The Flexible Ba7UM2S12.5O0.5 (M = V, Fe) Compounds: Syntheses, Structures and Spectroscopic, Resistivity, and Electronic Properties

Two new compounds, Ba7UV2­S12.5O0.5 and Ba7UFe2­S12.5O0.5, have been synthesized in fused-silica tubes by the direct combinations of V or Fe with U, BaS, and S at 1223 K. The compound Ba7UV2­S12.5O0.5 crystallizes at 100 K in the Cs7Cd3­Br17 structure type in space group D4h18–I4/mcm of the tetragonal system. The compound Ba7UFe2­S12.5O0.5 crystallizes at 100 K in space group D4h5–P4/mbm of the tetragonal system. The structures are very similar with V/S or Fe/S networks in which Ba atoms reside as well as channels large enough to accommodate additional Ba atoms and infinite linear US5O chains. Each U atom is octahedrally coordinated to four equatorial S atoms, one axial S atom, and one axial O atom. The Fe/S network contains a S–S single bond, whereas the V/S network does not. The result is that the Fe3+ compound charge balances with 7 Ba2+, U4+, 2 Fe3+, 10.5 S2–, S22–, and 0.5 O2–, whereas the V4+ compound charge balances with 7 Ba2+, U4+, 2 V4+, 12.5 S2–, and 0.5 O2–. Other differences between these two compounds have been characterized by Raman spectroscopy and resistivity measurements. DFT calculations have provided insight into the nature of their bonding. The overall structural motif of Ba7UV2­S12.5O0.5 and Ba7UFe2­S12.5O0.5 offers a remarkable flexibility in terms of the oxidation state of the incorporated transition metal.