Observation of Substitutional Site Preference in a Quasicrystal and Implication on Local Structure

A combination of magnetic susceptibility and Mössbauer measurements on quasicrystalline $i-{\mathrm{Al}}_{74}{\mathrm{Mn}}_{20-x}{\mathrm{Fe}}_x{\mathrm{Si}}_6$ ($0.02\le x\le 7.5$) establishes that Mn atoms in $i$-${\mathrm{Al}}_{74}$ ${\mathrm{Mn}}_{20}$ ${\mathrm{Si}}_6$ occupy two distinct classes of sites, and that Fe substitutes for only one of them. Thw two classes are distinguished by the possession or otherwise of a localized magnetic moment. The data are consistent with a structure of interconnecting Mackay icosahedra (MI) in which localized moments are possessed only by Mn atoms adjacent to "broken" MI connections. The implied connectivity of the resulting MI network is close to that anticipated for a packing of MI on a three-dimensional Penrose-tile lattice.