A strategy to achieve giant optical anisotropy via integrating linear pseudohalogen and stereochemical active lone pair cation
The precise control of optical polarization states underpins photonic engineering, yet commercial birefringent crystals such as MgF2, α-BaB2O4, and CaCO3 are restricted to specific wavelength ranges and exhibit insufficient birefringence for ultracompact systems. This study introduces a dual-functional group-driven polarization enhancement strategy integrating linear pseudohalogen units ([SCN] and [NCN]) with stereochemically active lone pair electrons (LPEs) of Sn2+. Four compounds—SnCN2, Sn2OCN2, Sn(SCN)2, and Sn(SCN)F—were synthesized and characterized. They exhibit exceptional birefringence values of 0.44–0.73 at 1064 nm, surpassing commercial benchmarks (α-BBO, YVO4, TiO2) by 1.7- to 6.1-fold. SnCN2 achieves the highest birefringence (0.73) due to maximal [NCN] density (0.048 Å−3) and Sn-polyhedra density (0.024 Å−3). Sn(SCN)2 exhibits 0.64 birefringence, four times that of CaCO3, despite lower active unit density, underscoring the pivotal role of spatial configuration. Electron density analysis reveals extreme anisotropy in [SCN] units of Sn(SCN)2, with LPE density concentrated along the nz axis. In contrast, Sn(SCN)F shows minimal electron density variation due to stereochemical inactivity and omnidirectional polarization contributions. This work establishes a fundamental design paradigm for high birefringence materials by maximizing electron density gradients along distinct optical axes.