Ferroelectric Mode and Phase Transitions in KNbo₃

Abstract

The dynamical properties of KNbO₃ in tetragonal phase are investigated by inelastic neutron scattering and Raman scattering experiments. The results are compared with those obtained in the other phases by different authors. Two main features emerge from this analysis. First, the lowest frequency F|u cubic mode, reflecting principally the dielectric properties in every phase, is the only sensitive to the structure change; it is the so- called ferroelectric mode. Its mode eigenvector is directly related to the cell distorsion and to the permanent electric dipole, which accompagny each phase change; the phase transitions are therefore considered of displacive-type. The frequency of the other zone center modes do not shift appreciably with temperature. On the other hand, the oscillating dipoles Nb-0 associated with the unstable mode are strongly correlated along the <100> directions; the dipole-dipole coupling explains the strong anisotropy of the TA and lowest TO phonon branches as well as the x-ray diffuse scattering planes.