First-principles study of instantaneous and averaged local potential in BaTiO3 -: art. no. 174116

The minima of the local mode's potential energy in the unit cell of BaTiO3 are studied by first-principles effective Hamiltonian numerical simulations. Instead of a constant potential with eight minima, we find a single minimum with position and depth changing in time. In agreement with the early view by Comes et al. [Solid State Commun. 6, 715 (1968)] and recent nuclear magnetic resonance experiments, we find that deeper values at the paraelectric phase are mainly found when the minimum is located at eight off-centered regions at the body diagonals of the unit cell. This situation changes at the ferroelectric phase, where four of the eight noncentered regions are privileged, and deeper values are found when the minimum is located in a surface around them. This order-disorder behavior of the instantaneous potential disappears when considering the long-term time-averaged potential: The paraelectric phase single minimum of the averaged potential is located at the center of the unit cell and it becomes displaced along the local dipole axis in the ferroelectric phase.