Halogen-ion-driven polymorphs for high-performance nonlinear optical crystalline materials

Noncentrosymmetric (NCS) crystalline materials are indispensable for nonlinear optical (NLO) applications, yet their rational design remains challenging due to thermodynamic preferences for centrosymmetric configurations. Herein, we demonstrate a halogen-driven strategy for obtaining NCS structures by leveraging competitive coordination between halide anions (X^-) and stereochemically active Sn²⁺ centers. Precise halogen substitution induced the formation of two new polymorphs of [N(C₂H₅)₄]SnBr₃ (Cc and Cmc2₁ phase) using different halide sources (i.e., [N(C₂H₅)₄]Cl or [N(C₂H₅)₄]Br). Compared to the Cc phase (2 × KH₂PO₄), the resulting Cmc2₁ phase exhibits exceptional second-harmonic generation (SHG) efficiency (5.6 × KH₂PO₄). This performance ranks among the highest for all reported Sn-based organic-inorganic hybrid NLO materials to date. Theoretical calculations indicate that the [SnBr₃]^- unit is the primary source of the strong SHG response. This work establishes halogen-driven symmetry control as a viable strategy for achieving a dramatically enhanced SHG response, thereby providing a valuable reference for the rational design of high-performance NLO materials.