Formation of Tl-1212 phase in Ga-substituted Tl(BaSr)Ca_1−xGa_xCu₂O₇ superconductor

The effect of Ga substitution on the superconductivity of Tl(BaSr)Ca_1-xGa_xCu₂O₇ (x = 0–0.5) is reported. The samples were prepared by solid-state reaction method. XRD patterns showed a single Tl-1212 phase for the x = 0 and 0.1 samples. Unreacted Ga₂O₃ and other impurity phases were observed in the higher substituted (x = 0.2 to 0.5) samples. The temperature-dependent resistance measurements showed metallic normal state behavior for all samples with onset transition temperature T_c-onset between 94 and 66 K and zero transition temperature T_c-zero between 86 and 55 K. AC susceptibility measurement showed the highest superconducting transition (95 K) and peak temperature (89 K) for non-substituted sample. The intergranular critical current density J_c at T_p for all samples was between 16 and 22 A cm⁻². This work showed that the intergranular coupling and flux pinning energy were suppressed with Ga substitution. Some Ga⁺² were most likely incorporated at Ca⁺² for x = 0.1 and some resided at the grains for x ≥ 0.2 samples. This work showed that a single superconducting Tl-1212 phase was formed for x ≤ 0.1 samples. This work also showed that the internal lattice constrain which is the ratio of the lattice parameter (a/c) is important in improving the transition temperature of the Tl-1212 phase.