THALLIUM SOLUBILITY RANGE IN TL2-YBA2CAN-1CUNO2N-X(4)

The Tl2-yBa2Can-1CunO2n+4-x phases corresponding to n = 1, 2 and 3 hav e been synthesized at 860-880-degrees-C under sealed-tube conditions a nd characterized by wet chemical analysis, X-ray diffraction and magne tic susceptibility. A lower Tl solubility limit of (2 - y) = 1.25 was found for all n, but the upper limit decreased from 1.89 for n = 1 to 1.80 for n = 2 and 1.65 for n = 3. Under our controlled synthetic cond itions, which allowed variation of the Tl concentration under similar temperatures and oxygen partial pressures, the equilibrium oxygen cont ent was found to vary with the change in Tl concentration so as to kee p a constant oxidation of the CuO2 sheets. Over the entire Tl solubili ty ranges, the superconductive onset temperatures were nearly constant : T(on) = 117 +/- 1 K for n = 3, T(on) = 105 +/- 1 K for n = 2, and T( on) = 86 +/- 1 K for n = 1. The superconductive properties improve sen sitively with chemical homogenization, and homogenization was achieved most easily for n = 2. Processing of Tl-2212 superconductors with a c ontrolled T(c) under sealed-tube conditions appears to be feasible.