Domain formation and dielectric response in PbTiO3: A first-principles free-energy landscape analysis

We determine the relative thermodynamic stability of competing homogeneously and inhomogeneously ordered ferroelectric phases of PbTiO3 using its free-energy landscape, obtained from constrained-polarization moleculardynamics simulations with a first-principles effective Hamiltonian and thermodynamic integration. While we find that the tetragonal structure is thermodynamically most stable at temperatures below the ferroelectric transition temperature (T0 = 660 K), the free energy of an “orthorhombic-like” 90◦ domain phase relative to the tetragonal phase almost vanishes at T = 540 K and remains small at all temperatures below T0. In contrast to BaTiO3, the 90◦ domain structure is an order of magnitude lower in energy than the one with 180◦ domains. We show that the dielectric response contains signatures of such a domain structure and is significantly stronger than that of the uniformly polarized tetragonal phase of PbTiO3.