Effects of rare-earth elements and alkali metals on the superconductivity of (Tl0.7M0.3)Sr2Ca0.8Cr0.2Cu2O7 with MÂ = 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 Tl0.7M0.3Sr2Ca0.8Cr0.2Cu2O7 (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 Ca0.3Sr0.7CuO2 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, Tc-zero compared with the rare-earth elemental substitution. AC susceptibility measurements showed a higher superconducting transition, Tcχ′ 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 Tp of the imaginary part χ”, Jc(Tp) 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.