XRPD analysis of karrooite-like metastable nanocrystalline phases

A coprecipitated xerogel with Ti:Mg atomic ratio of ∼1.3:1 was annealed for 3 h, at constant temperature in the 500-1100°C range, at 100°C intervals. Rietveld refinement of the XRPD scans of the products points to the formation of karrooite-like metastable structures, with cell parameters, atomic positions and site occupancies that greatly deviate from those of conventional karrooite (MgO 2TiO₂). The chosen precursor composition yields a mixture of stoichiometric geikielite (MgO TiO₂) and nonstoichiometric karrooite with Ti:Mg ratio (2 - x):1; the x value calculated from the Mg and Ti occupancies found by the Rietveld refinement was 0.2 for thermal annealing at 600°C and reduced nearly to zero for annealing above 1000°C; the amount of geikielite increased as the annealing temperature increased. Excellent agreement was found between X-ray quantitative and structural analyses, Line broadening analysis shows considerable decrease of the microstrain as well as increase of crystallite size associated with the rise of the annealing temperature.