Superconducting thallium cuprates obtained by substitution of copper for thallium in the double-thallium layer cuprate (T12212)

Abstract

A new series of superconducting thallium cuprates of nominal composition, (Tl_2-xCu_x)Ba₂CaCu₂O₈ (0<x<1.0), where copper seems to substitute for thallium in Tl2212, is reported. These materials, which are synthesized at 920K-1120K in sealed silver tubes, exhibit T_cs in the range 110-99K. The phases are metastable, decomposing at higher temperatures (≈1150K) to a mixture of thallium cuprates, CuO and BaCuO₂. Significantly, x=1 member decomposes at 1150K to mixture of Tl2223, CuO and BaCuO₂. Chemical titrations involving oxidation of bromide ions reveal that the copper substituting for thallium in (Tl_2-xCu_x)Ba₂CaCu₂O₈ most likely occurs in the III oxidation state for x≤0.25 and in a mixed state (II, III) state for x>0.25.