Far infrared dielectric dispersion in Sb2S3, Bi2S3 and Sb2Se3 single crystals

Far infrared reflectivity of Sb2S3, Bi2S3 and Sb2Se3 single crystals in the spectral region 25-400 cm-1 and temperature region 100-400°K were measured in polarized light. The reflectivity spectra were subjected to the Kramers-Kronig analysis to obtain dielectric response functions. The far infrared dispersion is of a very complex nature and continues down to the submillimetre and probably also to the millimetre region. The group analysis including the construction of symmetry vectors was performed and provided a basis for discussing the experimental results. A new type of dielectric phase transition with no symmetry changes is proposed for interpretation of known anomalies in Sb2S3 around 300°K.