Magnesium titanate phases from xerogels by hot stage X-ray powder diffractometry

Coprecipitated xerogel precursors of nanocrystalline magnesium titanates, with Mg:Ti stoichiometric ratio varying from 1:1 to 2:1, were subjected to thermal treatment at constant temperature in the range from 550 to 1200 °C, in air, using a hot-stage X-ray powder diffractometer. The kinetics during the first hour showed dependence on temperature and the Mg:Ti stoichiometric ratio. At low temperatures, for compositions near 2:1, a single nonstoichiometric metastable nanocrystalline qandilite-like phase is formed; however, when the Mg content is lowered a solubility limit is reached, below which a nonstoichiometric qandilite, of fixed composition depending on the temperature, is in equilibrium with stoichiometric geikielite. The limit moves to higher Mg contents as the temperature rises. In the approximate 900-1000 °C range the metastable qandilite-like phases decompose into geikielite and periclase. From 1100 °C on stoichiometric qandilite is obtained in equilibrium with geikielite. At low temperatures nanocrystalline qandilite is formed much faster than nanocrystalline geikielite, probably owing to the isotropic chemical structure of both the amorphous xerogels and qandilite. A phase diagram is proposed for the metastable nanocrystalline phases formed at low temperatures in the composition range of the present study.