Growth of ‘124’ and ‘247’ phases studied by high-resolution transmission electron microscopy in HoBa2Cu3O7 –x ceramics prepared under normal oxygen pressure

Abstract

High-resolution transmission electron microscopy (HREM) has been used to study the structure of superconductor HoBa₂Cu₃O_{7 –x}(‘123’) ceramics prepared under normal oxygen pressure. HREM images reveal that lattice disorder as well as high densities of stacking faults occur locally in the perfect ‘123’ structure. HREM micrographs obtained from thin areas of those faulted regions show a good correspondence to projections of the HoBa₂Cu₄O₈(‘124’) and Ho₂Ba₄Cu₇O₁₅(‘247’) structures proposed from neutron diffraction studies. The b/2 shift between double Cu–O chains occurring in both structures is confirmed by comparison between experimental and simulated HREM images corresponding to different projections of these structures. This demonstrates that owing to departure from nominal composition, stacking sequences occur locally in the specimens, which can give rise to growth of ‘124’ and ‘247’ phases normally obtained under higher oxygen pressure.