Z phase is one of the three basic units by which
the Frank−Kasper (F–K) phases are generally assembled.
Compared to the other two basic units, that is, A15 and C15 structures, the Z structure is rarely experimentally observed because of
a relatively large volume ratio among the constituents to inhibit
its formation. Moreover, the discovered Z structures
are generally the three-dimensional ordered Gibbs bulk phases to conform
to their thermodynamic stability. Here, we confirmed the existence
of a metastable two-dimensional F–K Z phase
that has only one unit-cell height in the crystallography in a model
Mg–Sm–Zn system, using atomic-scale scanning transmission
electron microscopy combined with the first-principles calculations.
Self-adapted atomic shuffling can convert the simple hexagonal close-packed
structure to the topologically close-packed F–K Z phase. This finding provides new insight into understanding the
formation mechanism and clustering behavior of the F–K phases
and even quasicrystals in general condensed matters.