Ab initio investigations of optical properties of MgS and CaS beyond the harmonic approximation.*

Using first-principles calculations based on density functional theory (DFT), we study the infrared (IR) optical properties of MgS and CaS. Firstly, we assess the efficacy of the harmonic approximation in the modeling of their vibrational spectra. Secondly, by comparing our results to available experimental data [1], we show that a more accurate description of the IR optical properties requires the inclusion of anharmonic effects. A more precise determination of the dielectric constant of these binary sulfides could help in the effective computation of microscopic mechanisms correlated to their macroscopic radiative behavior. Our results could, on one hand, shed light on using more effectively MgS and CaS towards energy storage materials [2], and on the other hand, help in the interpretation of data collected by future missions to Mercury to study the planet's surface, which is hypothesized to have an abundance of volatiles such as sulfur [3]. 

[1] Y. Kaneko, K. Morimoto and T. Koda, J. Phys. Soc. Jpn., 51, 2247-2254 (1982)

[2] S. Pervez and M. Z. Iqbal, Int. J. Energy Res., 46, 8093 (2022)

[3] I. Varatharajan, A. Maturilli, J. Helbert, et al., Earth Planet. Sci. Lett., 520, 127-140 (2019)