Materials Science

Order of a Sort

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Science  14 Jan 2011:
Vol. 331, Issue 6014, pp. 127
DOI: 10.1126/science.331.6014.127-c
CREDIT: HIRATA ET AL., NAT. MATER. 10, 28 (2011)

In amorphous solids and liquids, there is no long-range translational or rotational ordering of the atoms, whereas in crystalline materials, the positions of all the atoms can be ascertained by symmetry from knowing the positions of only a few of them. In between these extremes are glassy materials, which have short- and medium-range order that spans from nearest neighbors to about 100 nm. The nature of this ordering has been a mystery and is very hard to probe experimentally, because any interference peaks that might be generated from a small volume are smeared out under normal diffraction conditions. Hirata et al. studied the structure of a Zr2/3Ni1/3 metallic glass using nanobeam electron diffraction combined with ab initio molecular dynamics simulations. The electron beam could be tuned in size down to a minimum diameter approaching 0.3 nm. At a diameter of 0.72 nm, the authors were able to see distinct diffraction spots; these took on a twofold symmetry similar to that of a crystal when probed with a 0.36-nm-diameter beam. Voronoi polyhedra were used to design a local atomic environment from which the authors were able to compare their diffraction data with simulations, including the case of a pair of clusters that shared a common face. The overlapping of these data appears to confirm the recently proposed cluster model for the description of short- and medium-range order in a metallic glass.

Nat. Mater. 10, 28 (2011).

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