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 D(4h)(15)-P42/nmc. 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 S2– anions. Coordination about the An center, which has symmetry 4m2, consists of two S2(2–) ions and four S2– 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.