ON THE MORPHOLOGY OF INCLUDED CRYSTALS ; BICRYSTALLINE KALEIDOSCOPES

Abstract
The morphology of one crystal entirely surrounded by a different one, referred to here as an included crystal, is considered in this paper. It is shown that the morphology of such an included crystal can be profoundly affected by the presence of interfacial defects. On the basis of the Principle of Symmetry Compensation, it is shown that a cavity in the matrix crystal, which is bounded by degenerate surfaces, can always be filled by a contiguous included crystal in such a way that the initial surfaces become degenerate interfaces, provided the geometrically necessary defects are present. These defects can be predicted using a recently developed theory /4/. Included crystals of this form exhibit symmetry consistent with that of the matrix crystal, and such morphologies are referred to as being fully compensated. Morphological forms exhibiting lower symmetry may also occur, and require fewer defects to be present. Experimental observations are presented illustrating an example of a fully compensated morphology, and other lower symmetry forms. It is also pointed out that the free energy of a system containing an included crystal with a given volume is not necessarily minimised for a defect free configuration, because the presence of defects can enhance the area of low energy interface.