A transition phase can be defined as a structure formed at relative low temperature, irreversibly transforming at higher temperature into an equilibrium phase of the same elemental composition  Among the transition aluminas, gamma-alumina, usually produced from boehmite, is an extremely important technological material. It is easy to produce gamma-alumina by sol-gel technique. This alumina, which is formed by firing the xerogel at temperature range between 500 to 800 °C, has a nanocrystalline cubic spinel-like structure entirely different from the rhombohedral hR10 structure of alpha-alumina, that  formed by thermal treatment at higher temperatures. This work deals with nanocrystalline M₂O₃ oxides formed by firing xerogels at relatively low temperatures.  The following nanocrystalline binary oxide systems were studied: MgO-TiO₂, ZnO-TiO₂,  NiO-TiO₂,  ZrO₂-Al₂O₃, and Y₂O₃-Ln₂O₃, and Y₂O₃-Ln₂O₃ (Ln = rare earth element). Some pure or doped unary oxides were also studied. The xerogels were heated at constant T (200 to 1600 °C) for 3 to 6 hours. There was a threshold temperature for oxide formation and in many cases the products were transition nanocrystalline phases, depending on grain size and composition, including doping.  The binary MM’O₃ oxide phases of NiTiO₃, MgTiO₃, and ZrTiO₃ are ilmenite type structures with rhombohedral hR10 structure similar to alpha-alumina, however since they have more than one component it is possible to study nanocrystalline solid solutions.  It was found that firing the xerogels at temperature range between 300 and 600 °C transition nanocrystalline phases were obtained.  The phases have several cubic types.  In contrast to the ilmenite-like compounds (NiTiO₃ for example) which are stoichiometric, the transition phases exist at wide range of non-stoichiometric compositions. A model correlating the size effect with the unusual solid solutions and structures is proposed, correlating the size effect with the structure and enhanced solubility.

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