It was recalled that complex structures could be regarded as being derived from a few basic structures, via local twinning at the atomic level. Although quasi-crystals and approximants were believed to be B2 derivatives, the relevant B2 twinning mode was not known. This problem was approached by analyzing the twinning properties of Al-Cu approximants in a Cu4Al3 sample. The analysis led to the recognition that the B2 chemical-twinning mode was 180º/<111>B2 and was responsible for the pentagonal atomic configuration in Al-Cu approximants as well as for the pseudo 5-fold B2 twinning which was observed on quasi-crystal surfaces.

Chemical-Twinning Mode of the B2 Structure in Relation to Al-Cu Approximants. C.Dong, Q.Zhang, D.Wang, Y.Wang: Philosophical Magazine A, 1999, 79[3], 501-10