Abstract
Far infrared reflectivity measurements of vapour-grown single crystals SbSI, SbSeI, BiSI, and BiSeI have been performed in the region of 50-400 cm-1 at 25 and -170°C. The results are analysed by Kramers-Kronig relations to obtain the dielectric response functions. From the results the eigenvectors of all B1u normal modes have been calculated. For the soft mode in SbSI it is obtained that all atoms vibrate along the ferroelectric c axis. The Sb displacements are 1.3 times larger and of opposite direction to the S and I displacements.