Synthesis and study of Hg-based 1222-type superconducting cuprates (Hg, M)Sr₂(Ln, Ce)₂Cu₂O(z)

A novel series of '1222-type' copper oxides described by the formula (Hg, M)Sr₂(Ln, Ce)₂Cu₂O₂ (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 isostrucrural 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 (Hg_0.75W_0.25)Sr₂(Ln_1.5Ce_0.5)Cu₂O(z), Ln = Sm-Er. Except for Cr-containing compound, bulk superconductivity is observed in all the synthesized compounds. The value of T(c) 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 (Hg_0.75W_0.25)Sr₂(Gd_1.5Ce_0.5)Cu₂O(z) recorded the highest T(c) up to 75 K. (C) 2000 Academic Press.