Effects of rare-earth elements and alkali metals on the superconductivity of (Tl0.7M0.3)Sr2Ca0.8Cr0.2Cu2O7 with M = Gd, Er, La, Li, Na, K, and Rb

The effects of rare-earth elements M = Gd, Er, and La and alkali metals M = K, Li, Na, and Rb substitutions on Tl_0.7M_0.3Sr₂Ca_0.8Cr_0.2Cu₂O₇ (Tl-1212) were investigated. The characterization includes X-ray diffraction method, scanning electron microscopy, electrical resistance and AC susceptibility measurements. X-ray diffraction patterns showed that almost all samples consisted of major Tl-1212 and minor Tl-1201 and Ca_0.3Sr_0.7CuO₂ phase. Rare-earth elemental substitution improved the formation of the Tl-1212 phase but suppressed the transition temperature. Scanning electron micrographs showed smaller grain size in the substituted samples compared with non-substituted sample. The temperature-dependent electrical resistance measurements showed metallic normal state behavior for all samples. Alkali metals substitutions showed higher zero transition temperature, T_c-zero compared with the rare-earth elemental substitution. AC susceptibility measurements showed a higher superconducting transition, T_cχ′ for alkali metals substitutions (84–93 K) compared with the rare-earth elemental substitutions (50–61 K). The inter-grain critical current density at the peak temperature T_p of the imaginary part χ", J_c(T_p) measured using the Bean's model was between 17 and 22 A cm⁻². The effects of rare-earth elements and alkali metals substitutions were discussed in terms of ionic radius and the concept of average Cu valence.