Abstract
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.
Highlights
Frank-Kasper phase is a class of ordered crystals composed of the topologically close-packed atomiclayers [1, 2], in which the atoms with a smaller radius form as the close-packed planes and the atoms with a larger radius embed in the tetrahedral gaps
We confirmed the existence of a metastable two-dimensional (2D) Frank-Kasper Z phase with one unit-cell height in the crystallography in a model Mg-Sm-Zn system, by using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) combined with density functional theory (DFT) calculations
Since the Al-Mn icosahedral quasicrystal discovered in 1984 by Shechtman et al [12], the complex architectures of FrankKasper phases are considered as a link between the traditional simple periodic structures and quasicrystals [10, 13]
Summary
Frank-Kasper phase is a class of ordered crystals composed of the topologically close-packed atomiclayers [1, 2], in which the atoms with a smaller radius form as the close-packed planes and the atoms with a larger radius embed in the tetrahedral gaps. The Frank-Kasper phases can be considered as ordered approximates of quasi-periodic crystals due to some shared construction rules from atomic clustering and stacking [11].
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