Anharmonic effects on the reflectivity of CaS and MgS: A first-principles based study

TL;DR

This study uses first-principles calculations to show that including anharmonic effects significantly improves the accuracy of modeling the infrared reflectivity of CaS and MgS, validated by experimental comparison.

Contribution

It introduces a perturbative approach to incorporate anharmonicity and isotopic disorder in first-principles reflectivity modeling of CaS and MgS.

Findings

Anharmonic effects are crucial for accurate infrared reflectivity modeling.

Good agreement between theoretical predictions and experimental data.

Explicit anharmonic modeling improves understanding of optical properties.

Abstract

We employ systematic calculations based on density functional theory to model the reflectivity of CaS and MgS in the infrared region. We show that in addition to the modeling using the harmonic approximation, an accurate spectral description requires the inclusion of anharmonic effects. Due to their conceptual simplicity, CaS and MgS are excellent systems for the explicit consideration of the anharmonicity, which we include here using a perturbative approach up to three-phonon scattering processes, and the consideration of isotopic disorder. All physical quantities, such as Born effective charges and dielectric constant, necessary for the calculation of the reflectivity within the Lorentz model are extracted from our first-principles computations. To validate our predicted optical and transversal modes, and reflectivity spectra, we compare them to available experimental results. We find that the overall agreement is good, which supports the importance of the inclusion of anharmonic terms in the modeling of optical properties in the infrared region.