Nature of the octahedral tilting phase transitions in perovskites: A case study of CaMnO_3

摘要

The temperature-induced antiferrodistortive (AFD) structural phase transitions in CaMnO_3, a typical perovskite oxide, are studied using first-principles density functional theory calculations. These transitions are caused by tilting of the MnO_6 octahedra that are related to unstable phonon modes in the high-symmetry cubic perovskite phase. Transitions due to octahedral tilting in perovskites normally are believed to fit into the standard soft-mode picture of displacive phase transitions. We calculate phonon-dispersion relations and potential-energy landscapes as functions of the unstable phonon modes and argue based on the results that the phase transitions are better described as being of order-disorder type. This means that the cubic phase emerges as a dynamical average when the system hops between local minima on the potential-energy surface. We then perform ab initio molecular dynamics simulations and find explicit evidence of the order-disorder dynamics in the system. Our conclusions are expected to be valid for other perovskite oxides, and we finally suggest how to predict the nature (displacive or order-disorder) of the AFD phase transitions in any perovskite system.