Syntheses, structures, physical properties, and electronic properties of some AMUQ₃ compounds (A = Alkali metal, M = Cu or Ag, Q = S or Se)

The seven new isostructural quaternary uranium chalcogenides KCuUS ₃, RbCuUS₃, RbAgUS₃, CsCuUS₃, CsAgUS₃, RbAgUSe₃, and CsAgUSe₃ were prepared from solid-state reactions. These isostructural materials crystallize in the layered KZrCuS₃ structure type in the orthorhombic space group Cmcm. The structure is composed of UQ₆ octahedra and MQ₄ tetrahedra that share edges to form _∞²[UMQ ₃^-] layers. These layers stack perpendicular to [010] and are separated by layers of face- and edge-sharing AQ₈ bicapped trigonal prisms. There are no Q-Q bonds in the structure, so the formal oxidation states of A/U/M/Q may be assigned as 1+/4+/1+/2-, respectively. CsCuUS₃ shows semiconducting behavior with thermal activation energy E_a = 0.14 eV and σ₂₉₈ = 0.3 S/cm. From single-crystal absorption measurements in the near IR range, the optical band gaps of these compounds are smaller than 0.73 eV. The more diffuse 5f electrons play a much more dominant role in the optical properties of the AMUQ₃ compounds than do the 4f electrons in the AMLnQ₃ compounds (Ln = rare earth). Periodic DFT spin band-structure calculations on CsCuUS₃ and CsAgUS₃ establish two energetically similar antiferromagnetic spin structures and show magnetic interactions within and between the layers of the structure. Density-of-states analysis shows M-Q orbital overlap in the valence band and U-Q orbital overlap in the conduction band.