Phase formation and superconducting properties of (Tl,M)(Sr, Ba)Ca3Cu4Oz compounds with M = Ca, Mg, and Cu

초록

Polycrystalline samples of (Tl1-xMx)SrBaCa3Cu4Oz ((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 (Tl1-xCax)SrBaCa3Cu4Oz (0.3 <= x <= 0.5) and (Tl0.7Ca0.3)(Sr2-yBay)Ca3Cu4Oz (1.0 <= y <= 1.25). The (Tl0.7Ca0.3)(Sr2-yBay)Ca3Cu4Oz (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 (Tl0.7Ca0.3)SrBaCa3Cu4Oz 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.

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