Ca SUBSTITUTION AND OXYGEN DEFICIENCY IN Y₁Ba₂Cu₃O_7-d. P. Berastegui, S.-G. Eriksson, L.-G. Johansson, Dept. of Inorganic Chemistry, Chalmers University of Technology and University of Göteborg, S-41296 Göteborg, Sweden

The structural changes that occur upon the non-isovalent substitution of Ca for Y, were studied in Y₁Ba₂Cu₃O_7-d samples with d~0, 0.4 and 1. Polycrystalline samples with stochiometry Y_1-xCa_xBa₂Cu₃O_7-d (x= 0.0, 0.1, 0.2) were prepared by the solid-state reaction route and annealed in controlled partial oxygen pressures in order to obtain samples with the desired oxygen content. Neutron powder diffraction data were collected at room temperature using the medium resolution-high intensity powder diffractometer POLARIS, at the ISIS spallation source, Rutherford Appleton-Laboratory. Data analysis were carried out using the modified time-of-flight Rietveld program TF15LS, in the space groups Pmmm (samples with d~0, 0.4) and P4/mmm (samples with d~1). Site occupancies were constrained at their nominal values except the oxygen occupancies at the CuO chain.

Ca doping in the fully oxidised high-T_c superconductor Y₁Ba₂Cu₃O_7-d has been observed to result in a decrease of T_c. The differences in ionic size and valence induce changes in the interatomic distances around this site but have also a drastic effect on the rest of the structure. In both superconducting samples (d~ 0, 0.4), a decrease of the oxygen content can be observed with increasing Ca substitution, while the whole structure responds to the change in charge at this site. On the contrary, the nonsuperconducting sample (d~ 1) shows a constant oxygen content with Ca doping. This results in the CuO₂ planes being doped with holes, while the chain region remains unaffected.

The effect that the substitution has on the superconducting transition and calculations of the Bond Valence Sum for the different ions will also be discussed.