Abstract
The enthalpy of formation of pseudobrookite, Fe2TiO5, from Fe2O3 and TiO2 is found by oxide melt solution calorimetry to be +1.99 ± 0.15 kcal mol-1 (+8.33 ± 0.63 KJ mol-1). Thermochemical data for other compounds with the pseudobrookite structure are summarized. Pseudobrookites are “entropystabilized” high temperature phases with partially disordered cation distributions. Their decomposition at low temperatures to assemblages of binary oxides (for A23+B4+O5 pseudobrookites) or to assemblages of phases of ilmenite and rutile structure (for A2+B24+O5 pseudobrookites) reflects the balance between endothermic enthalpies of formation from these assemblages and the substantial positive entropies of formation arising from the substitutional disorder. A simple cation distribution model is applied to the distribution of iron between 4c (A) and 8f (B) sites in the FeTi2O6-Ti3O5 system, from which high Ti3O5 is predicted to have the cation distribution (Ti4+0.06Ti3+0.95)A(Ti4+0.96Ti3+1.05)BO5. Interchange enthalpies (or free energies) for the reactions: Mg2+A + Ti4+B = Mg2+B + Ti4+A, Fe2+A + Ti4+B = Fe2+B + Ti4+A, and Ti3+A + Ti4+B = Ti3+B + Ti4+A are +8.6, +13, and +10 kcal mol-1 respectively (36.0, 54, and 42 KJ mol-1, respectively). MgTi2O5 exhibits somewhat disordered pseudobrookite structures derived from a “ normal” structure stable at low temperature, while Ti3O5 and probably also Fe2TiO5, Al2TiO5, and Ga2TiO5 have somewhat disordered structures derived from an “inverse structure stable at low temperature. Thus Ti4+ appears to have a strong preference for the 8f(B) sites in all these compounds of pseudobrookite structure.