An in situ neutron diffraction study of cation disordering in synthetic qandilite Mg 2 TiO 4 at high temperatures

Temperature-dependent cation order-disorder has been studied in many 2+ - 3+ oxide spinels but 4+ - 2+ spinels have been found to be either completely normal or completely inverse when examined at room temperature. Here we report the temperature dependence of the cation distribution in the 4-2 spinel synthetic qandilite (Mg 2 TiO 4 ) from in situ time-of-flight neutron powder diffraction experiments to 1416 °C. At room temperature, Mg 2 TiO 4 is confirmed to have completely inverse cation distribution, with Ti atoms occupying half the octahedrally coordinated cation sites. Cation disordering becomes observable above about 900 °C, with 4% of the Ti occupying the tetrahedral site by 1416 °C. The rate of reordering on cooling is fast, such that high-temperature disorder is not preserved on cooling to room temperature. The thermodynamics of the change in cation distribution with temperature can be described by an enthalpy of Mg-Ti disorder of -46.1 ± 0.4 kJ/mol.