Abstract
Crystalline materials can act as hosts to specific types of interruption of their ordered arrangements. The topological nature of such admissible discontinuities in the bulk, or on the external surface of crystals, or at interfaces in bicrystals can be established by means of symmetry arguments. Theory indicates that dislocation, disclination and dispiration lines can arise in the bulk of crystals, and also domain boundaries can occur in non-holosymmetric crystals. On surfaces, facet junctions, steps, and demisteps can arise, and also surface dislocations and domain lines if surface relaxation occurs. At interfaces, the range of discontinuities that is permissible encompasses all the types of linear feature than can arise at external surfaces and in the bulk. The object of the present paper is to illustrate distinctive types of discontinuities in hexagonal materials, particularly h.c.p. metals and wurtzite-structure crystals.