First-principles calculations for stability of atomic structures of Al-rich AlX (X=Sc–Zn) alloys, including AlMn quasicrystal: II. Medium-ranged interactions of X pairs in Al

The AlMn quasicrystal (QC) is a metastable phase with quasiperiodic arrangement of Mackay icosahedrons (MIs) (Al42Mn12, a vacancy at the center) and is stabilized by the the addition of Si. We give qualitative discussions for the micromechanism of the stability of the atomic structures of MI in AlMn QC, as well as the Al-rich AlX (X=Sc, V, Cu, Zn) alloys, by using the ab initio calculations for point defects in Al(fcc), where the minority elements in the AlX alloys are considered as impurities. We show: (1) the atomic structure of MI may be stabilized by the medium-ranged interaction of Mn pairs, which is strongly attractive around the interatomic distance of 4.9 Å, which is close to the observed interatomic distance for the first-neighbor Mn–Mn pairs in MI. (2) Magnetism plays an important role around the interatomic distance 7.5 Å, which is close to the interatomic distance of second-neighbor Mn–Mn pairs in MI. We also discuss the effect of a vacancy and Si impurities in MI.