Optical phonons and magneto-elastic coupling in the ionic conductor AgCrSe$_2$

TL;DR

This study investigates the optical phonons and magneto-elastic interactions in AgCrSe₂, revealing temperature-dependent phonon behavior, weak magneto-elastic coupling, and insights into phonon decay processes relevant to its super-ionic conduction.

Contribution

It provides the first detailed analysis of optical phonons and magneto-elastic effects in AgCrSe₂ across a wide temperature range, highlighting their role in super-ionic conduction.

Findings

Phonon parameters show temperature dependence near the Néel temperature.

Weak magneto-elastic coupling is observed.

Three-phonon processes dominate phonon decay.

Abstract

AgCrSe$_2$ is an example of a super-ionic conductor that has recently attracted attention for its low thermal conductivity. Here we investigate the optical properties of AgCrSe$_2$ in the ordered phase between 14 K and 374 K using reflectivity experiments. The far infrared optical response is dominated by three phonon modes, while six interband transitions are observed in the visible range. From our analysis we find that the phonon parameters display an interesting temperature dependence around the N\'eel temperature, pointing to a small magneto-elastic 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 super-ionic transition. Finally, we detect a small free charge carrier response through the analysis of Fabry-Perot interference fringes in our reflectivity data.