The direct study by electron microscopy of crystal lattices and their imperfections

Abstract

Thin crystals of copper and platinum phthalocyanine have been studied in the transmission electron microscope. The (20$\overline{1}$) planes of the crystal lattice have been resolved and the distance between them (12$\cdot $0 $\mp $ 0$\cdot $2 angstrom) is in close agreement with the X-ray value of 11$\cdot $94 angstrom. Imperfections in the lattice have been directly observed. Dislocations have been photographed including both complex arrays and unit edge dislocations. Unit steps 12 angstrom high have been observed on the edge of a crystal. In a slightly deformed crystal the deformation of the (20$\overline{1}$) planes corresponds geometrically to the deformation of the surface of the crystal as would be expected with elastic deformation. One crystal displaying a feature resembling an incipient cleavage has been observed. The fracture appears to be displaced laterally from one cleavage plane to its neighbour as it traverses the crystal. The mechanism of the formation of the image is discussed in terms of the Abbe theory of image formation in the optical microscope. The image of the planes is formed as a result of interference between the zero-order and first-order spectrum from the (20$\overline{1}$) 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.