Nanocrystalline powders of binary oxides were prepared by the sol-gel technique and studied mostly by powder X-ray diffraction. Some of the materials are single metal oxides doped with a second cation, other were prepared as non-stoichiometric binary oxides. The following oxides systems were studied: Mg-Ti and Zr-Al. The oxides were obtained from xerogels calcined for 3 to 6 hours, at various temperatures in the range between 200 and 1600°C. The sol-gel method was carried out in two different ways: either by the action of superheated steam on metallorganic precursors dissolved in an aprotic solvent or, or by precipitation taking place on raising the pH of aqueous solution of salts. For each composition there was a minimum temperature for the formation of oxides, however, in many cases the oxides were metastable nanocrystalline phases. The existence of these metastable phases is strongly dependent on the grain size and the composition. In oxide systems such as those of Mg-Ti and Zr-Al, the phases formed at low temperatures (below 900˚C) are significantly different than the high temperature products. New ranges of solid solution and formation of high temperature structures were found. A model which correlates the size effect with the unusual phases and structures is proposed

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