Dielectric absorption observed in polycrystalline pentachloronitrobenzene has been interpreted as due to orientational disorder of the polar molecules in the lattice as was revealed by the X-ray crystal structure analysis; the space group is R3 and the apparent symmetry of the molecule is 6. A molecule can take one of the three possible orientations of equal potential energy in the lattice at random, with an axis of rotation perpendicular to the plane of the molecule. The crystal of p-chloronitrobenzene, which is known also as in a disordered state (Mak and Trotter, Acta Cryst., 15, 1078 (1962) ), has been found to show dielectric absorption in the low frequency region. The molecule seems to make a complete turnover in the lattice, under an applied electric field, from one of the equilibrium position to the other which is 180° apart from the first with a potential barrier of appreciable magnitude between them. A brief mention has been made of Frohlich's theory of the dielectric absorption based on the disordered arrangement of the dipoles in the crystalline lattice.
By use of high resolution electron microscopy the arrangements of heavy atoms within the unit cells have been visualized. It has increasingly become a powerful means, in particular, for studying crystal defects occurring at unit cell level. Feasibility of an intuitive interpretation of the lattice images, when the crystals are less than 100A thick, has been verified theoretically by n-beam dynamical treatment for electron diffraction and by taking into account of various electron optical parameters. Such calculations of the image contrast of crystals will provide more details of defect stru-tures derived from lattice images. An application of this new technique to the study of crystal defects in various non-stoichiometric niobium oxides is described.
A general introduction of the disclination is given. Many micrographic examples and bubble raft models of disclinations are used as illustrations. Definitions and historical aspects of the subject are reviewed in the first section. Disclination is defined as a line defect in the rotational symmetry element of the crystal lattice. It is an old brother of dislocation only recently drawing attention. As discussed in section 2, disclination occurs in (1) small or thin crystals, (2) crystals with unquantized elements, (3) the pattern with weak interaction force. It also occurs as (4) imperfect disclination or (5) dipole of disclinations. Structure of disclinations is described in section 3 using bubble raft model and electron micrographs. Stresses of the disclinations calculated by linear elastic approximation are presented. Various configuration of disclinations are considered in section 4. Growing tip of a deformation twin are approximated by a dipole of partial disclinations, while an array of the disclination dipoles are proposed by Li as a model of grain boundary structure in metals. A densely packed state of disclinations may correspond to the liquid state. The motion of disclinations are analysed in section 5. A wedge disclination moves by generating or absorbing dislocations, and a disclination dipole moves by exchanging dislocations between the two. A grain boundary consisting of an array of disclination dipoles migrates by the same process.