Influence of Se and Te substitutions at Tl-site of Tl(Ba,Sr)CaCu2O7 superconductor on the AC susceptibility and electrical properties.

The Sr and Ba bearing Tl-1212 phase, Tl(Ba,Sr)CaCu 2 O 7 is an interesting superconductor. The Sr only bearing TlSr 2 CaCu 2 O 7 is not easily prepared in the superconducting form. The Ba only bearing TlBa 2 CaCu 2 O 7 on the other hand does not show improvement in the transition temperature with elemental substitution. In this work the influence of multivalent Se (non-metal) and Te (metalloid) substitutions at the Tl-site of Tl 1- x M x (Ba,Sr)CaCu 2 O 7 (M = Se or Te) superconductors for x = 0–0.6 was studied. The samples were prepared via the conventional solid-state reaction method. XRD patterns showed a single Tl-1212 phase for x = 0 and 0.1 Se substituted samples. The critical current density at the peak temperature, T p of the imaginary (χ") part of the AC susceptibility (χ = χ' +χ"), J c-inter (T p) for all samples was between 15 and 21 A cm−2. The highest superconducting transition temperature was shown by the x = 0.3 Se-substituted sample (T c-onset = 104 K, T c-zero = 89 K, T cχ' = 104 K and T p = 80 K). Te suppressed the superconductivity of Tl-1212 phase. The order of highest transition temperatures are as follows: Tl 1- x Te x (Ba,Sr)CaCu 2 O 7 <Tl(Ba,Sr)CaCu 2 O 7 <Tl 1- x Se x (Ba,Sr)CaCu 2 O 7. This work showed that Se was better than Te in improving the transition temperature and flux pinning of the Tl-1212 phase. The roles of ionic radius of Se and Te on the superconductivity of Tl(Ba,Sr)CaCu 2 O 7 are discussed in this paper. [ABSTRACT FROM AUTHOR]