Inelastic neutron scattering in orthorhombic KNbO3

Inelastic neutron scattering measurements on single-domain orthorhombic KNbO3 at room temperature show no evidence of overdamped scattering, in contrast with previous neutron results in the cubic phase. As in the case of KTaO3, the observed diffuse x-ray planes are shown to correspond to a highly anisotropic transverse acoustic branch with an unusually flat dispersion in the 'soft' plane. In addition, a well defined optic branch with a polarization related to the orthorhombic-rhombohedral phase transition is seen to become soft as q to 0 in the soft plane. A mode-eigenvector determination confirms the x-ray result that essentially only one atom (niobium) moves in the flat transverse acoustic mode. The connection with other experimental results and with the independent chain model is discussed.