[C(<scp>NH₂</scp>)₃]<scp>₂SiF₆</scp>: Design of Deep Ultraviolet Crystals with Large Birefringence Based on a Fluorination‐Oriented Strategy

Comprehensive Summary Birefringent crystals exhibit significant advantages in the field of polarization state modulation due to their unique optical anisotropy properties. The property optimization between bandgap and birefringence depends on the crystal structure, which is always a key scientific problem in the field of functional crystal materials design. In this work, the fluorination‐oriented strategy was applied to the design of crystals with large birefringence and deep ultraviolet (DUV) cutoff edges. By introducing the fluorinated [SiF 6 ] octahedron, the arrangement orientation of the π‐conjugated [C(NH 2 ) 3 ] groups was accurately regulated, thereby achieving the design and preparation of a new crystal [C(NH 2 ) 3 ] 2 SiF 6 with a large bandgap (6.34 eV) and large birefringence (0.16 @546 nm). In addition, [C(NH 2 ) 3 ] 2 SiF 6 (GSF) can obtain large‐sized crystals through a simple water solution evaporation method, and it has unique advantages such as low cost and easy growth. The synergistic mechanism between the core factors of significant birefringence and wide bandgap in [C(NH 2 ) 3 ] 2 SiF 6 was revealed through experimental characterization and theoretical calculation. This work provides promising candidates with exceptional properties for DUV applications.