Abstract
In order to investigate the phase transition in terms of lattice dynamics, the symmetry modes of the paraβ and ferroelectric phases of the semiconductor SbSI are analysed by means of group theory. Starting from the simplified C^2^~2h~ space group with two formula units per unit cell, the symmetry modes in the different phases are deduced and their correlation from one phase to another is also established. Raman scattering measurements of the crystal in the two phases and in the phase transition region were obtained. These results are compared with available IR and Raman data. On the basis of the proposed model which takes into account the accidental degeneracies of the modes, the main features of the observed IR and Raman spectra are satisfactorily explained; the assignment of observed frequencies to determined symmetry coordinates is performed. An effort is also made to identify the temperature dependence of the mode frequencies in the vicinity of the ferroelectric transition temperature. An optical band whose frequency is strongly temperature dependent is observed in the Raman investigation and is in agreement with earlier works.