Structural Evolution of Hybrid Bismuth-Based Halides toward Enhanced Second-Harmonic Generation Response and Birefringence through Multisite Chemical Substitution

Exploring new material systems with an enhancement of the birefringence and second harmonic generation (SHG) effect represents a highly worthwhile pursuit. Herein, three bismuth-based halides, centrosymmetric (CS) (SCN₂H₅)₆BiBr₆·Br₃ (1), noncentrosymmetric (NCS) (NC₂H₈)₄BiBr₆·Br (2) and (NC₂H₈)₂(N₂H₄CS)BiBr₅ (3), were successfully designed through structural motif engineering. The regular full-halogen BiBr₆ octahedron in 1 and 2 was successfully transferred to highly distorted heteroligand BiSBr₅ octahedron in 3 via chemical substitution. Notably, the birefringence and SHG response of 3 are significantly enhanced to 0.118@546 nm and 2.1 × KDP, which are 5.4 and 2.6 times that of 2, respectively. Structural analysis and theory calculations reveal that the enhanced SHG intensity and birefringence of 3 can be attributed to the synergistic effect of unique heteroligand-coordinated [(N₂H₄CS)BiBr₅]^2- anions and organic NC₂H₈⁺ cations. This work not only discovers 3 as a promising nonlinear optical crystal but also provides an effective synthetic route for new metal halides with excellent optical properties.