Phonon dispersion and anharmonicity in cubic KNbO3

Inelastic neutron scattering measurements of cubic KNbO3 have been performed to study the temperature dependence and anharmonicity of the phonon branches believed to be associated with the subsequent phase transitions. Our results show that the cubic phase displays an anomalously flat and low energy transverse acoustic mode propagating in the [100] directions, absent from the previous study on the cubic phase. The transverse optic mode along the same directions shows large damping which is consistent with the previous report. A strong temperature dependent coupling occurs between these two modes, similar to the behavior reported in BaTiO3. The behavior of the other phonon branches, in contrast, show normal dispersion behavior and the phonon groups are well behaved. Computer simulations of lattice dynamics demonstrate that the phonon branches have largely harmonic behaviour except for the two anomalous modes. The transverse acoustic and lowest energy optic mode in the [100] directions can be successfully modeled with an anharmonic, non-linear polarizability model for a reduced system. This model is intrinsically anisotropic and indicates that the origin of the anomalous behavior arises from anisotropic oxygen polarizability. This anisotropy also manifests itself in the anomalous thermal diffuse X-ray scattering that has been shown to display a planar distribution in the reciprocal space.