Multiple beam direct lattice imaging of the hexagonal ferrites

A wide variety of stacking sequences has been studied by means of transmission electron microscopy in the hexagonal ferrites of the type M _nY _m. On comparing, for a given specimen, the stacking sequences as deduced from X-ray diffraction with the image obtained in the electron microscope it was possible to establish simple "imaging codes" applicable under different diffraction conditions. The observations are in agreement with the structural features derived from X-ray diffraction. A number of known stacking sequences has been confirmed and a number of new ones has been found as well. The structural features of the hexagonal ferrites are reinterpreted in terms of repeated, polysynthetic twinning, the twin plane being localized in the center of the M block. Such an interpretation is suggested by the electron microscopic observations. Twinning is accompanied by a change in composition: it is therefore called nonconservative twinning. It is in fact analogous to crystallographic shear in accommodating deviations from simple stoichiometry. The generation of regular twinning sequences is tentatively interpreted in terms of the growth process.