Stability and synthesis across barium tin sulfide material space

Abstract

Barium tin sulfide (Ba–Sn–S) is a ternary phase space with interesting material candidates for optoelectronic and thermoelectric applications, yet its properties have not been explored in-depth experimentally, and no thin films have been synthesized. This study uses combinatorial sputtering and theoretical calculations to survey the phase space of Ba–Sn–S materials. We experimentally find that at deposition temperatures up to 600 °C, phases of rocksalt-derived BaS structures (Fmm), layered SnS derived structures (Aem2), and heavily distorted rocksalt solid solutions (possibly P12₁/m) dominate phase space, with amorphous films crystallizing in the middle of the composition space (SnBa). Upon annealing with a capping layer, ternary phases of Ba₂SnS₄ (Pna2₁) and Ba₇Sn₅S₁₅ (P6₃cm) are observed. However the theoretically predicted 0 K thermodynamically stable phase of BaSnS₂ (P2₁/c) does not crystallize. These differences are explained with temperature-dependent computed phase diagrams, which show that BaSnS₂ becomes unstable at high temperatures while Ba₂SnS₄ (Pna2₁) becomes stabilized. Lastly, we compute electronic and optical absorption properties of selected observed and predicted Ba–Sn–S phases, showing band gaps ranging from 1.67–2.5 eV, electron effective masses from 0.5–1 m₀, and hole effective masses from 0.6–1.3 m₀. These findings motivate future research into materials within this chemical space for solar energy harvesting and other semiconductor applications.