Ag₂GeS₃: A Diamond‐Like Chalcogenide as an IR Nonlinear Optical Material with Outstanding Second‐Harmonic Generation Response

Abstract Diamond‐like (DL) metal chalcogenides have attracted significant attention in recent years because of their complex structural compositions and exceptional nonlinear optical (NLO) capabilities in the infrared (IR) range. Despite notable advancements in this field, there is still a lack of systematic research on high‐performance and simply composed IR‐NLO chalcogenides with DL structures. In this study, the insightful structural characteristics and IR‐NLO properties of a ternary Ag‐based chalcogenide Ag₂GeS₃ are investigated for the first time at both experimental and theoretical levels. The compound belongs to the non‐centrosymmetric Cmc2₁ space group (No.36) and displays a 3D DL structure comprising highly oriented [AgS₄] and [GeS₄] tetrahedra. Due to its distinct structure, the compound may produce significant second harmonic generation (SHG) response (5.6 × AgGaS₂ at 2050 nm), the biggest value reported among Ag‐based IR‐NLO chalcogenides to date, and phase matchability in the IR range with a calculated birefringence of Δn = 0.09 at 2050 nm. According to theoretical calculations, the combined impacts of the distorted [AgS₄] and [GeS₄] NLO‐active motifs inside the 3D DL framework provide the SHG tensor d₂₄ of Ag₂GeS₃ at 51.9 pm V⁻¹ at 2050 nm. The prospective uses of Ag₂GeS₃ in the field of IR‐NLO are highlighted in this work.