Some recent studies of domain structures in crystals

The geometrical aspects of domain formation in crystals are discussed, in particular in relation to the observation of interfaces by means of electron microscopy. The contrast effects which allow one to distinguish different types of interfaces are briefly summarized and it is shown how the image characteristics can be used to determine the geometry of the interfaces. A number of case studies are presented. We first describe the domain structure in Ni₄Mo and we show how the different types of interfaces can be identified. The antiphase boundary configurations in Ni₃Mo lead us to introduce the concepts of a "dissociated antiphase boundary". The domain formation due to small displacements of atoms or ions in the interstices of the structure are discussed in the cases of niobium ditelluride and in the manganese spinels. In this last class of compounds it is assumed that the domains result from the cooperative Jahn-Teller effect. Finally, we also describe the domain structure which results from the ordering of structural vacancies in pyrrhotite.