STRUCTURAL-CHANGES OF PBBASR(Y0.8CA0.2)CU3O7+Z ON OXYGEN INTRODUCTION INTO BLOCK LAYERS

On oxygen annealing, Pb and Cu atoms in block layers become disordered much more readily in PbBaSrYCu3O7+z than in Pb2Sr2YCu3O8+z. In this work, the structural properties of PbBaSr(Y1-xCa(x))Cu3O7+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(Y1-xCa(x))Cu3O8+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 arrangment 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(Y1-xCa(x))Cu3O7+z and Pb2Sr2(Y1-xCa(x)) Cu3O8+z.