Far-IR study of electron-phonon interaction and phase transition in α-In2S3−xSex (2.4 ⩽ x ⩽ 3.0) crystals

The electron-phonon interaction and phase transition in layered α-In2S3−xSex (2.4 ⩽ x ⩽ 3.0) compounds have been investigated using far-IR spectroscopy. It is found that the electron-phonon interaction in unannealed α-In2S3−xSex (x ≈ 3.0) samples is greater than that for annealed α-In2Se3−xSex (x ≈ 2.4) samples. The threshold temperature for the α- to β-phase transition is much greater for the latter samples than for the former ones. The phase transition from α to β is investigated in situ heating samples between 20 and 450 °C. While cooling down from the β-phase to α-phase, the intermediate β′-phase is observed. To explain this behaviour we proposed a simple model which presumes sulphur occupancy of vacant sites available in the In cationic sublattice or substituted in place of Se. It is suggested that a mixed structure formation may be responsible for the increase in transition temperature.