Positional flexibility: Syntheses and characterization of six uranium chalcogenides related to the 2H hexagonal perovskite family

Six new uranium chalcogenides, Ba₄USe₆, Ba₃FeUSe₆, Ba₃MnUSe₆, Ba₃MnUS₆, Ba_3.3Rb_0.7US₆, and Ba_3.2K_0.8US₆, related to the 2H hexagonal perovskite family have been synthesized by solid-state methods at 1173 K. These isostructural compounds crystallize in the K₄CdCl₆ structure type in space group D3_d⁶-R3¯c of the trigonal system with six formula units per cell. This structure type is remarkably flexible. The structures of Ba₃FeUSe₆, Ba₃MnUSe₆, and Ba₃MnUS₆ consist of infinite ∞1[MUQ₆^6-] chains (M = Fe or Mn; Q = S or Se) oriented along the c axis that are separated by Ba atoms. These chains are composed of alternating M-centered octahedra and U-centered trigonal prisms sharing triangular faces; in contrast, in the structures of Ba₄USe₆, Ba_3.3Rb_0.7US₆, and Ba_3.2K_0.8US₆, there are U-centered octahedra alternating with Ba-, Rb-, or K-centered trigonal prisms. Moreover, the Ba₄USe₆, Ba₃FeUSe₆, Ba₃MnUSe₆, and Ba₃MnUS₆ compounds contain U⁴⁺, whereas Ba_3.3Rb_0.7US₆ and Ba_3.2K_0.8US₆ are mixed U^4+/5+ compounds. Resistivity and μ-Raman spectroscopic measurements and DFT calculations provide additional insight into these interesting subtle structural variations.