Superconducting Thallium Cuprates Obtained by Substitution of Copper for Thallium in the Double-Thallium Layer Cuprate (T12212)

A new series of superconducting thallium cuprates of nominal composition, $(Tl_{2-x}Cu_x)Ba_2CaCu_2O_8 (0<x<1.0),$ where copper seems to substitute for thal- lium in Tl2212, is reported. These materials, which are synthesized at 920K- l120K in sealed silver tubes, exhibit Tcs in the range 110-99K. The phases are metastable, decomposing at higher temperatures $(\approx1150K)$ to a mixture of thallium cuprates, CuO and $BaCuO_2.$ Significantly, x=l member decomposes at l150K to mixture of T12223, CuO and $BaCu0_2.$ Chemical titrations involv- ing oxidation of bromide ions reveal that the copper substituting for thallium in $(T1_{2-x}Cu_x)Ba_2CaCu_2O_8$ most likely occurs in the Ill oxidation state for $x_\leq0.25$ and in a mixed state (lI, Ill) state for x>0.25