Hg 16 O 12 (NO 3 ) 6 F 2 (H 2 O): A Mercury-Rich Nitrate Oxyfluoride with Diverse Triple-Coordinated Mercury-Based Units That Exhibits an Unparalleled [(Hg 16 O 12 F 2 (H 2 O)) 6+ ] ∞ Cationic Framework and a Large Birefringence.

Designing new compounds based on anion regulation has been widely favored due to the production of diverse crystal structures and excellent optical properties. Here, a new nitrate oxyfluoride, Hg 16 O 12 (NO 3 ) 6 F 2 (H 2 O), has been obtained through a hydrothermal reaction. It crystallizes in the centric Ibca space group and shows a novel three-dimensional [(Hg 16 O 12 F 2 (H 2 O)) 6+ ] ∞ cationic framework composed of interconnected HgO 2 F, HgO 3 , and HgO 2 (H 2 O) units, with isolated NO 3 - groups as balanced anions to build the whole structure. Notably, the HgO 2 F and HgO 2 (H 2 O) units are first presented here among mercury (Hg)-based compounds. Additionally, Hg 16 O 12 (NO 3 ) 6 F 2 (H 2 O) exhibits a large birefringence of 0.17 at 546 nm. This work enriches the multiformity of Hg-based compounds and provides a route for developing promising birefringent materials.