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 MQ3 and distorted MQn (n ≥ 4) polyhedral units, a "linear-group" design strategy is proposed, targeting heavy-metal halides with HgX2 (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, RbHg5Br11 with linear HgBr2 units demonstrates excellent optical properties, including a wide bandgap (3.73 eV) and large Δn( cal. ) (0.35@546 nm). Importantly, the compound displays a good crystal growth habit, and the high-quality RbHg5Br11 single crystal can be grown by the simple solution method. Theoretical calculations reveal that the strong optical anisotropy arises from the aligned HgBr2 linear units. The results demonstrate that RbHg5Br11 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.
Du et al. (Tue,) studied this question.