Out-of-plane effect on the superconductivity of Sr2-xBaxCuO3+δ with Tc up to 98 K

A series of new Sr2-xBaxCuO3+delta (0 < x < 0.6) superconductors were prepared using high-pressure and high-temperature synthesis. A Rietveld refinement based on powder x-ray diffraction confirms that the superconductors crystallize in the K2NiF4-type structure of a space group I4/mmm similar to that of La2CuO4 but with partially occupied apical oxygen sites. It is found that the superconducting transition temperature T-c of this Ba substituted Sr2CuO3+delta superconductor with constant carrier-doping level, i.e., constant delta is controlled not only by order/disorder of apical-O atoms but also by Ba content. T-c(max)=98 K is achieved in the material with x=0.6 that reaches the record value of T-c among the single-layer copper oxide superconductors and is higher than T-c=95 K of Sr2CuO3+delta with optimally ordered apical-O atoms. There is Sr-site disorder in Sr2-xBaxCuO3+delta which might lead to a reduction in T-c. The result indicates that another effect surpasses the disorder effect that is related either to the increased in-plane Cu-O bond length or to elongated apical-O distance due to Ba substitution with larger cation size. The present experiment demonstrates that the optimization of local geometry out of the Cu-O plane can dramatically enhance T-c in the cuprate superconductors.