Theoretical investigation of the structural, elastic, electronic, and dielectric properties of alkali-metal-based bismuth ternary chalcogenides

The past decade has witnessed the rapid introduction of organic-inorganic hybrid compounds in photovoltaic applications. Motivated by the strong demand for stable and nontoxic materials in this class, we report a theoretical study on the structural, elastic, electronic, thermodynamic and dielectric properties of alkali-metal-based bismuth ternary chalcogenides. In particular, we employ state-of-the-art density functional theory to explore the potential of $A\mathrm{Bi}{X}_{2}$ and $A\mathrm{Bi}{X}_{3}$ ($A=\mathrm{Na}$, K and $X=$ O, S) as light-absorbing media. All the compounds under investigation are found to be thermodynamically and mechanically stable, with a semiconductor band structure. The Kohn-Sham band gaps range between 0.80 eV and 1.80 eV, when calculated with semilocal functionals, values that increase to 1.24--2.47 eV with hybrid ones. Although all but ${\mathrm{NaBiO}}_{2}$ and ${\mathrm{KBiO}}_{2}$ are indirect band-gap semiconductors, the onset of the imaginary part of their dielectric functions, the optical gap, is only marginally larger than the quasiparticle gap. This is due to the generally flat nature of both the conduction and the valence bands. We then expect these compounds to absorb light in the upper part of the visible spectrum. In all cases the valence band is dominated by $\mathrm{O}\text{\ensuremath{-}}p$ and S-$p$ orbitals and the conduction one by Bi-$p$, suggesting the possibility of excitons with low binding energy. The only exceptions are ${\mathrm{NaBiO}}_{2}$ and ${\mathrm{KBiO}}_{2}$ for which the $\mathrm{O}\text{\ensuremath{-}}p$ states dominate the density of states at both sides of the band gap.