[CN4H8]SiF6: Fluorination-guided design of deep-UV crystal with large birefringence

Birefringent crystals exhibit significant advantages in the field of polarization modulation due to their intrinsic optical anisotropy. However, the synergistic optimization of bandgap and birefringence remains a pivotal scientific challenge, as both properties are critically dependent on the underlying crystal structure. Herein, a fluorination-guided strategy was employed to design a crystal featuring both large birefringence and deep-ultraviolet transparency. Consequently, a crystal, [CN 4 H 8 ]SiF 6 , was successfully synthesized by incorporating fluorinated [SiF 6 ] 2− octahedron and π-conjugated [CN 4 H 8 ] 2+ moiety, as well as the spatial orientation of [CN 4 H 8 ] 2+ moiety was regulated. As expected, the [CN 4 H 8 ]SiF 6 possesses a wide bandgap of 6.57 eV and a large birefringence of Δ n  = 0.062 @ 546 nm. By combining experimental characterization and the first principles calculations, we elucidate that the highly polarizable [CN 4 H 8 ] 2+ cation and the dense hydrogen-bonding networks of [SiF 6 ] 2− anions jointly dictate the optical properties of the material. This work not only identifies [CN 4 H 8 ]SiF 6 as a superior candidate with an optimal trade-off between bandgap and birefringence, but provides a design paradigm for expanding the deep-ultraviolet material through coupling anisotropic organic template with fluoro-inorganic framework.