Effects of alkali metals on the formation of Tl-1212 type phase (Tl_0.85Cr_0.15)Sr_2−x A _x CaCu₂O₇ with A = Li, Na and Rb

Abstract

The effects of alkali metals A = Li, Na and Rb on (Tl_0.85Cr_0.15)Sr_2−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⁺. Ca_0.3Sr_0.7CuO₂ 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.