Structural order and disorder in Co-based layered cuprates CoSr ₂(Y,Ce)_sCu₂O_5+2s (s=1-3)

Crystal structures of a homologous series of Co-based layered cuprates, CoSr₂(Y,Ce)_sCu₂O_5+2s (s=1-3), have been investigated by high-resolution electron microscopy (HREM) and electron diffraction (ED) techniques. For all the three phases ED patterns showed double periodicity along a direction parallel to the CoO layers, indicating a regular alternation of two types of CoO₄-tetrahedra chains within the layers. Also seen was ordering of the chains along the layer-stacking direction for the s=1 phase (Co-1212); ED patterns simulated based on the proposed superstructure model well reproduced the observed patterns. For the s=2 (Co-1222) and s=3 (Co-1232) phases in which an additional fluorite-type layer-block is inserted between two CuO₂ planes, HREM and ED analysis revealed complete disorder of the CoO₄ chains along the layer-stacking direction. This implies that the interlayer ordering is mainly controlled by the distance between the neighboring CoO layers.