Critical Raman scattering in orthorhombic KNbO3

The Raman spectrum of a single-domain orthorhombic KNbO3 sample exhibits the appearance of a central peak beside the soft phonon when the transition to the tetragonal phase is approached. The low-frequency Raman profile is well reproduced by a coupled relaxation-phonon model. It is pointed out that the orthorhombic-tetragonal phase transition is related to a critical slowing-down of the phonon renormalized by the interaction with the relaxation, whereas the orthorhombic-rhombohedral transition is associated solely with a smooth softening of the decoupled phonon.