Acoustic Anomalies of Potassium Lithium Niobate Single Crystals near the Ferroelectric Phase Transition Temperature

The acoustic properties of two potassium lithium niobate single crystals have been investigated by using Brillouin spectroscopy. The temperature dependence of the Brillouin shifts corresponding to two elastic constants, C11 and C33, was obtained over a wide temperature range including the phase transition temperature (Tc). Consistent with the uniaxial nature of the tetragonal tungsten- bronze structure and the Ising-type order parameter along the c-axis, only the C33 showed a large acoustic anomaly at Tc while C11 did not show a substantial change in both crystals. This result indicates that the longitudinal acoustic wave propagating along the polar direction couples strongly to the order parameter fluctuations, resulting in a significant softening of C33, as well as a substantial increase in the acoustic damping in the paraelectric phase. Changes in the acoustic properties above Tc have been partly ascribed to the electrostrictive coupling between the strain and the squared order parameter of polar nanoregions and fluctuation damping, which seem to be correlated with the diffuse nature of the ferroelectric phase transition observed in the present study. However, another microscopic origin seems to contribute to the softening of C33 at temperatures above Tc, which is unclear at present. KCI Citation Count: 5