Mixed‐Valence Engineering in Ge 2+ Chalcogenides: Structural Modulation for Balanced Infrared Nonlinear Optics

Resolving the intrinsic trade‐off between strong second‐harmonic generation (SHG) and a wide band gap remains a central challenge in infrared nonlinear optical (NLO) materials. Here, we report a mixed‐valence engineering strategy to realize a previously unexplored Ge 2+ ‐containing chalcogenide NLO system, Ag 4 Ge 3 S 7 . This strategy not only stabilizes the stereochemically active Ge 2+ but, more importantly, induces structural modulation through flexible mixed‐valence [Ge 3 S 7 ] chains, which activate the adjacent Ag–S sublattice by amplifying local structural distortion. As a result, Ag 4 Ge 3 S 7 exhibits a type‐I phase‐matchable SHG response of 1.15 × AgGaS 2 at 2100 nm, together with a wide optical band gap of 2.43 eV and broad mid‐infrared transparency up to 8.6 µm. Combined experimental and theoretical analyses reveal that the activated Ag–S sublattice dominates the NLO response, while the stereochemically active [GeS 3 ] 4 − units provide a secondary contribution. This work not only addresses the longstanding instability of Ge 2+ ‐based chalcogenides but also establishes mixed‐valence‐induced sublattice activation as a promising strategy for high‐performance infrared NLO materials.