Controlling the off-center positions of anions through thermodynamics and kinetics in flexible perovskite-like materials.

Due to the network flexibility of their BX sub-lattice, a manifold of polymorphs with potential multiferroic applications can be found in perovskite-like ABX materials under different pressure and temperature conditions. The potential energy surface of these compounds usually presents equivalent off-center positions of anions connected by low energetic barriers. This feature facilitates a competition between the thermodynamic and kinetic control of the transitions from low to high symmetry structures, and explains the relationship between the rich polymorphism and network flexibility. In the rhombohedral phase of iron trifluoride, our first-principles electronic structure and phonon calculations reveal the factors that determine which of the two scenarios dominates the transition. At the experimentally reported rhombohedral-cubic transition temperature, the calculated fluorine displacements are fast enough to overcome forward and backward a barrier of less than 30 kJ mol, leading to an average structure with cubic symmetry. In addition, lattice strain effects observed in epitaxial growth and nanocrystallite experiments involving BX compounds are successfully mimicked by computing the phase stability of FeF under negative pressures. We predict a transition pressure at -1.8 GPa with a relative volume change around 5%, consistent with a first-order transition from the rhombohedral to the cubic structure. Overall, our study illustrates how, by strain tuning, either a thermodynamic or a kinetic pathway can be selected for this transformation.