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Ultrafine iron oxide particles with a mean diameter of 6.9 nm and a very narrow size distribution have been studied by electron microscopy, Mössbauer spectroscopy, and X-ray diffraction. Electron micrographs and electron diffraction patterns show that the particles form ‘super crystals’, that is, three-dimensional ordered close-packed structures, with a nearest neighbour distance of 9.1 nm.
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Bentzon et al. (1989) studied this question.
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