The Inversion Twin: Prototype in Beryllium Oxide

The inversion twin is an uncommon fault that potentially can be found in the wurtzite- and sphalerite-type crystal structures and related rhombohedral structures, all of which are hemimorphic and possess a polar axis parallel to the c-axis. The twin structure involves exact inversion of the sense of polarity across a transition region (boundary) of variable orientation and complexity. Although several examples of possible but uncertain occurrence of the twin have been noted in other materials, only in beryllium oxide crystals has it been found to occur with abundance and on a macroscopic scale. The inversion twin, as it occurs in BeO, is described in detail. Existence and geometry of the twin is evident from crystal morphology and chemical and mechanical properties. It is suggested that the twin boundary may be stabilised and its energy lowered by the presence of aliovalent impurities along the boundary. Relation of the inversion twin to other types of faults is considered briefly. Previous discussions and presentation of new data in up-to-date assessment of the inversion twin are reviewed.