Syntheses, structures, and electronic properties of Ba₃FeUS ₆ and Ba₃AgUS₆

The compounds Ba₃FeUS₆ and Ba₃AgUS ₆ have been synthesized by the reactions of BaS, U, S, and M (= Fe or Ag) at 1223 K. These two isostructural compounds crystallize in the K ₄CdCl₆ structure type in the trigonal system in space group D3d⁶-R3Ì...c. Both structures feature infinite ∞[MUS₆^6-] chains along c that are separated by Ba atoms. The ∞1[FeUS₆^6-] chains are formed by the face-sharing of US₆ trigonal prisms with FeS₆ octahedra; in contrast, the ∞1[AgUS₆^6-] chains are formed by the face-sharing of US₆ octahedra with AgS₆ trigonal prisms. The Ba₃FeUS₆ compound charge balances with 3 Ba²⁺, 1 Fe²⁺, 1 U⁴⁺, and 6 S^2-, whereas Ba₃AgUS₆ charge balances with 3 Ba²⁺, 1 Ag¹⁺, 1 U⁵⁺, and 6 S^2-. This structure offers a remarkable flexibility in terms of the oxidation state of the incorporated uranium depending on the oxidation state of the d-block metal. DFT calculations performed with the HSE functional have led to band gaps of 2.3 and 2.2 eV for Ba₃FeUS₆ and Ba₃AgUS₆, respectively. From resistivity measurements, the Arrhenius activation energies are 0.12(1) and 0.43(1) eV for Ba₃FeUS₆ and Ba₃AgUS ₆, respectively.