NEUTRON-SCATTERING STUDY OF THE FERROELECTRIC PHASE-TRANSITION OF SBSI

The paraelectric-ferroelectric phase transition of SbSI has been studied using neutron scattering. Several low frequency phonon branches have been observed. In particular, it has been shown that the [100] and [010] transverse acoustic modes polarized along the [001] direction with the same symmetry as the soft mode, exhibit very anisotropic dispersion relations in reciprocal space. For large q values, an anomalous dispersion relation and a very large temperature dependence have been observed. All these effects are shown to be due to a coupling with a soft mode-central component entity. This entity was observed away from the zone center, but at the zone center, in contrast with the light scattering measurements, no well-defined soft mode response could be observed. © 1979.