Syntheses and Characterization of Six Quaternary Uranium Chalcogenides A₂M₄U₆Q₁₇ (A = Rb or Cs; M = Pd or Pt; Q = S or Se)

The A2M4U6Q17 compounds Rb2Pd4U6S17, Rb2Pd4U6Se17, Rb2Pt4U6Se17, Cs2Pd4U6S17, Cs2Pd4U6Se17, and Cs2Pt4U6Se17 were synthesized by the high-temperature solid-state reactions of U, M, and Q in a flux of ACl or Rb2S3. 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 MQ4, monocapped trigonal-prismatic UQ7, and square-antiprismatic UQ8 polyhedra with A atoms in the voids. Rb2Pd4U6S17 is a typical semiconductor, as deduced from electrical resistivity measurements. Magnetic susceptibility and specific heat measurements on single crystals of Rb2Pd4U6S17 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 Rb2Pd4U6S17 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.