New Quaternary Thiostannates and Thiogermanates A₂Hg₃M₂S₈ (A = Cs, Rb; M = Sn, Ge) through Molten A₂S_x. Reversible Glass Formation in Cs₂Hg₃M₂S₈

Cs 2 Hg 3 M 2 S 8 (M = Ge, Sn) were synthesized in 70% and 65% yields by reacting HgS, and Sn or Ge, in molten Cs 2 S x at 520 °C. Rb 2 Hg 3 Sn 2 S 8 and Rb 2 Hg 3 Ge 2 S 8 were synthesized in 42% and 73% yield by reacting the same reagents in molten Rb 2 S x at 350 °C. Cs 2 Hg 3 M 2 S 8 crystallize in the triclinic space group P 1̄. Cs 2 Hg 3 Sn 2 S 8: a = 7.878(2) Å, b = 9.157(3) Å, c = 6.803(2) Å, α = 92.96(2)°, β = 109.45(2)°, γ = 107.81(2)°, V = 434.1(2) Å 3, and D calc = 5.207 g/cm 3 . The unit cell of isostructural Cs 2 Hg 3 Ge 2 S 8 is a = 7.808(2) Å, b = 9.164(2) Å, c = 6.612(2) Å, α = 92.02(2)°, β = 108.65(2)°g, γ = 108.10(2)°, and V = 419.9(2) Å 3 . Rb 2 Hg 3 M 2 S 8 crystallize in the monoclinic space group P 2 1 / c, Rb 2 Hg 3 Sn 2 S 8: a = 10.132(2) Å, b = 6.540(2) Å, c = 13.434(2) Å, β = 97.93(1)°, V = 881.7(6) Å 3, and D calc = 4.770 g/cm 3 . The unit cell of isostructural Rb 2 Hg 3 Ge 2 S 8 is a = 9.938 (3) Å, b = 6.352 (2) Å, c = 13.117 (3) Å, β = 97.33 (2)°, and V = 821.3 (4) Å 3 . The structure of Cs 2 Hg 3 M 2 S 8 consists of [Hg 3 M 2 S 8 ] 2 - layers separated by Cs + cations. The layers contain tetrahedral Sn 4+ or Ge 4+ centers and two types of Hg 2+, two coordinate linear and three-coordinate pseudotrigonal centers. The structure of Rb 2 Hg 3 M 2 S 8 consists of a 3-dimensional [Hg 3 M 2 S 8 ] 2 - framework with Rb + cations located within channels of the structure. This structure also contains tetrahedral Sn 4+ or Ge 4+ centers and two types of Hg 2+, the two-coordinate linear type and a four-coordinate “seesaw” geometry. Optical band gaps, determined from single-crystal UV/vis spectroscopy, range from 2.52 eV in the tin-based compounds to 2.89 eV in the germanium-based analogues. Cs 2 Hg 3 M 2 S 8 become glasses upon melting. Infrared and Raman spectroscopic characterization of the glasses are reported.