Order and Disorder in (Nd,Ce)nO2nSr2GaCu2O5 and YSr2CoCu2O7

The structures of the compounds YSr2CoCu2O7 and (Nd/Ce)Sr2GaCu2O5 and (Nd/Ce)2Sr2GaCu2O9 [Nd/Ce)nSr2GaCu2O5+2n, n = 1, 2,…] are considered as derivatives of the prototype 123-phase. They were previously found to contain layers of corner-sharing M O4 tetrahedra. In this paper we describe the superstructures which consist of a regular alternation of the two types of symmetry related M O-chains leading to period doubling along the direction normal to the chains. These conclusions are based on electron diffraction and on direct image evidence from high resolution electron microscopy. Both compounds are fragmented in domains resulting from the two crystallographically equivalent orientations of the chains which differ by 90°. Both compounds contain linear defects confined to one MO-layer. Twin "lines" are due to orientation changes of the chains within a single MO-layer and antiphase boundary lines result from the parallel shift over one interchain distance, of two parts of a layer.