Cation distribution of pseudobrookite-type titanates and their phase stability

Abstract

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.