Quasicrystal growth observed in the lab

First spotted in 1982 by Dan Shechtman, quasicrystals have a subtle long-range order that enables them to evince rotational symmetries never seen in conventional crystals. (See Physics Today, December 2011, page 17 .) When a quasicrystal grows, it incorporates material in small bits. For decades, theorists have been working to understand how the local incorporation and long-range order can be compatible. Now a team led by the University of Tokyo’s Keiichi Edagawa has imaged the quasicrystalline alloy Al_70.8Ni_19.7Co_9.5 as it adds on new material. They found that the growth process includes episodes in which the quasicrystal’s 10-fold rotational symmetry threatened to become lost. But during those episodes, the quasicrystal became strained; the molecular rearrangements that relaxed the strain defused the threat. Edagawa and colleagues reached those conclusions from a geometric analysis of images such as the one here, obtained with high-resolution transmission electron microscopy (HRTEM). The A and B regions both include quasicrystal, but the material in B is invisible to HRTEM because none of its symmetry axes are aligned with the HRTEM beam. The figure to the right reproduces the locations of the atoms in the HRTEM image. Nearest neighbors are connected to show the tiling structure of the quasicrystal. As time passed, the quasicrystal in region A incorporated material from region B, occasionally in a manner incompatible with the quasicrystal’s rotational symmetry. But then atoms shifted, the tiling structure subtly rearranged, and symmetry-preserving growth followed. For quasicrystals at 1183 K, that correction process typically took about 1 s. But cooling the material a mere 60 K increased the repair time by more than a factor of 10. (K. Nagao et al., Phys. Rev. Lett. 115, 075501, 2015, doi:10.1103/PhysRevLett.115.075501 .)