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 compositi on, (Tl2-xCux)Ba2CaCu2O8 (0<x<1.0), where copper seems to substitute f or thallium in T12212, is reported. These materials, which are synthes ized at 920K-1120K in sealed silver tubes, exhibit T(c)s in the range 110-99K. The phases are metastable, decomposing at higher temperatures (almost-equal-to 1150K) to a mixture of thallium cuprates, CuO and Ba CuO2. Significantly, x = 1 member decomposes at 1150K to mixture of Tl 2223, CuO and BaCuO2. Chemical titrations involving oxidation of bromi de ions reveal that the copper substituting for thallium in (Tl2-xCux) Ba2CaCu2O8 most likely occurs in the III oxidation state for x less-th an-or-equal-to 0.25 and in a mixed state (II, III) state for x>0.25.