Finite-temperature properties of the prototypical perovskite CaTiO$_3$ from second-principles effective interatomic potential

We introduce a second-principles effective interatomic potential for the perovskite $\rm CaTiO_3$ (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 CTO, in particular successfully reproducing the structure, energy, and dynamical properties of the nonpolar $Pbnm$ ground state as well as the ferroelectric $R3c$ phase. The finite-temperature simulations suggest that the sequence of structural phase transitions over heating of CTO is: $Pbnm (a^-a^-c^+) \rightarrow C2/m \ (a^-b^-c^0) \rightarrow I4/mcm \ (a^-c^0c^0) \rightarrow Pm\bar{3}m \ (a^0a^0a^0)$, during which the oxygen octahedral rotations around the three pseudocubic axes vanish progressively. The model also provides the opportunity of investigating the properties of the ferroelectric $R3c$ phase, which is a metastable phase free of lattice instability at zero Kelvin. Our model-based simulations confirm that the $R3c$ phase remains stable below a certain finite temperature. Additionally, we find that the minimum energy path connecting the $Pbnm$ and $R3c$ phases involves localized layer-by-layer flipping of octahedral rotations. A similar mechanism is also observed in the thermal destabilization process of the $R3c$ phase toward the $Pbnm$ ground state in our simulation.