Thin crystals of copper and platinum phthalocyanine have been studied in the transmission electron microscope. The (20T) planes of the crystal lattice have been resolved and the distance between them (12.0 + 0.2 A) is in close agreement with the X-ray value of 11.94 A. Imperfections in the lattice have been directly observed. Dislocations have been photographed including both complex arrays and unit edge dislocations. Unit steps 12 A high have been observed on the edge of a crystal. In a slightly deformed crystal the deformation of the (20T) planes corresponds geometrically to the deformation of the surface of the crystal as would be expected with elastic deformation. One crystal displaying a feature resembling an incipient cleavage has been observed. The fracture appears to be displaced laterally from one cleavage plane to its neighbour as it traverses the crystal. The mechanism of the formation of the image is discussed in terms of the Abbe theory of image formation in the optical microscope. The image of the planes is formed as a result of interference between the zero-order and first-order spectrum from the (20T) planes. The very high resolution arises from the fact that the diffracted beam from a small crystal traverses a very narrow zone of the objective lens so that the effect of spherical aberration is not severe. Experiment has confirmed the general validity of this approach. It is suggested that this method may be extended to the study of crystals of even smaller lattice dimensions than the phthalocyanines, making possible the direct study of imperfections in a wide range of materials in relation to properties known to be affected by them such as strength, plastic flow and fracture.
The direct resolution of a crystal lattice has already been obtained in the electron microscope, with lattice spacings down to about 7 Å. Although a metal lattice has not so far been resolved in this way, a method has been developed to allow this to be carried out in an indirect manner, by means of moiré patterns. In this way, indirect resolution of crystal planes of even less than 1Å in spacing may be obtained, and the atomic array in metal lattices may be studied. The moiré patterns are obtained on transmission electron micrographs from pairs of overlapping single-crystal films of metals, prepared in a suitable manner. The mode of formation of these moiré patterns has been interpreted in terms of the kinematical theory of electron diffraction, and the relation between the patterns and the individual lattices has been considered. If a dislocation line passes through one of the metal crystals, it may be readily detected on the moire pattern. It is shown that both edge and screw dislocations in one of the crystals will give rise to terminating half-lines in the moiré pattern: the number of these terminating half-lines has been deduced, for the f. c. c. system, for any general dislocation line. The results are completely consistent with the observations, and several examples of dislocations are illustrated in the paper. Partial dislocations and stacking faults are also observed, and a full interpretation of these is given. The moiré patterns have also been used to record dislocation movements. The smallest periodicity which we have so far succeeded in resolving in the electron microscope is a moiré pattern of spacing 5.8 Å, obtained from overlapping nickel and gold crystals. The potentialities and limitations of the technique of moiré patterns are discussed in relation to their application to various problems in the metal physics of thin films, with particular reference to the study of lattice imperfections and their effect on mechanical properties.
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