Role of Twinning in Perovskite - Lithium Niobate Phase Transitions

Abstract

A number of ABO₃ compounds having the ilmenite structure at low pressures have been shown to transform to the orthorhombic perovskite structure at high pressures and temperatures. The compounds that exhibit this behavior contain cations having a relatively restricted range of radii and radius ratios, and include MnTiO₃, FeTi_O3, MgTiO₃, MgGeO₃, and MnSnO₃. In addition, two silicates with mixed cation content show similar behavior. When these materials are quenched to ambient conditions, the products often are twinned crystals having the LiNbO₃ structure. It was thought originally that twinning was required in this transition, but examination of TEM photographs indicates the presence of untwinned regions of the LiNbO₃-type phase. However, this structure appears to be metastable with respect to ilmenite. A₂O₃ compounds such as Fe₂O₃ also transform similarly to the perovskite structure at high pressures and temperatures, but the intermediate metastable form is different from the ABO₃ type. It has been predicted that this could be the Rh₂O₃ II structure.