Effects of alkali metals on the formation of Tl-1212 type phase (Tl0.85Cr0.15)Sr2−x A x CaCu2O7 with A = Li, Na and Rb

Abstract

The effects of alkali metals A = Li, Na and Rb on (Tl0.85Cr0.15)Sr2−x A x CaCu2O7 (Tl-1212) superconductors with x = 0–0.1 were studied. The samples were prepared via the solid-state reaction method. X-ray diffraction patterns showed that the formation of Tl-1212 phase was favorable when the ionic radius of the alkali metal was closer to the radius of Tl1+. Ca0.3Sr0.7CuO2 was present as a major or minor phase in some of the samples. Scanning electron micrographs showed that the grain size increased with Li, Na and Rb substitutions. The temperature-dependent electrical resistance measurements showed metallic normal state behavior for all samples with onset transition temperature, T c onset between 96 and 105 K and zero-resistance temperature, T c zero between 92 and 99 K. AC susceptibility measurement showed superconducting transition, \({T_{{c_{{{\upchi}}}}{{'}}}}\) between 92 and 97 K. The peak temperature T p of the imaginary part of the susceptibility χ″ was the highest for Na and Rb with x = 0.1 samples (T p = 92 K). The inter-grain critical current density, J c at T p for x = 0 sample was J c (T p = 68 K) = 14 A cm−2 and for Li substituted sample with x = 0.1, J c (T p = 76 K) = 22 A cm−2. This work showed that Rb greatly enhanced the Tl-1212 phase formation due to the proximity of the Rb1+ and Tl1+ ionic radius. Na and Rb improved the transition temperature. Li, Na and Rb improved the inter-grain critical current density and increased the grain size of the samples.