Crystal structure of Tl0.5Pb0.5Sr2Ca2Cu3O9 at 300 K and around Tc (118 K)

The crystal structure including the cation distribution, of a polycrystalline sample of nominal composition Tl_0.5Pb_0.5Sr₂Ca₂Cu₃O₉ with T_c = 118.2 K has been determined using resonant synchrotron X-ray diffraction data collected at the Cu K, Tl L_III and Sr K edges and time-of-flight powder neutron diffraction data. No oxygen deficiency was observed, but cation disorder at all the non copper sites according to the formula (Tl_0.60Pb_0.40)(Sr_1.60Ca_0.40)(Ca_1.93Tl_0.07) Cu₃O₉ gives a mean hole concentration of 0.18(1) per Cu atom for the three CuO₂ planes, consistent with the high T_c for this material. Analysis of five time-of-flight powder neutron diffraction data between 80 and 150 K have revealed a possible discontinuity in the variation of the c lattice parameter at T_c, due to an anomaly in the position of the apical oxygen atoms.