SYNTHESIS OF ALKALI-SUBSTITUTED SR,CU OXYCARBONATES - SUPERCONDUCTIVITY IN SR2-XKXCUO2CO3 LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.7)

Sr(2-x)M(x)CuO(2)CO(3), M = Na or K, 0 less than or equal to x less th an or equal to 1, samples were obtained by a high-pressure high-temper ature technique using SrCO3, SrCuO2 and Na(K)CuO2 as initial reagents. The alkali-doped oxycarbonates were found to exist with Na for 0 less than or equal to x less than or equal to 1, and with K for 0 less tha n or equal to x less than or equal to 0.7. These oxycarbonates have a tetragonal structure derived from that of Sr2CuO2CO3. The a parameter decreases with increasing alkali ion content from a=3.9062(1) Angstrom (x=0) to a=3.8854(2) Angstrom for x(Na)=1 and to 3.8884(7) Angstrom f or x(K)=0.7. Weak superstructure reflections corresponding to doubling of the cell parameters (a=2a, c=2c) were found by electron diffractio n for K substituted oxycarbonate, Sr1.6K0.4CuO2CO3. The EDS analysis p erformed on this sample revealed that the crystallites contained an in homogeneous K distribution and the average K concentration was found t o be smaller than the nominal composition, x=0.16 and x=0.4, respectiv ely. Superconductivity by AC susceptibility measurements was found to exist only for K substituted samples with 0.25 less than or equal to x less than or equal to 0.7, The critical temperature was found to vary in the range of 13-23 K depending on the K content. No superconductiv ity was observed for Na substituted samples.