Structural Symmetry and Mixed‐anion Engineering in Halopnictides for Extraordinary Second‐Harmonic Generation

Achieving high nonlinear optical (NLO) coefficients in NLO materials remains a significant challenge. Herein, based on the [Cd₄P₂] flexible host pore framework, through the structural symmetry and mixed-anion engineering, different tetrahedral units as the guest are selected as structural directing agents for embedding, resulting in host-guest halopnictides [Cd₄P₂][ZnCl₄] (space group P2₁, 1), [Cd₄P₂][MnCl₄] (P2₁2₁2₁, 2), [Cd₄P₂][ZnBr₄] (Pna2₁, 3), [Cd₄P₂][Mn_0.6Cd_0.4Br₄] (Pna2₁, 4), and [Cd₄P₂][ZnCl₃I] (Pna2₁, 5). Notably, we enhance sharply second-harmonic generation (SHG) responses (0∼6.6 × AgGaS₂@1700 nm) by synergistically disrupting local symmetry through host framework porosity tuning (54.1% to 63.1%) and constructing polarizable polyhedra via mixed-anion modulations. Furthermore, millimeter-sized single-crystal (3.0 × 5.0 × 8.0 mm³) 3 is successfully obtained, and possesses a strong SHG response (2.5 × AgGaS₂), a high laser-induced damage threshold (3.0 × AgGaS₂@1064 nm), and broad infrared transmittance (2.5-14.9 µm), indicating the possible promise as a NLO material. This work offers valuable insights for the development of NLO materials through combined structural symmetry and mixed-anion engineering.