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−1CunO2n+5 at 1250°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) A and a=3.857(1), c=25.067(7) A , respectively. Electron microscopic observations indicated that Cu and C are located alternately along the a-axes of the subcells resulting in orthorhombic superstructures with as=2a, bs=b and cs=c. The n=4 compound showed superconducting transition at 113 K. The n=5 compound appeared to be superconducting below ∽110 K, as well.