Far-infrared studies of the intermediate phases in In2Se3

This work reports on a study of the infrared active long-wavelength optical phonons of the layered semiconductor In2Se3. The temperature dependence behaviour is investigated between 10 K and 500 K, and the intermediate phases α′- and β′-In2Se3 are evidenced by reflectivity measurements for the E ⊥ c polarization in the spectral range 10–600 cm−1. At low temperature, the anomaly in the frequency and life-time of the LO mode is related with the α′-phase which is described as the frozen-in configuration of the longitudinal mode. During cooling down from the β-In2Se3 phase to the α-phase, the intermediate β′-In2Se3 phase is formed in the temperature range 473-363 K. The formation of the superlattice is observed and a model in which the β → α phase transition is driven by acoustic phonons is proposed.