AC susceptibility and electrical properties of Cr and Mo substituted Tl(BaSr)CaCu${}_2$O${}_7$ Tl-1212 type phase superconductors

The Ba and Sr bearing Tl(BaSr)CaCu${}_2$O${}_7$ (Tl-1212) phase is an interesting superconductor. Optimizing the average copper valence can lead to a high transition temperature, T${}_c$. Single elemental substitution can be used to optimize the Cu valence. This work investigated the effects of multivalent Cr and Mo substitutions at the Tl site of Tl${}_{1-x}$M${}_x$(BaSr)CaCu${}_2$O${}_7$ (M = Cr and Mo). The samples were prepared via the solid-state reaction method. The structure and lattice parameters were determined using the X-ray diffraction method (XRD). A field emission scanning electron microscope (FESEM) was utilized to observe the microstructure. The electrical resistance and AC susceptibility were measured to determine the transition temperature. XRD pattern showed a single Tl(BaSr)CaCu${}_2$O${}_7$ phase for the non-substituted sample. Other phases, such as Tl-1201 and Ca${}_{0.3}$Sr${}_{0.7}$CuO${}_2$ were also observed in the Cr and Mo substituted samples. The non-substituted sample showed onset transition temperature, T${}_{c-onset}$ of 108 K. The highest T${}_{c-onset}$ was observed in the Tl${}_{0.9}$Mo${}_{0.1}$(BaSr)CaCu${}_2$O${}_7$ (115 K) and Tl${}_{0.5}$Cr${}_{0.5}$(BaSr)CaCu${}_2$O${}_7$ (111 K) samples. The intergranular critical current density was between 17 and 22 A cm${}^{-2}$. Cr substitutions for x = 0.1 to 0.5 and Mo substitutions for x = 0.1 showed the highest transition temperature. These results can be used as guides to improve the superconducting properties of the Tl-1212 phase.