CATION DISTRIBUTION IN THE SUPERCONDUCTING TL, PB-1223 PHASE (TL0.5PB0.5SR2CA2CU3O9) FROM RESONANT SYNCHROTRON POWDER X-RAY-DIFFRACTION

The cation distribution in a polycrystalline sample of nominal composi tion ''Tl0.5Pb0.5Sr2Ca2Cu3O9'' with T(c) = 118 K has been determined f rom resonant synchrotron X-ray diffraction data collected at the Cu K, Tl L(III), Pl L(III) and Sr K edges. Cation disorder is found at all the non-copper sites according to the formula (Tl1-xPbx) (Sr2-yCay) (C a2-zTlz) Cu3O9 where x = 0.53 (2), y = 0.42 (2) and z = 0.064 (4) for our sample. The overall composition Tl0.53Pb0.53Sr1.58Ca2.36Cu3O9 conf irms that an excess of Ca and a deficiency of Sr can stabilise the Tl, Pb-1223 structure.