NEW OXYCARBONATE SUPERCONDUCTORS (CU0.5C0.5)BA2CAN-1CUNO2N+3 (N=3,4) PREPARED AT HIGH-PRESSURE

In the process of searching for the phases of the Ag-Ba-Ca-Cu-O system at 1200-degrees-C under 5 GPa, we found the new oxycarbonate superconductors (Cu0.5C0.5)Ba2Can-1CunO2n+3. Two members of the series with n=3 and n=4 were isolated as almost single phases; their structures are closely related to those of Tl (or Hg) based 1223 and 1234, respectively. The T1 (Hg) sites are, in the present series, replaced by Cu and C. Disregarding ordering of Cu and C, the present compounds take tetragonal subcells; a(t)=3.859 (1), c(t)=14.766 (5) angstrom for n=3 and a(t)=3.855 (1), c(t)=17.930(6) angstrom for n=4. Electron microscopic observation indicated that Cu and C are, in both cases, located alternately along the a-axes of the subcells resulting in superstructures having a(s)=2a(t), b(s)=b(t) and c(s)=2c(t). The n=3 member showed a superconducting transition at 67 K, while the n=4 member at 117 K which is the highest T(c) among the oxycarbonate superconductors found thus far.