Surface effects and ferroelectric phase transitions in BaTiO₃ ultrathin films

We present a finite-temperature study of ferroelectric phase transitions in BaTiO₃ ultrathin films using an atomic-level simulation approach. This is based on a shell model for BaTiO₃ with parameters obtained from first-principles calculations. While transitions involving in-plane electric polarization are practically unperturbed by the presence of the surface, surface atomic relaxations act as a strong perturbation on the ferroelectric order perpendicular to the surface. However, a strain induced ferroelectric polydomain phase with an out-of-plane orientation of polarization is stabilized in a not short-circuited film as thin as 20 Å. A reduction of the out-of-plane polarization at the film surface is observed.