The direct observation of imperfections in crystals

A review is given of the various methods which are now available for the direct observation of lattice imperfections in crystals. After a brief historical introduction a summary of some of the more important properties of lattice imperfections is given, with particular emphasis on properties which allow the imperfections to be made visible. Optical methods for revealing dislocations are described briefly, including studies on growth spirals, etching studies and internal decoration techniques. More detailed attention is given to the application of transmission electron microscopy, which is currently of such widespread interest. An outline is given of the basic diffraction mechanisms which give rise to image contrast, and various examples are given to illustrate the application of the technique. The treatment includes consideration of the formation and use of periodic images, which provide direct information on imperfections in the lattice periodicities. The various x-ray methods are described and compared with the electron microscope method. Finally, the potentialities of the field ion microscope for revealing lattice imperfections are considered, and a description is given of the manner in which dislocations are made visible by this technique.