A NEW SERIES OF OXYCARBONATE SUPERCONDUCTORS (CU0.5C0.5)2BA3CAN-1CUNO2N+5(N=4,5) PREPARED AT HIGH-PRESSURE

We prepared a new series of oxycarbonate superconductors, (Cu0.5C0.5)2Ba3Can-1CunO2.+5 at 1250-degrees-C under 5 GPa. Members of the series with n = 4 and n = 5 were prepared in bulk. In addition, an intermediate phase in which n=4 and n=5 slabs were stacked alternately was found by electron microscopic observations. X-ray patterns of n = 4 and n = 5 members could be indexed assuming tetragonal subcells with a = 3.855(3), c = 21.87(1) angstrom and a = 3.857(1), c = 25.067(7) angstrom, respectively. Electron microscopic observations indicated that Cu and C are located alternately along the a-axes of the subcells resulting in orthorhombic superstructures with a(s) = 2a, b(s) = b and c(s) = c. The n = 4 compound showed superconducting transition at 113 K. The n = 5 compound appeared to be superconducting below approximately 110 K, as well.