Direct observation of domains and discommensurations in Mn-Si-Al octagonal quasicrystal by transmission electron microscopy

Abstract

An alloy with nominal composition Mn80Si]&A15 was obtained by melting high-purity manganese, silicon, and aluminum in argon atmosphere in an arc furnace. Quasicrystal thin films were then prepared by remelting and rapid solidification of the alloy by the piston-and-anvil method as reported previously [7]. Specimens for TEM were prepared by ion milling the thin films. A Philips EM 420 electron microscope was employed for taking electron-diAraction patterns and images.The Mn-Si-Al octagonal quasicrystal is a two-dimensional quasicrystal with a periodic eightfold axis (A8) and sixteen orthogonal twofold axes (e.g. , A2P and A2D), as shown in Fig. 1

Structural model for octagonal quasicrystals derived from octagonal symmetry elements arising inβ-Mncrystallization of a simple monatomic liquid

“…A detailed comparison between the simulated pattern for this structure to one of the highest quality electron-diffraction images measured to date 19 can be found in Fig. 9.…”

“…In one, we tried to optimize the fit to the experimental diffraction pattern. 19 In doing this, we assumed perfect 8 3 screws at each vertex and dumbbells centered at the edges; we then varied the internal distances in and rotations of these elements, while making sure reasonable interatomic distances were preserved. We will call the result of this the idealized decoration.…”

“…Two different structure models, those of Kuo et al 15,18,19 and Hovmöller et al 20 have been proposed making use of the apparently close structural relationship between the octagonal quasicrystals and the ␤-Mn structure. Both models extract square and rhombus units from the ␤-Mn structure, and then use these as tiles in an octagonal tiling, the difference being in the definition of the square and rhombic units.…”

Comparison of quantum spin Hall states in quasicrystals and crystals

“…Without classical crystallographic restriction, quasicrystals may exhibit rotational symmetries that are forbidden in conventional crystals [2]. For example, an eightfold rotational symmetry in octagonal quasicrystals was observed in CrNiSi, VNiSi, and MnSiAl alloys [3][4][5]. Due to the lack of periodicity, quasicrystals cannot be constructed from a single unit cell as for periodic crystals.…”

Discommensurate microstructures in phason-strained octagonal quasicrystal phases of Mo-Cr-Ni

“…While the symmetry of the Mn-Si-Al quasicrystal is eightfold [4], the symmetry of the Mo-Cr-Ni phase is fourfold (Fig. 1).…”