High-pressure synthesis and properties sf a new oxycarbonitrate superconductors in the Sr-Ca-Cu-N-C-O system

Recently, oxycarbonitrate superconductors (Cu, N, C)Sr2Can-1CunOy were prep ared under high pressure. They have M-12(n - 1)n type structures with the M sites occupied by mixed atoms of (Cu, N, C). The phases with n = 3-6 have been isolated, identified and characterized. However, the n = 1 and 2 membe rs have not been reported and it is even unknown whether they exist or not. In the present study, we performed systematic high-pressure: synthesis exp eriments for the Sr-Ca-Cu-N-C-O system to find the n = 1 and 2 members. The n = 1 phase has a somewhat different characteristic, i.e. its M site is oc cupied only by C and N without partial replacement of Cu, having a composit ion of (N, C)Sr2CuOy. On the other hand, in the n = 2 phase the M site is o ccupied by the three atoms Cu, N and C, as in the n = 3-6 members. These tw o new phases show superconductivity below 33 and 91 K, respectively. The su perstructure due to an ordered arrangement of Cu, C and N was studied using a high-resolution electron microscope. A superstructure with 4a x b x 2c r epresenting a simple tetragonal cell with a, b, c was observed in the n = 2 phase.