The U⁵⁺ compound Ba₉Ag₁₀U₄S₂₄: Synthesis, structure, and electronic properties

Black crystals of Ba₉Ag₁₀U₄S₂₄ have been made by direct combination of BaS, Ag, U, and S at 1273 K. This compound crystallizes in a new structure type in the space group C4v10-I4cm of the tetragonal system with four formula units in a cell with lattice constants a=13.9189(6) Å and c=23.7641(11) Å (V=4604(5) ų). Multiphoton Luminescence Spectroscopy measurements are consistent with the noncentrosymmetric nature of the structure. In the structure each U atom is octahedrally coordinated by six S atoms, whereas three of the five crystallographically independent Ag atoms are tetrahedrally coordinated to four S atoms, another has a seesaw coordination to four S atoms, and the last has a triangular coordination to three S atoms. The overall structure consists of the three-dimensional stacking of the US₆, AgS₄, and AgS₃ polyhedra to leave channels in which Ba atoms reside. Based on the values of the U-S interatomic distances, the compound Ba₉Ag₁₀U₄S₂₄ contains U⁵⁺ and charge balance is achieved with the formal oxidation states of 9 Ba²⁺, 10 Ag¹⁺, 4 U⁵⁺ and 24 S^2-. DFT calculations predict an antiferromagnetic ground state and a band gap of 2.1 eV. Resistivity measurements indicate that the compound is a semiconductor with a complex activation mechanism and activation energies ranging from 0.03(1) eV to 0.08(1) eV.