On the precipitation of coherent spinel nanoparticles in Ti-doped MgO

The Mg2TiO4/MgO composites prepared by reactive sintering of MgO and TiO2 powders (9:1 molar ratio) at 1600 °C and then air-cooled or further aged at 900 °C were studied by X-ray diffraction and analytical electron microscopy in order to characterize the microstructures and formation mechanism of nanosized Mg2TiO4 spinel precipitated from Ti-doped MgO. Expulsion of Ti4+ during cooling caused the formation of (001)-specific Guinier–Preston zone under the influence of thermal/sintering stress and then the spinel precipitates, which were about 30 nm in size and nearly spherical with {111} and {100} facets to minimize coherency strain energy and surface energy. Secondary nanosized spinel was precipitated and became site saturated during aging at 900 °C, leaving a precipitate-free zone at the grain boundaries of Ti-doped MgO. The intergranular spinel became progressively Ti-richer upon aging at 900 °C and showed 〈110〉-specific diffuse scatter intensity likely due to short-range ordering and/or onset decomposition.