The U$^{5+}$ compound Ba$_9$Ag$_{10}$U$_4$S$_{24}$: Synthesis, structure, and electronic properties

International audienceBlack crystals of Ba$_9$Ag$_{10}$U$_4$S$_{24}$ 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 $C_{4v}^{10}$$-I4cm$ of the tetragonal system with four formula units in a cell with lattice constants $a$=13.9189(6)angstrom and $c$=23.7641(11) angstrom (V=4604(5) angstrom$^3$). 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 US$_6$, AgS$_4$, and AgS$_3$ polyhedra to leave channels in which Ba atoms reside. Based on the values of the U$-$S interatomic istances, the compound Ba$_9$Ag$_{10}$U$_4$S$_{24}$ contains U$^{5+}$ and charge balance is achieved with the formal oxidation states of 9 Ba$^{2+}$, 10 Ag$^{1+}$, 4 U$^{5+}$ 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