Realizing large birefringence via S-substitution and anisotropic arrangement optimization

Abstract

Despite the widespread use of birefringence crystals in optical instruments, the birefringence of commercially available crystals is generally limited (Δn < 0.3), making them less suitable for demanding optical requirements. In this work, three novel birefringent crystals, namely (C₂N₅H₈)(H₂C₃N₃O₃)·H₂O (1), (C₂N₅H₈)₃(H₂C₃N₃S₃)(HC₃N₃S₃)·H₂O (2), and (C₂N₅H₈)(H₂C₃N₃S₃)·H₂O (3), were successfully synthesized via a two-step strategy. In their anionic structures, a displaced parallel arrangement is observed among three crystals, combined with a partially distinct arbitrary intersecting arrangement. Further research findings indicate that sulfur (S) substitution (from 1 to 2) and the optimized arrangement of functional groups (from 2 to 3) lead to a significant enhancement in the birefringence. The experimental birefringence values at 550 nm increased from 0.259 (1) to 0.347 (2), reaching 0.403 (3). Remarkably, the birefringence performance of compound 3 ranks third among all reported [C₃N₃S₃]-based materials. Theoretical calculations reveal that its high birefringence is primarily attributed to S-substitution and the optimized arrangement of functional groups. This work provides critical guidance for further exploration of the impact of structure on birefringence performance and opens up new research directions for designing high-performance optical materials.