Optical phonons and magnetoelastic coupling in the ionic conductor AgCrSe₂

AgCrSe₂ is an example of a superionic conductor that has recently attracted attention for its low thermal conductivity. Here we investigate the optical properties of AgCrSe₂ in the ordered phase between 14 K and 374 K using reflectivity experiments and compare them to the optical conductivity obtained from electronic structure calculations. The far infrared optical response is dominated by three phonon modes, while six interband transitions are observed in the visible range. We find good agreement between the experimental interband transitions and the calculated optical transitions. From our analysis we find that the phonon parameters display an interesting temperature dependence around the Néel temperature, pointing to a small magnetoelastic coupling. In addition, the lifetimes of the modes indicate that three-phonon processes dominate and the optical phonons decay into low-energy acoustic modes involved in the superionic transition. Finally, we detect a small free charge carrier response through the analysis of Fabry-Perot interference fringes in our reflectivity data.