Ba₂An(S₂)₂S₂ (An = U, Th): Syntheses, Structures, Optical, and Electronic Properties

The compounds Ba₂An­(S₂)₂S₂ (An = U, Th) have been synthesized by reactions of the elements with BaS and S at 1273 and 1173 K, respectively. These isostructural compounds crystallize in a new structure type in the tetragonal space group <i>D</i>_4<i>h</i>¹⁵-<i>P</i>4₂/<i>nmc</i>. The structure comprises Ba²⁺ cations and _∞²[An­(S₂)₂(S)₂^4–] layers. The An⁴⁺ cations in these layers are arranged linearly and are bridged by S^2– anions. Coordination about the An center, which has symmetry 4̅<i>m</i>2, consists of two S₂^2– ions and four S^2– ions. Thus, the compounds are charge-balanced with An⁴⁺. No other alkali-metal actinide chalcogenides are known that contain chalcogen–chalcogen bonds. Optical measurements on Ba₂Th­(S₂)₂S₂ indicate a direct band gap of 2.46(5) eV. Density functional theory calculations, performed with the HSE exchange-correlation potential, lead to band gaps of 2.2 and 1.8 eV for Ba₂Th­(S₂)₂S₂ and Ba₂U­(S₂)₂S₂, respectively, thus demonstrating the utility of applying this functional to 5f-electron systems