Recent Progress in Hg-Based Oxysalt Crystals for Nonlinear Optics and Birefringence Applications

Nonlinear optical (NLO) and birefringent crystals are core materials for modern optoelectronic technology. Optimizing their performance and developing novel compounds remain key frontiers in solid-state chemistry and optical materials science. Hg-based compounds are particularly promising for breaking structural symmetry and achieving strong NLO responses and large birefringence. Their potential originates from a combination of unique electronic configurations, flexible coordination geometries (e.g., HgO₃, HgO₄, [Hg-Hg]²⁺ units), and strong synergistic effects with anisotropic oxyanions. Together, these features provide a valuable basis for the targeted design of high-performance optical crystals. This review systematically surveys 33 Hg-based oxysalt crystals reported in the last five years. Classified by anionic groups (including nitrites, sulfates, phosphates, selenites, and tellurites), it analyzes their structural features, summarizes their NLO and birefringence properties, and explores the structure-property relationships governed by the synergy between Hg-based functional units and anisotropic oxyanions. Finally, building on the emerging trend from optimizing single-anion compounds to constructing new structures via mixed-anion engineering, we discuss prospective research directions and challenges for mercury-based oxysalt optical crystals.