Structure of twist units in a Ge Σ=27 〈110〉 tilt grain boundary

General high-angle tilt grain boundaries may be described by an arrangement of repeating structural units. A particular type of defect can occur in this repeating structural-unit arrangement in a Σ=27 related 〈110〉 〈110〉 tilt grain-boundary in Ge. This defect accommodates a slight misorientation of the boundary away from the perfect tilt configuration; it introduces a twist component into the tilt grain-boundary. The defect, which can be described as a twist unit, is characterised by a short segment of (111) facet at the interface, and is associated with a screw dislocation in the boundary plane.