Phase formation and superconducting properties of (Tl,M)(Sr, Ba)Ca₃Cu₄O_z compounds with M = Ca, Mg, and Cu

Polycrystalline samples of (Tl_1−xM_x)SrBaCa₃Cu₄O_z ((Tl, M)-1234), where M = Ca, Mg and Cu, were prepared by using a solid state reaction method to find the environmentally friendly element that promotes the formation of the 1234 phase. X-ray diffraction data showed that Ca substitution was very effective in forming the 1234 phase at normal pressure, compared to either Mg or Cu substitution. Nearly single-phase samples were obtained for the nominal compositions of (Tl_1−xCa_x)SrBaCa₃Cu₄O_z (0.3 ≤ x ≤ 0.5) and (Tl_0.7Ca_0.3)(Sr_2−yBa_y)Ca₃Cu₄O_z (1.0 ≤ y ≤ 1.25). The (Tl_0.7Ca_0.3)(Sr_2−yBa_y)Ca₃Cu₄O_z (1.0 ≤ y ≤ 1.25) samples were found to have a relatively homogeneous microstructure by scanning electron micrography. Bulk superconductivity with T_c of 102 K was observed in the (Tl_0.7Ca_0.3)SrBaCa₃Cu₄O_z sample. The experimental results suggest that Ca can be a very useful element in the charge reservoir layer to form a single-phase Tl-1234-type compound with appropriate substitution of Ba into the Sr sites in the bridging layer.