ATOMIC STRUCTURE, GROWTH, AND FACETING OF ICOSAHEDRAL QUASICRYSTALS. Vladimir E. Dmitrienko and S. B. Astaf'ev A. V. Shubnikov Institute of Crystallography, 59 Leninski prospekt, 117333, Moscow, Russia

A realistic mechanism of the three-dimensional atomic growth is developed and simulated for icosahedral quasicrystals. Instead of large icosahedral clusters, previously used for such simulations, the presented mechanism exploits the atomic motives found in related crystalline structures which are called crystalline approximants. The simplest model includes atoms of two sizes, Large and Small, and the ratio of atomic radii is equal to that one in the CsCl-type structures when the S-atom in the centre of the cubic unit cell just fits the hole between L-atoms. This ratio supports the Dodecahedral Local Ordering (DLO) of atoms. The ideal DLO means that every S-atom has its closest neighbours (L-atoms) positioned at the vertices of a regular pentagon-dodecahedron, and vice versa. In crystals, the DLO is exemplified by the CsCl (B2), FeSi (B20), Hg (A10) and many other structures including also the approximants with large unit cells. To suppress the growth of crystalline DLO-structures, the oscillating interatomic potentials, typical for metallic alloys, are used. More sophisticated models include atomic positions with icosahedral coordination, phasonic relaxation, etc. The grown clusters (up to 2 million atoms) have narrow Bragg peaks and rather pronounced faceting. Some preliminary result have been published elsewhere (V.E.Dmitrienko and S.B.Astaf'ev: Phys. Rev. Lett. v.75, 1538-1541, 1995). The report will be accompanied with computer demonstrations.