High-resolution study of incoherent twin boundaries and of isolated wedge microtwins in rare-earth monoclinic sesquioxides (Ln2O3-B)

It has been shown previously by Schiffmacher, Caro and Boulesteix (1976) that the incoherent twin boundaries of monoclinic rare-earth sesquioxides are made of a succession of short coherent boundaries separated by small incoherent steps. This result is confirmed in more detail by the present work, where the high-resolution method of observation was used with the electron beam parallel to the pseudo-hexagonal crystallographic axis. We show that three kinds of steps can occur, whose heights are equal to the period of the coincidence lattice or to the half of this value. In the latter case each step acts as a dislocation. In the case of isolated wedge microtwins the piling up of steps acts as an edge dislocation wall, i.e. a disclination.