Structure and composition of the nanocrystalline phases in a MgO-TiO₂ system prepared via sol-gel technique

Samples in the MgO-TiO₂ system were prepared via the sol-gel technique with titanium and magnesium ethoxides as precursors and HNO₃ as hydrolysis catalyst. The analyzed magnesia to titania weight ratios were 0.1:0.9, 0.5:0.5, and 0.9:0.1 (0.05, 0.5, and 4.0 MgO: TiO₂ molar ratios). Samples were characterized with X-ray powder diffraction, differential thermal analysis, and thermogravimetry. To quantify the concentration and the crystallography of the phases in the samples, their crystalline structures were refined by using the Rietveld method. In addition to periclase, anatase, and rutile, three intermediate compounds were observed: karooite (MgTi₂O₅), geikielite (MgTiO₃), and qandilite (Mg₂TiO₄). The formation of the above crystalline phases in the samples occurred after an exothermic reaction at 200°C, and their dehydroxylation above 300°C. Karooite was abundant only in titania-rich samples, while geikielite and qandilite concentrations were high in the samples with equal magnesia to titania weight percent. Phase concentrations depended on the annealing temperature of the sample and its MgO content. Magnesium was soluble in anatase, but not in rutile; the solubility, however, was low.