A new phase transition in MnTiO₃: LiNbO₃-perovskite structure

Abstract

The stable polymorph of MnTiO₃ at room temperature and pressure has the ilmenite structure. At high temperatures and pressures, MnTiO₃ ilmenite transforms to a LiNbO₃ structure through a cation reordering process (Ko and Prewitt 1988). Single crystals of both phases have been studied with X-ray diffraction to 5.0 GPa. We have obtained the first experimental verification of the close relationship between the LiNbO₃ and perovskite structures, first postulated by Megaw (1968). MnTiO₃ with the LiNbO₃ structure (MnTiO₃ II) transforms directly to an orthorhombic perovskite structure (MnTiO₃ III) between 2.0 and 3.0 GPa. The transition involves a change of volume of -5%, is reversible and has pronounced hysteresis. Only pressure is required to drive the transition because it involves no breaking of bonds; it simply involves rotation of the [TiO₆] octahedra about their triad axes accompanied by displacement of the Mn cations to the distorted twelve-coordinated sites formed by the rotations. An unusual aspect of this transition is that twinned MnTiO₃ II crystals transform to untwinned MnTiO₃ III crystals with increasing pressure. The twin plane of MnTiO₃ II, $\left( {10\bar 1\bar 2} \right)$, corresponds to the (001) mirror plane of the orthorhombic perovskite structure. MnTiO₃ III examined at 4.5 GPa is very distorted from the ideal cubic perovskite structure. The O(2)-O(2)-O(2) angle describing the tilting in the ab plane is 133.3(7)°, in contrast to 180° for a cubic perovskite and the O(2)-O(2)-O(2) angle describing the tilting in the ac plane is 109.3(4)°, as opposed to 90° in a cubic perovskite.