Thermodynamics and structural chemistry of compounds in the system MgOTiO₂

Thermochemical and crystallographic properties of MgTi₂O₅, MgTiO₃, and Mg₂TiO₄ have been studied to characterize stability relations, structural variations, and order-disorder phenomena. Enthalpies of formation, decomposition, and order-disorder transitions were determined by high-temperature solution calorimetry and transposed-temperature-drop calorimetry on synthetic powders. X-Ray lattice parameter measurements and Rietveld refinements of neutron diffraction data were used to evaluate intracrystalline cation distributions and their variation with quenching temperature. MgTiO₃ is the most stable phase and apparently retains a fully ordered MgTi distribution to at least 1673 K. Both MgTi₂O₅ and Mg₂TiO₄ are stable only at high temperature because of the configurational entropy arising from cation disorder. The disorder in MgTi₂O₅ appears to vary continuously throughout the range 773-1373 K and is accompanied by changes in lattice parameters. Mg₂TiO₄ undergoes a cubictetragonal transition at 933 ± 20 K involving the appearance of long-range order on octahedral sites. However, thermochemical evidence suggests that MgTi octahedral short-range order changes gradually, perhaps over an interval of several hundred degrees. Models for describing the order-disorder and accompanying enthalpy changes are discussed.