Rotation boundaries and crystal growth in the hexagonal system

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Abstract
A rotating crystal x-ray diffraction technique has been employed for the measurement of rotation boundary angles in natural graphite crystals, and the values obtained have been compared with those predicted by coincidence boundary theory. It is shown that for natural graphite crystals these values do not correlate with theory, while previous measurements for graphite in dendritic form (Oron and Minkoff 1964) were in good agreement with predicted values. The results may be evaluated by considering attachment kinetics at an interface which has, in the case of graphite, the step of a rotation boundary of varying angle. For the dendritic growth form, which may be identified with a maximum in the growth rate, atom attachment occurs at the step of a rotation boundary tending to a maximum in coincidence. The rotation boundary values actually obtained are related to the structure and density of coincidence lattices. For graphite, atoms on the coincidence lattice are shared between the two rotated planes, and the actual density of sites is only one-half that for an equivalent rotation in the h.c.p. lattice. This leads to maximum and next maximum coincidence values in graphite, and a precise maximum in cadmium (h.c.p.). Under conditions of growth remote from those giving a maximum in kinetics, rotation boundaries may have any value.

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