Synergistic Distortion Engineering: Activating Nonlinear Optical Response in Quaternary Chalcogenides via Lattice‐Induced [MnS ₄ ] Asymmetry

ABSTRACT The rational design of infrared nonlinear optical (NLO) chalcogenides with balanced performance—large second harmonic generation (SHG) response and wide band gap—remains a formidable challenge. This work proposes “synergistic distortion engineering” to transcend the limitations of simple cation replacement. Specifically, we exploit the structural flexibility of high‐spin Mn ²⁺ ( d ⁵ ): although lacking intrinsic Jahn–Teller driving force for distortion, its electronic isotropy allows it to accommodate severe lattice‐induced asymmetries imposed by the surrounding framework. Guided by this concept, we successfully synthesized two quaternary chalcogenides, SrMnM ^IV S ₄ (M = Ge, Sn), which exhibit an exceptional combination of properties: strong phase‐matchable SHG responses (0.6 and 1.0 × AgGaS ₂ ), wide band gaps (3.05 and 2.47 eV), high laser‐induced damage thresholds (6.2 and 3.3 × AgGaS ₂ ), and notably large birefringence (0.166 and 0.204). First‐principles calculations are employed to investigate whether these forced geometric distortions within the [MnS ₄ ] tetrahedra can effectively synergize with adjacent [M ^IV S ₄ ] units to amplify the macroscopic nonlinearity. The experimental results confirm that this structural modulation leads to a remarkable balance of NLO properties. This study demonstrates that manipulating the microscopic distortion of flexible motifs, rather than mere compositional variation, serves as a critical degree of freedom for the discovery of high‐performance NLO materials.