Band-Resolved Optical Properties of Rock-Salt BaSe and BaS from FP-LAPW Density Functional Theory

Abstract

The optical properties of the alkaline-earth chalcogenides barium selenide (BaSe) and barium sulfide (BaS) are investigated from first principles within density functional theory (DFT), using the full-potential linearized augmented plane-wave plus local-orbitals (FP-LAPW+lo) method as implemented in the WIEN2k code, building on the structural and electronic-structure results reported in a companion paper. The interband contribution to the optical response — which underlies the imaginary part of the frequency-dependent dielectric function, ε₂(ω), and from which the real part, the refractive index, the absorption coefficient, and the reflectivity can in turn be obtained via the Kramers–Kronig relations — was calculated for photon energies up to 14 eV. The principal spectral features were identified and assigned to specific valence-to-conduction-band transitions at high-symmetry points and lines of the Brillouin zone, including transitions at Γ, Λ, W, K, and L. The fundamental absorption onset occurs at lower photon energy in BaSe than in BaS, consistent with the smaller band gap expected for the larger, more polarisable Se²⁻ anion, while the ordering of the higher-energy spectral features between the two compounds varies from peak to peak, reflecting differences in the higher conduction-band dispersion. Both compounds show strong interband optical activity extending from the visible into the ultraviolet, consistent with their potential use in optoelectronic devices such as light-emitting diodes and semiconductor lasers, and the results are shown to be consistent with the wider body of experimental and computational work on the barium and alkaline-earth chalcogenides.