8 : a ) 7.878(2) Å, b ) 9.157(3) Å, c ) 6.803(2) Å, R ) 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, R ) 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 P2 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 ] 2layers 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 ] 2framework 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.
