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Materials

Demystifying Quasicrystal Growth

Microscopy: Growth-and-repair mechanism yields nonperiodic order

by Jyllian Kemsley
August 31, 2015 | APPEARED IN VOLUME 93, ISSUE 34

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Credit: Phys. Rev. Lett.
TEM analysis of an Al-Ni-Co quasicrystalline alloy shows that the crystal grows along a growth edge (red line) by a tiling mechanism that results in a pattern of ordered pentagons.
09334-scicon-alloyquasicrystal-690.jpg
Credit: Phys. Rev. Lett.
TEM analysis of an Al-Ni-Co quasicrystalline alloy shows that the crystal grows along a growth edge (red line) by a tiling mechanism that results in a pattern of ordered pentagons.

Ordered crystalline materials that lack periodicity are called quasicrystals. Such materials, which were first identified in the 1980s, include metal alloys, polymers, and liquid crystals. One fundamental piece of information about quasicrystals—how they grow into their particular structured forms without simply repeating a unit cell—has largely remained a mystery. Transmission electron microscopy studies now provide the first atomic-scale look at the growth of a quasicrystalline alloy of aluminum, nickel, and cobalt, reports a team led by Keiichi Edagawa of the University of Tokyo (Phys. Rev. Lett. 2015, DOI: 10.1103/physrevlett.115.075501). The alloy grows in a so-called Penrose tiling pattern with fivefold rotational symmetry. Edagawa and colleagues found that the growth occurs in part through an error-and-repair mechanism in which clusters of atoms sometimes add initially in an “incorrect” formation, then within one second rearrange into the quasicrystalline pattern. The initial addition adds strain to the quasicrystal, and the strain is relieved through the repair process, the authors say. Other metal alloy quasicrystals may grow similarly, Edagawa says, but polymeric and other materials may have different mechanisms.

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