“All-four-in-one”: a novel mercury tellurite–nitrate Hg3(TeO3)(Te3O7)(NO3)2 exhibiting exceptional optical anisotropy

Abstract

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 a 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 a [TeO₃]²⁻ triangular pyramid, [(Te₃O₇)²⁻]_∞ chains, and 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 a 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.