Effect of alkali metal Na on the formation and superconductivity of Tl-1212-type phase (Tl_0.85Cr_0.15)Sr_2−xNa_xCaCu₂O₇

Abstract

The effect of Na substitution on the formation and superconductivity of Tl-1212-type phase (Tl_0.85Cr_0.15)Sr_2−xNa_xCaCu₂O₇ (x = 0–0.30) is reported. The samples were prepared via the solid-state reaction method. X-ray diffraction patterns showed the presence of Tl-1212-type phase or Ca_0.3Sr_0.7CuO₂ (CSCO) as a major or minor phase in the samples. The resistance versus temperature curves showed onset transition temperature, T_c-onset between 97 and 106 K and zero-resistance temperature, T_c-zero between 88 and 99 K. AC susceptibility measurement showed superconducting transition ${T_{{\text{c}}{\chi ^\prime }}}$ between 89 and 98 K. The peak temperature, T_p, of the imaginary part of the susceptibility, χ″, was the highest for x = 0.02 sample (T_p = 96 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 x = 0.02, 0.20 and 0.30 J_c (T_p) at 96, 96, and 88 K, respectively, is 19 A cm⁻². Na substitution increased the grain size of the Tl-1212 samples. All Na-substituted samples showed higher transition temperature, enhanced critical current density, and improved inter-grain coupling compared with the non-substituted sample. This work showed that other than rare-earth elements, monovalent alkali metal such as Na could also be used to enhance the superconducting properties of the Tl-1212 phase.