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.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Condensed Matter Physics
- Physical and Theoretical Chemistry
- Inorganic Chemistry
- Materials Chemistry