Order at a Distance

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Science  26 Feb 2010:
Vol. 327, Issue 5969, pp. 1060-1061
DOI: 10.1126/science.327.5969.1060-c

The classic view of crystallization is the addition of atoms, colloids, or other small segments to a growing ordered crystal, where anisotropies in the growth are caused by differences in the thermodynamic or kinetic influences at different crystal faces. For some systems, such as metal oxides, nanocrystals are believed to form ordered aggregates through a fusion process. It has been proposed that these primary crystals form a crystallographically aligned aggregate, even though each crystal unit is separated from the other by solvent. Yuwono et al. observe this process in the slow growth of crystals in solutions of ferrihydrite nanoparticles, using cryogenic transmission electron microscopy to preserve the structure of the sample at various ages. They observe the slow aggregation of primary crystals, with the formation of rodlike assemblies, which they speculate are composed of oriented goethite nanocrystals. They also note occasional twinned aggregates in which the angle at the junction matches that observed in a twinned goethite crystal. Under the beam intensity required for high-resolution imaging, the water between the crystals sublimes and the authors are able to see the crystallographic alignment of each of the primary units.

J. Am. Chem. Soc. 132, 10.1021/ja909769a (2010).

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