Twinning in Cs2YbNb6Br18 and the atomic structure of the twin interface

In the search for defects in crystals of the cluster compound Cs2YbNb6Br18, polysynthetic microtwinning was observed by electron diffraction and high- resolution electron microscopy (HREM), and the complete twin law could be d etermined. The twinning element is a twofold screw axis along [001], and th e perfectly smooth twin interface is parallel to (110) of the rhombohedral unit cell. From the twin law a structure model of the twin intergrowth zone was deduced, The model assumes a fully coherent twin interface. Within the twin intergrowth zone, the Nb6Br18 clusters as well as all the coordinatio n relations between clusters, Yb atoms, and Cs atoms are preserved-and thus the chemical composition, Computer-simulated HREM images of the tn in inte rface were generated on the basis of this model, Comparison with experiment al images showed excellent agreement, Crystallographic relations between th e pseudocoincidence lattices of twins are discussed, as well as the Br coor dination polyhedra of interfacial Yb and Cs atoms. (C) 1998 Academic Press.