La2U2Se9: an ordered lanthanide/actinide chalcogenide with a novel structure type.

The compound La(2)U(2)Se(9) was obtained in high yield from the stoichiometric reaction of the elements in an Sb(2)Se(3) flux at 1123 K. The compound, which crystallizes in a new structure type in space group Pmma of the orthorhombic system, has a three-dimensional structure with alternating U/Se and La/Se layers attached via three independent, infinite polyselenide chains. The U atom has a monocapped square antiprismatic coordination of Se atoms, whereas one La atom is bicapped square prismatic and the other La atom is trigonal prismatic. La(2)U(2)Se(9) displays an antiferromagnetic transition at T(N) = 5 K; above 50 K, the paramagnetic behavior can be fit to the Curie-Weiss law, yielding a mu(eff) of 3.10(1) mu(B)/U. The low-temperature specific heat of La(2)U(2)Se(9) exhibits no anomalous behavior near the Neel temperature that might indicate long-range magnetic ordering or a phase transition. X-ray absorption near-edge structure (XANES) spectra have confirmed the assignment of formal oxidation states of +III for lanthanum and +IV for uranium in La(2)U(2)Se(9).