THE CRYSTAL-STRUCTURE OF (PB0.5CD0.5)SR2(Y1-XCAX)CU2O7

The crystal structures of (Pb0.5Cd0.5)Sr2YCu2O7, (Pb0.5Cd0.5)Sr2(Y0.6C a0.4)Cu2O7 and (Pb0.5Cd0.5)Sr2(Y0.5Ca0.5)Cu2O7 have been refined by th e Rietveld method for the X-ray diffraction data. The refinement resul ts indicate that both Pb and Cd atoms in the (Pb, Cd)O layers and O(3) atoms are displaced from their ideal sites, and that there is a compl icated occupation in the (Pb, Cd)O layers, i.e., other cations, such a s Sr2+, Cu2+ and Ca2+, can occupy the (Pb, Cd) site for these samples. This may be the reason why the lattice constants do not vary monotoni cally with the calcium content, and why there exist broad superconduct ing transitions in the system (Pb0.5Cd0.5)Sr2(Y1-xCax)Cu2O7. The refin ement results also indicate that, as the calcium content increases, th e O(2) atoms move close to the CuO2 planes and far from the (Pb, Cd)O layers. Thus, the increase of calcium content results in charge transf er from the charge reservoir layer (Pb, Cd)O to the CuO2 planes. This result is consistent with the bond valence sums of the Cu ions in the CuO2 planes.