Ba2GeS4 and Mg2SnS4: synthesis, structures, optical properties and electronic structures

Two ternary metal chalcogenides, Ba2GeS4 and Mg2SnS4, have been synthesized by a high-temperature solid-state reaction. Single-crystal X-ray diffraction analysis reveals that they crystallize in the same space group Pnma with a three-dimensional framework composed of discrete MS4 (M = Ge and Sn) tetrahedra, but they are not isostructural. The arrangements of isolated MS4 (M = Ge and Sn) units are obviously different in the Ba2GeS4 and Mg2SnS4 structures - they have their a and c axes interchanged - and this leads to different structures for the title compounds. The UV-Vis-IR diffuse reflectance spectra show that the optical band gap is about 2.05 eV for Mg2SnS4. IR spectra of the title compounds are also measured and show a wide IR transmission range. First-principle theoretical studies are used to aid the understanding of the electronic structures and linear optical properties.