High-Temperature Phase Transitions Preceding the Decomposition of Mixed Valence Aluminum Titanate Pseudobrookite

Dark blue single crystals of Al1.753+Ti1.04+Ti0.253+O5 were grown with a novel synthesis method based on the reaction of a Ti3+/Ti4+-containing langbeinite melt and porous alpha-alumina crucible. The obtained needles crystallize in the pseudobrookite structure and, upon heating, undergo two reversible phase transitions, from orthorhombic Cmcm to C2/m first and subsequently to C2 symmetry. Like the known aluminum titanate pseudobrookites, anisotropic thermal expansion is observed. The temperature evolution of the crystal structure reveals insights into the mechanism leading to the decomposition of the pseudobrookites above 725°C. The magnetic and electrical properties are discussed and compared to other reported aluminum titanate pseudobrookites.