Sol–gel synthesis, structural and superconducting properties of (Hg1−ySey)Sr2(Y1−xCax)Cu2O6+δ

A new series of Sr-based Hg-1212 superconducting cuprate (Hg1−ySey)Sr2(Y1−xCax)Cu2O6+δ (y=0.25; 0.0⩽x⩽0.7) have been successfully synthesized using a highly homogenous and reactive precursor Sr2(Y1−xCax)Cu2Oz prepared by the citrate sol–gel process. This chemical method is fast, cheap, reproducible and more efficient than the traditional solid-state reaction method. X-ray diffraction (XRD) and Energy dispersive X-ray analyses (EDX) studies have shown that Se is required for stabilization of the Sr-based Hg-1212 phase (Hg1−ySey)Sr2YCu2O6+δ; y≈0.25. On the other hand, electrical resistivity and magnetic susceptibility measurements indicated that substitution of Y by Ca is necessary to induce superconductivity in the 1212 (Hg0.75Se0.25)Sr2(Y1−xCax)Cu2O6+δ samples. Superconductivity is observed only for samples with x⩾0.3 and Tc increases with increasing Ca content as well as O2-annealing. A maximum Tc(onset) of 85 K is found in the (Hg0.75Se0.25)Sr2(Y0.3Ca0.7)Cu2O6.84 sample annealed in an oxygen atmosphere. The structure of O2-annealed samples was investigated by the Rietveld refinement. For all samples, it was found that Se substitutes at the Hg site. Each Se atom is surrounded by four oxygen atoms O(3), but these are not at the ideal image site. Rather, these oxygen atoms are shifted along the [110] direction ((0.3989, 0.3989, 0) in the case of x=0.5), implying a four-fold split site with an occupancy of 0.22(2) for each of them.