Thallium solubility range in Tl2 –yBa2Can– 1CunO2n+ 4 –xsuperconductors

Abstract

The Tl_{2 –y}Ba₂Ca_{n– 1}Cu_nO_{2n+ 4 –x}phases corresponding to n= 1, 2 and 3 have been synthesized at 860–880 °C under sealed-tube conditions and characterized by wet chemical analysis, X-ray diffraction and magnetic 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 conditions, which allowed variation of the Tl concentration under similar temperatures and oxygen partial pressures, the equilibrium oxygen content was found to vary with the change in Tl concentration so as to keep a constant oxidation of the CuO₂ sheets. Over the entire Tl solubility 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 sensitively with chemical homogenization, and homogenization was achieved most easily for n= 2. Processing of Tl-2212 superconductors with a controlled T_c under sealed-tube conditions appears to be feasible.