Stable Pyramids

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Science  22 Aug 2008:
Vol. 321, Issue 5892, pp. 1020-1021
DOI: 10.1126/science.321.5892.1020d

Lead sulfide commonly crystallizes into cubes, dominated by the stability of the {100} faces, with the occasional formation of an octahedron or tetrahedron that involves the growth of {111} faces. Though other growth faces such as the {113} are possible, their much lower stability means that they are not observed during fast crystallization processes. Fan et al. combined a toluene solution of lead diethyldithiocarbamate with an aqueous solution containing a slight stoichiometric excess of sodium sulfide. At the interface, they observed the slow growth of upright pyramids of PbS with only {113} facets. The pyramids were uniform in size and randomly distributed across the water/toluene fluid interface, as a result of the capillary and van der Waals forces overcoming the intrinsic dipolar repulsion of the highly charged crystals. The authors believe that at the interface, an emulsive supersaturated layer forms, which serves as the growth medium for the pyramids and also limits the number of growth directions. They found that the height of the toluene column was critical in determining the type of crystal that formed, thus offering an additional variable to consider for the growth of kinetically unfavored crystal habits. — MSL

J. Am. Chem. Soc. 130, 10892 (2008).

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