It was pointed out that a hidden icosahedral symmetry of the tetrahedral interatomic bonds was to be found in the BC8-Ia¯3 phase (where 75% of all bonds were directed almost exactly along 5-fold icosahedral axes). It was shown that this phase could be considered to be a crystalline approximant to a hypothetical icosahedral quasicrystal with a body-centered 6-dimensional lattice. All of the atomic positions in BC8 crystals could be obtained as projections from the 6-dimensional lattice. The diffraction pattern of the BC8 phase exhibited the hierarchical organisation which was typical of both approximants and quasicrystals. A specific type of topological disorder which was related to phason flips was suggested. An experimentally observed BC8-to-R8 phase transition could be considered to result from coherent phason flips of this type. The dual relationships with approximants of conventional quasicrystals were used to construct a higher-order approximant without dangling bonds.

Icosahedral Order and Disorder in Semiconductors V.E.Dmitrienko, M.Kléman: Philosophical Magazine Letters, 1999, 79[6], 359-67