NEUTRON-DIFFRACTION ON (TL, PB)-SR-CA-CU-O 1223

Materials of the composition (Tl, Pb)Sr2Ca2Cu3O9-delta have been synth esized with different O stoichiometries and their structure and superc onducting properties investigated. Powder neutron-diffraction studies reveal that the Tl and O atoms in the Tl-O layers are both disordered off the ideal sites and are partially vacant, producing a highly disto rted environment. As a result of changing the Tl and O stoichiometry t he apical Cu-O bond and cuprate layer spacing decrease with decreasing site occupancy. These structural changes are discussed in relation to the superconducting properties and comparisons drawn to the (Tl, Pb)S r2CaCu2O7-delta phase and the double-Tl-layer materials.