Structural, thermodynamic, and kinetic aspects of disordering in the pseudobrookite-type compound karrooite, MgTi ₂O₅

The pseudobrookite-type compound karrooite (MgTi₂O₅) has been studied to understand how cation disordering, coupled with tight constraints of symmetry, affects its structural and thermodynamic properties and to address the kinetics of ordering. X-ray powder diffraction has been used to study the changes in lattice parameters of quenched samples at room temperature and in situ at temperatures between 973 and 1473 K. Enthalpies of annealing have been measured, using transposed-temperature-drop calorimetry, on two sets of quenched samples having different degrees of disorder. An empirical model that determines the degree of cation disorder from the c lattice parameter, along with the enthalpy data, has allowed assessment of various thermodynamic models of disordering. The data suggest the need to incorporate a term representing changes in the vibrational heat capacity with increasing disorder. The sigmoidal change in lattice parameters with increasing quench temperature results from slow ordering at T < 973 K and from unquenchable cation distributions (i.e., extremely rapid ordering) at T > 1273 K. The activation energy of ordering at T < 973 K is 212 ± 10 kJ/mol