Structural, Optical, and Electronic Properties of Two Quaternary Chalcogenide Semiconductors: Ag 2 SrSiS 4 and Ag 2 SrGeS 4 .

Quaternary chalcogenide materials have long been a source of semiconductors for optoelectronic applications. Recent studies on I ₂ -II-IV-X ₄ (I = Ag, Cu, Li; II = Ba, Sr, Eu, Pb; IV = Si, Ge, Sn; X = S, Se) materials have shown particular versatility and promise among these compounds. These semiconductors take advantage of a diverse bonding scheme and chemical differences among cations to target a degree of antisite defect resistance. Within this set of compounds, the materials containing both Ag and Sr have not been experimentally studied and leave a gap in the full understanding of the family. Here, we have synthesized powders and single crystals of two Ag- and Sr-containing compounds, Ag ₂ SrSiS ₄ and Ag ₂ SrGeS ₄ , each found to form in the tetragonal I 4̅2 m structure of Ag ₂ BaGeS ₄ . During the synthesis targeting the title compounds, two additional materials, Ag ₂ Sr ₃ Si ₂ S ₈ and Ag ₂ Sr ₃ Ge ₂ S ₈ , have also been identified. These cubic compounds represent impurity phases during the synthesis of Ag ₂ SrSiS ₄ and Ag ₂ SrGeS ₄ . We show through hybrid density functional theory calculations that Ag ₂ SrSiS ₄ and Ag ₂ SrGeS ₄ have highly dispersive band-edge states and indirect band gaps, experimentally measured as 2.08(1) and 1.73(2) eV, respectively. Second-harmonic generation measurements on Ag ₂ SrSiS ₄ and Ag ₂ SrGeS ₄ powders show frequency-doubling capabilities in the near-infrared range.