Synthesis and Study of Hg-Based 1222-Type Superconducting Cuprates (Hg, M)Sr2(Ln, Ce)2Cu2Oz

A novel series of "1222-type" copper oxides described by the formula (Hg, M)Sr2(Ln, Ce) 2Cu2Oz (M=Ti, V, Cr, Mo, W, Re; and Ln=Sm, Eu, Gd, Dy, Ho, Er) have been synthesized under high pressure. The structure was determined by combination of X-ray powder diffraction, electron diffraction and high-resolution electron microscopy measurements. The materials are isostructural with (Pb, Cu)-1222, crystallizing in a tetragonal unit cell, space group I4/mmm. The compounds adopt the 1222-type structure with an intergrowth of three types of layers: oxygen-deficient perovskite (CuO5 pyramidal layers), double fluorite-type layers, and double rock-salt layers. A correlation between lattice parameters and effective ionic radii of the (Ln, Ce) site is determined for (Hg0.75W0.25)Sr2(Ln1.5Ce0.5)Cu2Oz, Ln=Sm-Er. Except for Cr-containing compound, bulk superconductivity is observed in all the synthesized compounds. The value of Tc is related to both the effective ionic radius of the (Ln, Ce) site and the valence of the M-element. Among these compounds, the sample with nominal composition of (Hg0.75W0.25)Sr2(Gd1.5Ce0.5)Cu2Oz recorded the highest Tc up to 75 K.