Direct observations of arrangements of carbonate groups in oxycarbonate superconductors by high-resolution electron microscopy.

Arrangements of CO3 groups in various types of oxycarbonate superconductors are examined by high-resolution transmission electron microscopy (HRTEM). Every other B-site of basic perovskite structure is replaced with CO3 groups in the first carbonate superconductor, (Ba0.56Sr0.44)2Cu1.1(CO3)0.9Oy. The 123-related oxycarbonate superconductor obtained in a Y-Ca-Sr-Cu-O system, (Y0.475Ca0.475Sr0.05)Sr2Cu2.4(CO3)0.6Oy, has a superstructure with 2a periodicity due to ordered replacements of Cu-site with CO3 groups. The non-superconducting counterpart with 123-related structure, (Y0.84Sr0.16)2Sr2Cu2.6(CO3)0.4Oy, on the other hand, shows more disordered arrangements of CO3 groups with nearly 3a periodicity. Similar superstructures, due to ordered replacements of Cu sites with CO3 groups, are also observed in the 223-related oxycarbonate superconductors, (Ln,Ce)2Sr2Cu2.5(CO3)0.5Oy (Ln = Ho, Dy). Homologous series of compounds, (CaSr)n+1Cun(CO3)Oy (n = 1-5), consist of alternate stacking of Sr2Cu(CO3)Oy and SrCuO2 (infinite-layer) types of blocks. They become superconductive by additionally doping the BO3 group. Another homologous series of Bi-based oxycarbonate superconductors, (Bi,Pb)2Sr2n+2Cun+1 (CO3)nOy (n = 1-3), contain alternate CuO2 and CO3 layers in between the two (BiO)2 layers. Both mercury (Hg)- and thallium (TI)-based oxycarbonate superconductors, MBa2Sr2Cu2(CO3)Oy (M = Hg or Tl) show quite unique modulation structures, where both HgO (or TlO) and CO3 layers repeat in the same plane, along [110] in the Hg compound and [100] in the Tl compound, to form long-period superstructures with wavy distortion of atom planes.