PerspectiveMaterials Science

Solving Amorphous Structures—Two Pairs Beat One

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Science  24 Feb 2012:
Vol. 335, Issue 6071, pp. 929-930
DOI: 10.1126/science.1218723

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Diffraction data can be used to determine atomic structures by creating and refining a structural model. If the calculated diffraction pattern is in sufficiently good statistical agreement with the data, we trust that a unique structure has been found. For single crystals, the data are well-defined diffraction spots that represent reflections off the repeating lattice of atoms. For amorphous materials, the data are transformed into a radial distribution function (RDF) that provides the distribution of interatomic distances, typically over the range from 1 to 10 Å. In such cases, structural models can be tested against the RDF by analyzing the distribution of distances between random pairs of atoms. It is tempting to treat a model that has good agreement with the RDF as a unique structure, but on page 950 of this issue, Treacy and Borisenko (1) show that we cannot always rely on the RDF to fingerprint the correct medium-range structure (5 to 30 Å) of a material that is disordered on the nanoscale. They show that more than one structure, including some that resemble crystals in their topology, can equally well fit the RDF of amorphous silicon (a-Si).