AC susceptibility and electrical properties of rare-earth- and alkali metal-substituted (Tl_0.7M_0.3)Sr₂CaCu₂O₇ with M = Er, Gd, La, Na, K and Rb

Abstract

The effects of rare-earth elements M = Er, Gd and La, and alkali metals M = Na, K and Rb substitution on (Tl_0.7M_0.3)Sr₂CaCu₂O₇ (Tl-1212) are reported. The rare-earth elements were chosen by considering the ionic radius, i.e., from the smallest (Er⁺³) to the largest (La⁺³) ion. The samples were prepared via the solid-state reaction method. X-ray diffraction patterns showed the presence of major Tl-1212 and minor Tl-1201 and Ca_0.3Sr_0.7CuO₂ phase in almost all samples. The resistance versus temperature curves for the rare-earth-substituted samples showed onset temperature between 87 and 90 K and zero resistance temperature between 69 and 81 K. Alkali metal substitutions exhibited Tl-1212 phase but showed no superconducting transition although the electrical resistivity was much lower than the rare-earth-substituted samples. AC susceptibility measurement showed superconducting transition T_cχ, between 59 and 80 K for the rare-earth element-substituted samples. The average grain size, the transition temperature, T_cχ and the peak temperature T_p of the imaginary part of the susceptibility χ″ decreased as the ionic radius of the rare earths was reduced.