Raman spectroscopy of PbTe1−xSx alloys

Raman scattering spectra as well as results of galvanomagnetic, X-ray and ultrasonic measurements of PbTe1−xSx (x = 0.02 and 0.05) mixed crystals in the temperature range from 10 to 300 K are presented. A structural phase transition at about Tc = 60 K for x = 0.05 was detected, whereas for samples with x = 0.02, the phase transition was not observed. The two TO/LO mode pairs of PbTe1−xSx mixed crystals are determined from Raman spectra at all temperatures, and according to that we found that the optical phonon modes of these mixed crystals showed a two-mode behaviour. The model of phonon behaviour based on the model of Genzel et al. was applied. One additional mode, at about 75 cm−1, the intensity of which is connected with off-center sulphur ions, is observed in all samples. Our results suggest that the only reason for complex phase transition in PbTe1−xSx systems may be the off-center sulphur ions.