Cation distribution of pseudobrookite-type titanates and their phase stability

Pseudobrookite-type titanate has two types of oxide octahedra for different cation types. The cation distributions in the octahedra of three pseudobrookite type oxides, Al2TiO5, MgTi2O5 and Fe2TiO5, and in a solid solution of 0.5Al2TiO5–0.5MgTi2O5 were studied by the Rietveld method. The analysis revealed that the cation distributions of Al2TiO5 and Fe2TiO5 did not depend on the annealing temperature, whereas that of MgTi2O5 depended on the annealing temperature. The estimated cation distributions determined by the Rietveld method agreed with those determined by bond valence sum estimation. The configuration entropies of Al2TiO5 and Fe2TiO5 were almost the same as that of the maximum value of the random distribution of cations. Although the configuration entropy stabilized Al2TiO5 thermodynamically, the decomposition of Al2TiO5 should be attributed to the stable nature of corundum, and the rather "stretched" structure of Al2TiO5 would also cause its instability at low temperature.