  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, have
revealed that these quasicrystals belong to a new structural group similar to
Cd-based quasicrystals. This group is characterized by a triple-shell
icosahedral cluster different from both Mackay- and Bergman-types. The presence
of the atomic cluster has been deduced from the structure model of the
approximant crystal, Zn17Sc3, in which the clusters are embedded in a periodic
network of so-called "glue atoms". Density measurement suggested the presence
of at least 2.7 Zn atoms in the first shell of the cluster in this approximant.
The substitutional relationship in these three quasicrystals indicates the
important role of Hume-Rothery rule for the formation of this type of
quasicrystal. The occurrence of a P-type icosahedral quasicrystal in Zn-Mg-Yb
alloy is also reported.
