New group of stable icosahedral quasicrystals: structural properties and formation conditions

摘要

Structural studies on the icosahedral quasicrystals in Zn-Mg-Sc, Cu-Ga-Mg-Sc,and Zn-Mg-Ti alloys as well as their corresponding 1/1 cubic approximants, haverevealed that these quasicrystals belong to a new structural group similar toCd-based quasicrystals. This group is characterized by a triple-shellicosahedral cluster different from both Mackay- and Bergman-types. The presenceof the atomic cluster has been deduced from the structure model of theapproximant crystal, Zn17Sc3, in which the clusters are embedded in a periodicnetwork of so-called "glue atoms". Density measurement suggested the presenceof at least 2.7 Zn atoms in the first shell of the cluster in this approximant.The substitutional relationship in these three quasicrystals indicates theimportant role of Hume-Rothery rule for the formation of this type ofquasicrystal. The occurrence of a P-type icosahedral quasicrystal in Zn-Mg-Yballoy is also reported.