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, (Ba_0.56Sr_0.44)₂Cu_1.1(CO₃)_0.9O_y. The 123‐related oxycarbonate superconductor obtained in a Y‐Ca‐Sr‐Cu‐O system, (Y_0.475Ca_0.475Sr_0.05)Sr₂Cu_2.4(CO₃)_0.6O_y, has a superstructure with 2α periodicity due to ordered replacements of Cu‐site with CO₃ groups. The non‐superconducting counterpart with 123‐related structure, (Y_0.84Sr_0.16)₂Sr₂Cu_2.6(CO₃)_0.4O_y, on the other hand, shows more disordered arrangements of CO₃ groups with nearly 3α periodicity. Similar superstructures, due to ordered replacements of Cu sites with CO₃ groups, are also observed in the 223‐related oxycarbonate superconductors, (Ln,Ce)₂Sr₂Cu_2.5(CO₃)_0.5O_y (Ln = Ho, Dy). Homologous series of compounds, (CaSr)_n+1Cu_n(CO₃)O_y (n = 1–5), consist of alternate stacking of Sr₂Cu(CO₃)O_y 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)₂Sr_2n+2Cu_n+1 (CO₃)_nO_y (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)O_y (M = Hg or TI) 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 longperiod superstructures with wavy distortion of atom planes. © 1995 Wiley‐Liss, Inc.