Crystal structure-Tc correlations in LaBa1.5Ca0.5Cu3Oδ

The structure of superconducting LaBa1.5Ca0.5Cu3Odelta (LaBCCO) was refined by the Rietveld method using powder neutron diffraction (ND) data. Data sets were collected at both room temperature and high temperature (HT). For the room temperature data, samples were heated isothermally over a range of temperatures to give a range of oxygen contents and then either cooled slowly or quenched. HT data were recorded in isothermal steps over the range 300-900 degrees C in vacuum. The structure is similar to that of YBCO but with the A-cation distribution [La0.5Ca0.5][La0.5Ba1.5]Cu3Odelta; variable oxygen content is accommodated by changes in occupancy of the basal plane sites and the samples remain tetragonal under all conditions. Layers of Cu(2)O-5 pyramids are connected via apical oxygens O(3) to Cu(1) atoms. The Cu(2)-O bond distances vary smoothly with delta, have similar values for all three data sets and are insensitive to critical temperature (T-c) variations seen between quenched and slowly cooled samples for a given delta. The Cu(1)-O(3) bond distances show a range of dependencies on delta and appear to correlate with T-c, even though the superconductivity cannot reside with Cu(1) due to the absence, in this tetragonal structure, of infinite Cu(1)-O chains which could have provided a long-range conduction pathway. These results, in which the composition dependence of the Cu(1)-O(3) bond length appears to reflects that of T-c, are in complete contrast to data on YBCO which show that Cu(2)-O(3) is the key structural parameter that mirrors T-c. (C) 1999 Elsevier Science B.V. All rights reserved.