Syntheses and characterization of six quaternary uranium chalcogenides A ₂M ₄U ₆Q ₁₇ (A = Rb or Cs; M = Pd or Pt; Q = S or Se)

The A ₂M ₄U ₆Q ₁₇ compounds Rb ₂Pd ₄U ₆S ₁₇, Rb ₂Pd ₄U ₆Se ₁₇, Rb ₂Pt ₄U ₆Se ₁₇, Cs ₂Pd ₄U ₆S ₁₇, Cs ₂Pd ₄U ₆Se ₁₇, and Cs ₂Pt ₄U ₆Se ₁₇ were synthesized by the high-temperature solid-state reactions of U, M, and Q in a flux of ACl or Rb ₂S ₃. These isostructural compounds crystallize in a new structure type, with two formula units in the tetragonal space group P4/mnc. This structure consists of a network of square-planar MQ ₄, monocapped trigonal-prismatic UQ ₇, and square-antiprismatic UQ ₈ polyhedra with A atoms in the voids. Rb ₂Pd ₄U ₆S ₁₇ is a typical semiconductor, as deduced from electrical resistivity measurements. Magnetic susceptibility and specific heat measurements on single crystals of Rb ₂Pd ₄U ₆S ₁₇ show a phase transition at 13 K, the result either of antiferromagnetic ordering or of a structural phase transition. Periodic spin-polarized band structure calculations were performed on Rb ₂Pd ₄U ₆S ₁₇ with the use of the first principles DFT program VASP. Magnetic calculations included spin-orbit coupling. With U f-f correlations taken into account within the GGA+U formalism in calculating partial densities of states, the compound is predicted to be a narrow-band semiconductor with the smallest indirect and direct band gaps being 0.79 and 0.91 eV, respectively.