Structural, Optical, and Electronic Properties of Two Quaternary Chalcogenide Semiconductors: Ag₂SrSiS₄ and Ag₂SrGeS₄.

Quaternary chalcogenide materials have long been a source of semiconductors for optoelectronic applications. Recent studies on I₂-II-IV-X₄ (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, Ag₂SrSiS₄ and Ag₂SrGeS₄, each found to form in the tetragonal I4̅2m structure of Ag₂BaGeS₄. During the synthesis targeting the title compounds, two additional materials, Ag₂Sr₃Si₂S₈ and Ag₂Sr₃Ge₂S₈, have also been identified. These cubic compounds represent impurity phases during the synthesis of Ag₂SrSiS₄ and Ag₂SrGeS₄. We show through hybrid density functional theory calculations that Ag₂SrSiS₄ and Ag₂SrGeS₄ 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 Ag₂SrSiS₄ and Ag₂SrGeS₄ powders show frequency-doubling capabilities in the near-infrared range.

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