A structural model of a face-centred icosahedral quasicrystal in Al-Mg-Li alloys is proposed. It has an ordered arrangement of two icosahedral atom clusters. One consists of 20 Mg and 24A1 and the other contains 20 Li and 24 Al. The clusters are situated at even- and odd-parity twelvefold vertices of the three-dimensional Penrose pattern, forming a face-centred icosahedral structure. The centres of the clusters are linked by three structural units: A prolate rhombohedron with two body diagonal positions occupied by Mg and Li atoms and two rhombic dodecahedra accommodating eight Mg atoms and eight Li atoms. The corresponding 1/1 crystal approximant has the Al24Mg20cluster at the origin and the Al24Li20cluster at the body centre and its space group is Pm3.
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