Structural changes of PbBaSr(Y_0.8Ca_0.2)Cu₃O_7+z on oxygen introduction into block layers

On oxygen annealing, Pb and Cu atoms in block layers become disordered much more readily in PbBaSrYCu₃O_7+z than in Pb₂Sr₂YCu₃O_8+z. In this work, the structural properties of PbBaSr(Y_1-xCa_x)Cu₃O_7+z (x=0.2; z=0.0 and 1.35) were studied by X-ray and time-of-flight neutron powder diffraction and compared with those of Pb₂Sr₂(Y_1-xCa_x)Cu₃O_8+z. In the superconductor (T_c=39 K) with z=0.0, Pb and Cu atoms in PbO-Cu block layers are perfectly ordered, in agreement with structural data reported by Rouillon et al. On the other hand, oxygen atoms are incorporated in the Cu layer in the oxygen-annealed sample with z=1.35, which gives rise to a disordered arrangement of Pb and Cu atoms in the block layers. Bond-valence-sum calculations show that the oxidation states of the Pb and Cu atoms respectively change from +2 to ca. +4 and from +1 to ca. +2 upon oxygen incorporation. The effects of lone electron pairs at Pb²⁺ ions and the structure of the block layer on metal disordering is discussed in connection with the difference in oxidation behavior between PbBaSr(Y_1-xCa_x)Cu₃O_7+z and Pb₂Sr₂(Y_1-xCa_x)Cu₃O_8+z.