Structures and bonding in K_0.91U_1.79S₆ and KU₂Se₆

The compounds K_0.91U_1.79S₆ and KU ₂Se₆, members of the AAn₂Q₆ actinide family (A = alkali metal or Tl; An = Th or U; Q = S, Se, or Te), have been synthesized from US₂, K₂S, and S at 1273 K and U, K ₂Se, and Se at 1173 K, respectively. KU₂Se₆ shows Curie-Weiss behavior above 30 K and no magnetic ordering down to 5 K. The value of μ_eff is 2.95(1) μ_B/U. Its electronic spectrum shows the peaks characteristic of 5f-5f transitions. It is a semiconductor with an activation energy of 0.27 eV for electrical conduction. Both K_0.91U_1.79S₆ and KU₂Se ₆ crystallize in space group lmmm of the orthorhombic system and are of the KTh₂Se₆ structure type. Both contain infinite one-dimensional linear Q-Q chains characteristic of the AAn₂Q ₆ family. Typical of the known AAn₂Q₆ compounds, in KU₂Se₆, there are two alternating Se-Se distances of 2.703(2) and 2.855(2) Å, both much longer than an Se-Se single bond. In contrast, in K_0.91U_1.79S₆, the first sulfide of this family to be characterized structurally, there are alternating normal S₂^2- pairs 2.097(5) Å in length. In K_0.91U_1.79S₆, the formal oxidation state of U is 4+.