The crystal structure of the 1212 nonsuperconductor phase (Pb_0.71Cu_0.29)Sr₂(Y_0.73Ca_0.27)Cu₂O₇

The reaction product from an oxide mixture of 1Pb:2Sr:0.75Y:25Ca:2Cu at 960 °C was nearly single phase. The crystal structure of a single crystal, selected from the reaction product, was determined from three-dimensional x-ray diffraction data. The stoichiometry was found to be Pb_0.71Cu_0.29Sr₂(Y_0.73Ca_0.27)Cu₂O₇. The tetragonal lattice constants are a = 3.8207(4) Å, c = 11.826(1) Å, space group P4/mmm, M_r = 655.6, Z = 1, D_x, = 6.31 g cm⁻³, λ(MoK_α) = 0.71069 Å. R = 0.0269 for 179 unique observed reflections. The crystal structure is isomorphous with the 1212 phases. The Pb ion at the origin has an occupancy of 0.705 (8) and a Cu ion is located at 0.221, 0, 0 with an occupancy of 0.295. The oxygen ion in the plane of the Pb ions is disordered over a fourfold site with an occupancy of one. An ordered arrangement of this oxygen ion results in a distorted environment for periodically repeated Pb ions. The Pb–O bond lengths to the two oxygen ions above and below the Pb plane are 1.968(14) Å. The Cu ion, displaced from the origin, can be considered as tetrahedrally coordinated. The four Cu–O bond distances in the CuO₂ plane are 1.917(6) Å and the bond length to a fifth oxygen atom is 2.304(14) Å at the apex of the square pyramid. A valence sum calculation for Cu in square coplanar coordination yields a value of 2.01. A similar calculation for Cu in the tetrahedral site yields 1.5. Sr is in a monocapped square antiprism coordination polyhedron. Y and Ca atoms occupy statistically the same site and are in cubic coordination. The structure can be described as consisting of CuO₂–(Y, Ca)–CuO₂ layers with PbO₆ octahedra flanking the layers and Sr in the oxygen interstice. The compound is metallic at room temperature and shows no superconducting transition to 20 K.