XRPD analysis of stable and metastable magnesium titanate phases

Three stable stoichiometric phases derived from magnesia and titania are known: Geikielite (magnesium metatitanate, MgO.TiO₂) is formed from conventional periclase and anatase powders above 600°C; qandilite (magnesium orthotitanate, 2MgO.TiO₂) and karrooite (MgO.2TiO₂) are formed from conventional powders only above 1200-1300°C, depending on whether MgO or TiO₂ is in excess, however, at such temperatures titania appears as rutile. Nanometric precursors that were prepared by sol-gel methods consist of hydroxides of magnesium and titanium, which lose water on heating to about 400°C, yielding nanocrystalline oxides. Geikielite, qandilite and karrooite are formed from coprecipitated precursor powder at about 600°C, depending on the relative stoichiometry of magnesia and titania in the precursor. However, from about 700°C up to about 1200°C qandilite and karrooite decompose, leaving geikielite as the only stable titanate, and they reappear as stable phases at about 1300°C or higher temperatures, depending on the stoichiometry of the original mixture. The XRPD analysis underlying the research of the thermal behavior of the nanocrystalline magnesia-titania system with Mg:Ti atomic ratio of 1.73 is discussed, including stuctural and quantitative phase analysis, and estimation of crystallite size and microstrain. The Reitveld refinement was applied to the crystalline components found in this work. Only nanocrystalline qandilite was obtained at 600°C, showing Mg relative ocupancy of 0.85 at both Mg sites of the spinel structure, while for nanocrystalline qandilite residuals in the 800-1100°C range, and ordinary qandilite obtained at ≥ 1200≥C relative ocupancy was full. Microstrain and crystallite size were calculated by the Willamson-Hall method. The qandilite obtined at 600≥C had 7 nm size and microstrain of 0.3%; on heating this material to 800°C the size increased to 40 nm and the microstain was reduced to 0.1%.