Molecular Pseudo-Halogen Engineering Enables Remarkable Birefringence Enhancement in Hybrid Perovskites

Abstract

Hybrid perovskites have emerged as promising candidates for birefringent crystals due to their structural diversity.Conventional strategies to enhance birefringence often rely on incorporating highly π-conjugated organic components; however, it remains challenging to reconcile high birefringence (>0.3) and large single crystal growth. Herein, we present the pseudo-halogen engineering that also enables giant birefringence enhancement. By substituting halogen Cl ^-with polar pseudo-halogen (SCN) ^-, we designed and synthesized a one-dimensional hybrid perovskite, namely (C₆N₂H₁₅)Cd(SCN)₃, which exhibits high birefringence of Δn_exp = 0.37@550 nm, over 12 times that of isostructural (C₆N₂H₁₆)Cd₂Cl₆ •2H₂O (Δn_exp = 0.03@550 nm). This birefringence value not only surpasses all commercial birefringent crystals, but also is even higher than those of many hybrid perovskites composed of π-conjugated cations. Moreover, large single crystals of (C₆N₂H₁₅)Cd(SCN)₃ were readily grown by the facile evaporation method.First-principles calculations reveal that the remarkable birefringence enhancement originates from a tens-of-times increase in polarizability anisotropy of the distorted Cd(SCN)₆ octahedra compared to CdCl₅ •H₂O counterparts. This work provides a novel molecular-level strategy towards designing high-performance birefringent crystals for polarized optics.