CATION DISTRIBUTION AND COMPOSITION OF THE TL-2223 SUPERCONDUCTOR FROM COMBINED POWDER NEUTRON AND RESONANT X-RAY-DIFFRACTION

A combined powder neutron and resonant X-ray diffraction study of a po st-annealed Tl-2223 (''Tl2Ba2Ca2Cu3O10'') superconductor with T(c)(0)= 124.5 K shows that Cu substitutes for Tl in the TlO layers and Tl subs titutes at the Ca sites. The resulting composition is (Tl1.72Cu0.28)Ba 2(Ca1.86Tl0.14)Cu3O10, and some indications of how the stoichiometry o f Tl2223 varies with starting composition have been obtained. Using ex cess Cu in the synthesis of Tl2223 gives more phase-pure material than other starting compositions, resulting in a T(c)=127 K sample after o nly 3.5 h synthesis time.