First-principles approach to the effect of c-axis elongation of BaTiO3 thin films

The effect of c-axis elongation in the tetragonal phase of perovskite BaTiO3 (BTO) thin films was investigated using the first-principles method with ultrasoft pseudopotentials. The total energy Ep, with the relaxation of the internal atoms' positions R-atom as a function of the lattice constant of the c-axis c, was drastically lowered by the polarization, i.e., the long-range dipole-dipole interaction, compared with the total energy E-NP without the relaxation of R-atom. The tensile elastic deformation in the direction of the c-axis was found to become much easier, due to the polarization. Changing of ferroelectric properties via the lattice constant c was also considered by investigating the distance between the Ti atom and the apex O atom as a function of the lattice constant c. Moreover, the polarizations were approximately estimated as a function of the lattice constant c utilizing the Born dynamical effective charge obtained by Axe for cubic BTO.