Synthesis of alkali-substituted Sr,Cu oxycarbonates superconductivity in Sr 2− xK xCuO 2CO 3 (0.25 ⩽ x ⩽ 0.7)

Abstract

Sr 2− x M x CuO 2CO 3, M  Na or K, 0 ⩽ x ⩽ 1, samples were obtained by a high-pressure high-temperature technique using SrCO 3, SrCuO 2 and Na(K)CuO 2 as initial reagents. The alkali-doped oxycarbonates were found to exist with Na for 0 ⩽ x ⩽ 1, and with K for 0 ⩽ x ⩽ 0.7. These oxycarbonates have a tetragonal structure derived from that of Sr 2CuO 2CO 3. The a parameter decreases with increasing alkali ion content from a = 3.9062(1) Å ( x = 0) to a = 3.8854(2) A ̊ for x(Na) = 1 and to 3.8884(7) Å for x(K) = 0.7. Weak superstructure reflections corresponding to doubling of the cell parameters ( a = 2 a, c = 2 c) were found by electron diffraction for K substituted oxycarbonate, Sr 1.6K 0.4CuO 2CO 3. The EDS analysis performed on this sample revealed that the crystallites contained an inhomogeneous K distribution and the average K concentration was found to be smaller than the nominal composition, x = 0.16 and x = 0.4, respectively. Superconductivity by AC susceptibility measurements was found to exist only for K substituted samples with 0.25 ⩽ x ⩽ 0.7. The critical temperature was found to vary in the range of 13–23 K depending on the K content. No superconductivity was observed for Na substituted samples.