“All-Four-in-One”: A Novel Mercury Tellurite-Nitrate Hg3(TeO3)(Te3O7)(NO3)2 Exhibiting Exceptional Optical Anisotropy

In recent years, birefringent crystals have attracted much attention in the field of optical materials and play a significant role in laser technology, and optical imaging. However, commercially available birefringent crystals are still relatively scarce and need improvement. Low-dimensional structures and well-oriented anisotropic units are conducive to obtaining excellent birefringent materials. Therefore, it is crucial to utilize molecular engineering strategy for designing crystal structures. Through screening, the tellurite-nitrate system caught our attention because most of them exhibit low dimensional structures. The structure of Hg₃(TeO₃)(Te₃O₇)(NO₃)₂ HTTN consists of unprecedented [(Hg₃Te₄O₁₀)²⁺]_∞ cationic layers built by [TeO₃]^2- triangular pyramid, [(Te₃O₇)^2-]_∞ chains, as well as novel [(Hg₃O₇)^8-]_∞ chains balanced by isolated NO₃⁻ anions. Here, (HTTN) with multiple functional units was obtained. HTTN has a birefringence value of 0.295@546 nm, which is significantly higher than those of all commercially available birefringent crystals and exhibits the highest value among tellurite-nitrate birefringent crystals. Structural analysis and theoretical calculations reveal that the synergistic interaction between [[TeO₃]^2- (5.19%) and [NO₃]^- (7.32%) groups, and [(Te₃O₇)^2-]_∞ (36%) and [(Hg₃O₇)^8-]_∞ (51.49%) chains, plays the crucial role in the optical anisotropy of HTTN. This study demonstrates that introducing functional units with high optical anisotropy is an effective strategy for developing high-performance birefringent materials.