RIETVELD REFINEMENT WITH XRD AND ND: ANALYSIS OF METASTABLE QANDILITE-LIKE STRUCTURES

Binary oxides of Mg and Ti were prepared by the sol-gel technique in various Mg/Ti atomic ratios, r. Metastable phases, produced between 873 and 973 K from xerogels with Mg/Ti atomic ratios in the range ~1.1 < r ≤ 2, were studied by XRD and ND. XRD analysis showed crystal structures similar to that of Mg2TiO4 (qandilite), an inverse spinel of cubic structure. The composition range where this occurred did not correspond to those of the conventional equilibrium phase diagrams. The data were processed by the Rietveld method. ND data led to improvements in the refinement of the atomic occupancies. Assuming some degree of tetragonal distortion was effective only in ND analysis. It was found by ND that the charge balance is kept only for a certain extent of mixed occupancy of the octahedral sites, leading to a molecular formula Mg2-2xTi1+xO4, were 0 < x < 0.25. Using this formula as a constraint in the Rietveld refinement with XRD improved the Rietveld refinement. An important contribution of XRD was in rejecting any attempt to introduce Ti in the tetrahedral sites.