A₂Hg _x (SeO₃) _y (A = K, Rb, Cs): Three Alkali Metal Mercury Selenites Featuring Unique 1D [HgO _m (SeO₃) _n ] Chains

Herein, three alkali metal mercury selenites, K2Hg2(SeO3)(3), Rb2Hg2(SeO3)(3), and Cs2Hg3(SeO3)(4), were successfully obtained by a hydrothermal method. The three compounds featured same one-dimensional (1D) [HgOm(SeO3)(n)](infinity) chain structure that consisting of distorted Hg-O polyhedra and SeO3 triangular pyramids with stereochemically active lone pair (SCALP) electrons. Interestingly, the rich coordination environment of Hg atoms and the size difference of alkali metal cations lead to diverse arrangement of SeO3 groups, which makes them exhibit different birefringence. The band gaps of the three compounds indicate that they are potential ultraviolet (UV) optical materials. Detailed theoretical calculations demonstrate that the combined effects of SeO3 triangular pyramids and Hg-O polyhedra are responsible for the optical characteristics of the reported compounds.