The flexible Ba₇UM₂S_12.5O_0.5 (M = V, Fe) compounds: syntheses, structures and spectroscopic, resistivity, and electronic properties

Two new compounds, Ba₇UV₂S_12.5O_0.5 and Ba₇UFe₂S_12.5O_0.5, 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 Ba₇UV₂S_12.5O_0.5 crystallizes at 100 K in the Cs₇Cd₃Br₁₇ structure type in space group D_4h¹⁸-I₄/mcm of the tetragonal system. The compound Ba₇UFe₂S_12.5O_0.5 crystallizes at 100 K in space group D4h 5 -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 S^2-, S₂^2-, and 0.5 O₂^-, whereas the V⁴⁺ compound charge balances with 7 Ba²⁺, U⁴⁺, 2 V⁴⁺, 12.5 S^2-, 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 Ba₇UV₂S_12.5O_0.5 and Ba₇UFe₂S_12.5O_0.5 offers a remarkable flexibility in terms of the oxidation state of the incorporated transition metal. (authors)