Crystal structure-T_c correlations in LaBa_1.5Ca_0.5Cu₃O_δ

The structure of superconducting LaBa_1.5Ca_0.5Cu₃O_δ (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 °C in vacuum. The structure is similar to that of YBCO but with the A-cation distribution [La_0.5Ca_0.5][La_0.5Ba_1.5]Cu₃ O_δ; 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)O₅ pyramids are connected via apical oxygens O(3) to Cu(1) atoms. The Cu(2)-O bond distances vary smoothly with δ, 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 δ. The Cu(1)-O(3) bond distances show a range of dependencies on δ 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.