First-principles study of the temperature-pressure phase diagram of BaTiO3

We investigate the temperature-pressure phase diagram of BaTiO_3 using a first-principles effective-Hamiltonian approach. We find that the zero-point motion of the ions affects the form of the phase diagram dramatically. Specifically, when the zero-point fluctuations are included in the calculations, all the polar (tetragonal, orthorhombic, and rhombohedral) phases of BaTiO_3 survive down to 0 K, while only the rhombohedral phase does otherwise. We provide a simple explanation for this behavior. Our results confirm the essential correctness of the phase diagram proposed by Ishidate et al. (Phys. Rev. Lett. 78, 2397 (1997)).