Critical behavior of hyper-rayleigh scattering in the centrosymmetric phase of hexagonal barium titanate

Abstract

Critical behavior of hyper-Rayleigh (second-harmonic) scattering was investigated for the first time for a new type of second-order displacive phase transition at T_a = 222 K in the hexagonal barium titanate, which is neither ferroelectric nor proper-ferroelastic in nature. It is found that the scattering intensity in a 90° scattering configuration increases strikingly in such a manner as I²ω ∼ (T-T_a)⁻², the square of the generalized susceptibility also observed, as T_a is approached in the centrosymmetric phase, only for those polarizations enabling one to observe the relevant nonpolar soft phonon, while that for other polarizations the 90 intensity remains almost constant. These aspects newly found can be well accounted for with a model that residual point defects acting as strongly localizing “field” should induce microclusters in their vicinity such that development of a locally condensed order-parameter, the square of which I²ω is proportinal to, follows the generalized susceptibility.