From Coplanarity to Coaxiality: A Linearity-Oriented Design Paradigm for Ultrahigh Ultraviolet Birefringent Inorganic Crystals

Birefringent materials capable of manipulating and detecting polarized light are indispensable to modern optical and photonic technologies. Achieving both wide band gaps and large optical anisotropy remains inherently challenging, as these properties are often mutually constrained yet essential for device miniaturization and ultraviolet (UV) applications. Although purely inorganic crystals offer superior chemical stability and mechanical robustness, those exhibiting large birefringence are rarely realized. Herein, we report a systematic exploratory synthesis of mercury halide/pseudohalide inorganics, in which stepwise substitution of chloride ligands by linear thiocyanate anions generates a series of UV-transparent birefringent crystals: NH4Cl·HgCl2 (1), HgCl(SCN) (2), Hg(SCN)2 (3), and NH4Hg(SCN)3 (4). Structurally, compounds 1−4 incorporate linear [HgXX’] (X, X’ = Cl or S) units and/or linear [SCN]− anions with pronounced geometric anisotropy arranged in distinct coaxial and coplanar fashions, providing ideal model systems to elucidate how orientational coherence of linear modules governs birefringence. Combined experimental measurements and first-principles calculations identify compounds 2 and 3 as high-performance UV birefringent materials, exhibiting ultrahigh birefringence values of Δn2 = 0.526 and Δn3 = 0.754 at 546 nm together with wide band gaps exceeding 3.20 eV (3.50 and 3.53 eV, respectively). Structural fingerprint and response electron distribution anisotropy analyses establish a comprehensive structure−property relationship for linear-module-dominant systems, revealing that coaxiality, rather than conventional coplanarity, constitutes the decisive structural criterion for constructing next-generation birefringent crystals based on linear functional modules.