Inelastic neutron scattering in tetragonal KNbO3

Inelastic neutron scattering experiments in tetragonal KNb${\mathrm{O}}_3$ show a high anisotropy in some regions of the phonon dispersion relation and the existence of strong one-dimensional intercell correlations between atomic motions along the [100] and [010] directions, in agreement with x-ray diffuse-scattering measurements in the same phase. The transverse phonon modes propagating normally to these directions of strong correlations, that is, with polarization vectors parallel to them, are relatively soft, not only near the zone center but all the way to the zone boundary. Comparison of these results with the Raman, neutron, and x-ray data obtained in the other phases suggests a description of the ferroelectric phase transition mechanism in KNb${\mathrm{O}}_3$ in terms of soft modes and dynamic correlations in each phase.