined by Rietveld re finement. The samples were prepared by annealing at different oxygen p artial pressures and for the undoped samples delta values of 0.03, 0.3 8, and 0.95 were obtained. A decrease in the oxygen content with Ca do ping is observed for the orthorhombic samples. T-c decreases due to ov erdoping 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 t he change in average charge and ion size at the Y site, In the fully r educed samples, only the oxygen ions at the CuO2 planes are significan tly affected by substitution. (C) 1996 Academic Press, Inc.