Hg2(SeO3)(SO4): the first sulfate selenite with large birefringence explored from d10 transition metal compounds

The exploration of new birefringent materials in the d10 transition metal sulfate selenite system resulted in three new compounds, namely, Hg2(SeO3)(SO4), La2Hg3(SeO3)4(SO4)2(H2O)2 and Ag2Cd(Se2O5)(Se0.3S0.7O4), using mild hydrothermal methods. Hg2(SeO3)(SO4) shows a 3D structure composed of 2D mercury selenite layers connected by 1D mercury sulfate chains; La2Hg3(SeO3)4(SO4)2(H2O)2 features a novel 3D network composed of 1D lanthanum sulfate chains and 2D mercury oxide layers, which are connected by a SeO3 triangular pyramid bridge; Ag2Cd(Se2O5)(Se0.3S0.7O4) exhibits a novel 3D framework that consists of selenate silver layers bridged by SO4 tetrahedra and CdO6 octahedra. These compounds can remain stable above 250 °C and their optical band gap values are larger than 3.5 eV. The birefringence (Δn) of Hg2(SeO3)(SO4) was calculated to be 0.133 at 532 nm and 0.126 at 1064 nm. Hg2(SeO3)(SO4) was proved as the first reported metal selenite sulfate exhibiting a large birefringence. In addition, the IR spectra, TG-DSC curves and theoretical calculations of the three compounds were studied.