Neutron-Scattering Analysis of the Linear-Displacement Correlations in KTaO3

In view of recent x-ray experiments which detected a surprisingly strong diffuse scattering in the {100} reciprocal sheets of KTa${\mathrm{O}}_3$, a new neutron-scattering investigation was performed to determine the origin of this scattering. It was found that a strong anisotropy in the dispersion of the low-lying phonon branches, which was unexpected from the earlier data, can account for the x-ray results. Both the transverse-acoustic and lower-optic phonons polarized along [0101] have particularly low frequencies in the reciprocal {100} sheets. This anisotropy of the dispersion relation can be understood as arising from dipolar-type forces along the directions, and optic-acoustic mode interaction restricted to the same directions.