Substitutional sulfur and its vibrational fingerprints in Sb${}_2$Se${}_3$

The configurational behavior of sulfur in antimony triselenide (Sb${}_2$Se${}_3$) is investigated by combining infrared absorption spectroscopy with density functional theory. Four sulfur-related local vibrational modes are identified at 249, 273, 283, and 312~cm${}^{-1}$ in melt-grown single crystals prepared from Sb${}_2$Se${}_3$ granulate. Their assignment to sulfur is confirmed through controlled indiffusion experiments using Sb${}_2$S${}_3$ and elemental sulfur, as well as isotope-substitution studies with ${}^{34}$S, which produce the expected frequency shifts. Polarization-resolved measurements, together with theoretical calculations of local vibrational modes, demonstrate that the observed spectral features are fully consistent with substitutional sulfur on the three inequivalent selenium sites of Sb${}_2$Se${}_3$.