Three‐dimensional arrays of SiO2 nanometer particles lead to Bragg diffraction effects of visible light—as seen for natural opals, see also this issue's cover—and applications such as photonic bandgap materials. Teh fabrication of the opalline structures is described and details are given of how to obtain ordered compacts. The Figure shows a fracture surface of a sintered sample comprising 390 nm‐diameter silica spheres. magnified image

It is well-known that stacking of hard spheres results in close-packed structures. However, until recently, it was not clear which of the various possible phases (cubic, hexagonal, mixed, or random) was the stable one. We have performed a microscopy characterization of solid crystals made of monodisperse SiO2 nanometric spheres. It was found that, for a wide range of particle diameters, the cubic phase is the only one present. This largely serves to confirm recent theoretical calculations by L. V. Woodcock which conclude that the cubic phase is the most stable one. This opens new prospects in the application of colloidal crystals to photonic band gap engineering.

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