Chemical characterization of the thallium cuprate superconductors

All the known copper-oxide superconductors have intergrowth structure consisting of superconductively active CuO2 sheets and other inactive oxide layers. Oxidation/reduction of the CuO2 sheets above/below the formal oxidation Cu2+ is one of the necessary conditions to suppress the long-range antiferromagnetic order and induce superconductivity. This process can be achieved by several mechanisms: (i) Suitable ionic substitutions in the inactive layer as in La2-ySryCuO4, Nd2-yCeyCuO4 and Nd2CuO4-xFx, (ii) insertion of excess x oxygen into the inactive layer as in La2CuO4+x and YBa2Cu3O6+x, and as shown below, (iii) by band overlap. We have been able to resolve the ambiguities by adopting a simple wet-chemical procedure for the determination of Tl content and a modified iodometric method to determine the oxygen content. Our results demonstrate that the oxidation of the CuO2 sheets in the Tl2Ba2Can-1CunO2n+4 family is primarily due to an overlap of the Tl:6s band with the CuO2:σsub(x2-y2)* conduction band. Our analytical data and thermogravimetric analysis results for the system Tl2Ba2CuO6 are given in this paper. We have also investigated the system Tl2Ba2-zLazCuO6 in an attempt to understand the variation of properties with doping. (orig./BHO)