A NEW PHASE IN THE SR-PB-CU OXIDE SYSTEM - THE CRYSTAL-STRUCTURE OF SR5-XPB3+XCUYO12-DELTA

A single crystal with compositions Sr4.79Pb3.21Cu0.66O11.12 was grown from a reaction mixture of nominal composition 2 Pb:1 Sr:1 Cu that was fired at 860°C in air.Mr = 1304.6,hexagonal,P-62m, a = 10.072(3)Å,c = 3.542(3)Å,V = 311.2(4)Å3,Z = 1,Dx = 6.96 g cm-3, λ = 0.71069A °, μ = 645 cm-1,F(000) = 553.34,room temperature,R = 0.040for 323 unique reflections5σ(Fo).The Pb atoms are in octahedral coordination; the octahedra form a chain parallel tocby edge-sharing. Sr is coordinated by nine oxygen atoms that form a capped trigonal prism. The trigonal prisms form a chain parallel tocby face-sharing. TheSr/Pbsite is surrounded by seven oxygen ions. The polyhedron can be described as a tricapped trigonal prism with one edge of the prism missing. The octahedra and trigonal prisms articulate by corner sharing into a three-dimensional framework. Two crystallographically independent Cu atoms are disordered and occupy tetrahedral interstices. One oxygen site is partially occupied. The presence of Pb2+ on the Pb site distorts the octahedron by lengthening two bonds to 2.44(3)A °; the average of the other four bond lengths is 2.137(19)A °. The Sr{single bond}O bond lengths to the six apices of the prism are 2.632(14) and 2.935(3)A °to the three capping atoms. The disorderedSr/Pb atom site has four oxygen neighbors at 2.544(17)A °, two at 2.621(5)A °, and one at 2.42(2)A °. The compound is an electrical insulator. © 1990 Academic Press, Inc. All rights reserved.