Optical modes and dielectric properties of ferroelectric orthorhombicKNbO3

Abstract

Both Raman scattering and infrared reflectivity measurements have been performed to determine the optic-mode spectrum of orthorhombic ${\mathrm{KNbO}}_3$ at room temperature. Pincipal-axis phonons and oblique phonons were observed with a combination of 90° scattering, back scattering, and forward scattering. Unobserved longitudinal modes and mode couplings were derived from the reflectivity measurements. The ferroelectric modes, namely those rendered highly anisotropic as a result of successive transitions, were identified and their spatial anisotropy determined; strong interference with more isotropic modes was pronounced. Dielectric constants were derived from these measurements and the $A_1$ polariton dispersion curve was also measured. The modes were grouped according to the cubic representations from which they are derived and compared with those of ${\mathrm{BaTiO}}_3$: In particular the modes were three time less damped. Of the original soft cubic ferroelectric mode, the component along the remaining soft direction was found at 56 ${\mathrm{cm}}^{-1\mathrm{}}$ and not overdamped ($\frac{\Gamma }{\omega }=1\mathrm{}$).