Low-frequency dielectric response in cubicKNbO3studied by hyper-Raman scattering

Hyper-Raman scattering is observed in the cubic phase of ${\mathrm{KNbO}}_3$ above the Curie temperature $T_C$≊702 K. The imaginary part of the low-frequency dielectric function is obtained as a function of temperature. In the spectral range from 2 to 150 ${\mathrm{cm}}^{\mathrm{-}1}$ it can adequately be described in terms of a single overdamped oscillator. In approaching $T_C$ from above, we find a mode softening or critical slowing down of the form ${\omega }_0^2$/γ∼T-$T_0$, where ${\omega }_0$ and γ are the frequency and damping constant of the overdamped oscillator and $T_0$ is the Curie-Weiss temperature. The mode softening seems to account for the Curie-Weiss law of the static dielectric constant. Our results differ from data deduced from far-infrared reflectivity measurements, the discrepancies being of the same kind as in the case of ${\mathrm{BaTiO}}_3$.