First-principles accurate total energy surfaces for polar structural distortions of BaTiO₃, PbTiO₃, and SrTiO₃: consequences for structural transition temperatures

Abstract

Specific forms of the exchange-correlation energy functionals in first-principles density-functional-theory-based calculations, such as the local-density approximation (LDA) and generalized-gradient approximations (GGA), give rise to structural lattice parameters with typical errors of −2% and 2%. Due to a strong coupling between structure and polarization, the order parameter of ferroelectric transitions, they result in large errors in estimation of temperature-dependent ferroelectric structural transition properties. Here, we employ a recently developed GGA functional of Wu and Cohen Phys. Rev. B 73 235116 (2006) and determine total energy surfaces for zone-center distortions of BaTiO₃, PbTiO₃, and SrTiO₃ and compare them with the ones obtained with calculations based on standard LDA and GGA. Confirming that the Wu and Cohen functional allows better estimation of structural properties at 0 K, we determine a set of parameters defining the effective Hamiltonian for ferroelectric transition in BaTiO₃. Using the new set of parameters, we perform molecular-dynamics simulations under effective pressures p=0.0 GPa, p=−2.0 GPa, and p=−0.005T GPa. The simulations under p=−0.005T GPa, which is for simulating thermal expansion, show a clear improvement in the cubic to tetragonal transition temperature and c/a parameter of its ferroelectric tetragonal phase, while the description of transitions at lower temperatures to orthorhombic and rhombohedral phases is marginally improved. Our findings augur well for use of Wu-Cohen functional in studies of ferroelectrics at nanoscale, particularly in the form of epitaxial films where the properties depend crucially on the lattice mismatch.