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

Arrangements of CO 3 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 CO 3 groups in the first carbonate superconductor, (Ba 0.56 Sr 0.44 ) 2 Cu 1.1 (CO 3 ) 0.9 O y . The 123‐related oxycarbonate superconductor obtained in a Y‐Ca‐Sr‐Cu‐O system, (Y 0.475 Ca 0.475 Sr 0.05 )Sr 2 Cu 2.4 (CO 3 ) 0.6 O y , has a superstructure with 2α periodicity due to ordered replacements of Cu‐site with CO 3 groups. The non‐superconducting counterpart with 123‐related structure, (Y 0.84 Sr 0.16 ) 2 Sr 2 Cu 2.6 (CO 3 ) 0.4 O y , on the other hand, shows more disordered arrangements of CO 3 groups with nearly 3α periodicity. Similar superstructures, due to ordered replacements of Cu sites with CO 3 groups, are also observed in the 223‐related oxycarbonate superconductors, (Ln,Ce) 2 Sr 2 Cu 2.5 (CO 3 ) 0.5 O y (Ln = Ho, Dy). Homologous series of compounds, (CaSr) n+1 Cu n (CO 3 )O y (n = 1–5), consist of alternate stacking of Sr 2 Cu(CO 3 )O y and SrCuO 2 (infinite‐layer) types of blocks. They become superconductive by additionally doping the BO 3 group. Another homologous series of Bi‐based oxycarbonate superconductors, (Bi,Pb) 2 Sr 2n+2 Cu n+1 (CO 3 ) n O y (n = 1–3), contain alternate CuO 2 and CO 3 layers in between the two (BiO) 2 layers. Both mercury (Hg)‐and thallium (TI)‐based oxycarbonate superconductors, MBa 2 Sr 2 Cu 2 (CO 3 )O y (M = Hg or TI) show quite unique modulation structures, where both HgO (or TlO) and CO 3 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.