Complete determination of elastic stiffness coefficients and local symmetry breaking in the paraelectric barium titanate

Abstract

The temperature dependence of all independent elastic stiffness coefficients of cubic BaTiO3 was determined over a wide temperature range from 400 °C to the Curie point (Tc ∼ 131 °C) by using Brillouin light scattering spectroscopy. Among the two elastic eigen-modes corresponding to the two measured transverse acoustic waves, (C11-C12)/2 showed a large softening in the paraelectric phase, while the change in C44 was very small. This result caused the elastic anisotropy A, defined as 2C44/(C11-C12), to increase from ∼2.1 to ∼2.7 upon cooling toward the phase transition temperature. The large elastic anisotropy was attributed to the tetragonal instability and the associated anisotropic polarization fluctuations of precursor polar clusters formed by the correlated Ti motions. The elastic anisotropy was fitted by using the Curie-Weiss law resulting in a significant deviation in a certain temperature range between Tc and ∼Tc + 50 °C, similar to the dielectric permittivity.