SUPERCONDUCTIVITY AT 110-K IN THE INFINITE-LAYER COMPOUND (SR1-XCAX)1-YCUO2

THE 'infinite-layer' parent structure 1 of the copper oxide superconductors (Fig. 1) is the simplest structure containing the CuO2 sheets that are apparently essential to high-transition-temperature (high-T(c)) superconductivity. At ambient pressure only Ca1-xSrxCuO2 with x almost-equal-to 0.1 can be stabilized in this structure 1,2, but at high pressures and temperatures compounds ranging from Ba1/3Sr2/3CuO2 to Ca2/3Sr1/3CuO2 through SrCuO2 can be synthesized 3. We have previously reported superconductivity with T(c) = 40-100 K in the Ba-Sr-Cu-O system 4,5, but have not until now been able to isolate a superconducting phase. Here we report the isolation of an alkaline-earth-deficient infinite-layer phase, (Ca1-xSrx)1-yCuO2 (y almost-equal-to 0.1), with T(c) up to 110 K. In contrast to Sr1-xRxCuO2 (with R a rare-earth element and T(c) less-than-or-equal-to 43 K), which from the composition dependence of the lattice constants is thought to be an n-type superconductor 6,7, our data suggest that the present superconductor is of p-type, with the carriers arising from calcium and strontium vacancies. High-resolution electron micrographs reveal defect layers, which we suggest are where the calcium and strontium vacancies are concentrated.