Pseudobrookite-type titanate has two types of oxide octahedra for different cation types. The cation distributions in the octahedra of three pseudobrookite type oxides, Al
2TiO
5, MgTi
2O
5 and Fe
2TiO
5, and in a solid solution of 0.5Al
2TiO
5–0.5MgTi
2O
5 were studied by the Rietveld method. The analysis revealed that the cation distributions of Al
2TiO
5 and Fe
2TiO
5 did not depend on the annealing temperature, whereas that of MgTi
2O
5 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 Al
2TiO
5 and Fe
2TiO
5 were almost the same as that of the maximum value of the random distribution of cations. Although the configuration entropy stabilized Al
2TiO
5 thermodynamically, the decomposition of Al
2TiO
5 should be attributed to the stable nature of corundum, and the rather “stretched” structure of Al
2TiO
5 would also cause its instability at low temperature.
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