Finite-temperature properties and the hidden ferroelectric R 3 c phase of bulk CaTiO 3 from second principles

A second-principles effective interatomic potential is introduced for the prototypical perovskite CaTiO₃ (CTO), relying on a Taylor polynomial expansion of the Born-Oppenheimer energy surface around the cubic reference structure, in terms of atomic displacements and macroscopic strains. This model captures various phases of bulk CTO and successfully reproduces, in particular, the structure, energy, and dynamical properties of the nonpolar Pbnm ground state as well as of the hidden ferroelectric R3c phase. Finite-temperature simulations suggest that the still debated sequence of structural phase transitions over heating is Pbnm4pt{0ex} (a^-a^-c^+) 2/m4pt{0ex} (a^-b^-c^0) 4/mcm4pt{0ex} (a^-c^0c^0) 3m4pt{0ex} (a^0a^0a^0), a sequence during which the oxygen octahedra rotations around the three pseudocubic axes vanish successively. Although never experimentally observed in bulk, the ferroelectric R3c phase appears to be metastable and at an energy only slightly above the Pbnm ground state at 0 K. The simulations confirm that, if induced in some way, the R3c phase remains stable up to about 300 K and shows ferroelectric properties. Furthermore, we find that the minimum energy path connecting the Pbnm and R3c phases involves localized layer by layer flipping of octahedral rotations, a mechanism which is shown to be at play during the thermal destabilization process of the R3c phase toward the Pbnm ground state. The proximity of the R3c phase with the Pbnm ground state suggests that the former could be stabilized under electric field. However, due to the large energy barrier, the field required for the Pbnm to R3c transition appears to be extremely large, consistent with the fact that bulk CTO was never reported to be ferroelectric nor antiferroelectric.