[HgX] Linear Group Enabled Ultraviolet Birefringent Crystal RbHgBr with Strong Optical Anisotropy and Wide Bandgap.

Birefringent crystals are pivotal for modern optical modulation technologies, yet developing high-performance birefringent materials with large birefringence (Δn), wide bandgaps, and scalable synthesis remains a significant challenge. Different from the traditional planar [MQ] and distorted [MQ] (n ≥ 4) polyhedral units, a "linear-group" design strategy is proposed, targeting heavy-metal halides with [HgX] (X = halides) coordination modes to exploit their inherent polarizability anisotropy. Through systematic experimental investigations in the ternary A-Hg-X (A = Rb, Cs; X = Br, I) system, six novel Hg-based halides were synthesized. Notably, RbHgBr with linear [HgBr] units demonstrates excellent optical properties, including a wide bandgap (3.73 eV) and large Δn (0.35@546 nm). Importantly, the compound displays a good crystal growth habit, and the high-quality RbHgBr single crystal can be grown by the simple solution method. Theoretical calculations reveal that the strong optical anisotropy arises from the aligned [HgBr] linear units. The results demonstrate that RbHgBr is a promising birefringent material and give some new insights for designing high-performance optical materials based on the linear units with high polarizability anisotropy.