Mixed Valence Pseudobrookite Al$_{1.75}$Ti$_{1.25}$O$_5$: High Temperature Phase Transitions, Magnetism and Resistivity

Dark blue single crystals of Al$_{1.75}^{3+}$ Ti$_{1.0}^{4+}$ Ti$_{0.25}^{3+}$O$_5$ were grown with a novel synthesis method based on the reaction of a Ti3+/Ti4+ containing langbeinite melt and Al$_2$O$_3$. The obtained needles crystallize in the pseudobrookite structure and undergo two reversible phase transitions from orthorhombic Cmcm to C2/m first and subsequently to C2 symmetry. Like the known aluminum titanate pseudobrookites, anistropic thermal expansion is observed. The temperature evolution of the crystal structure reveals some insights into the mechanism leading to the decomposition of the Al$_{1.75}$Ti$_{1.25}$O$_5$ above 725$^\circ$C. The magnetic and electrical properties are discussed and compared to other reported aluminum titanate pseudobrookites.