La<sub>2</sub>U<sub>2</sub>Se<sub>9</sub>: An Ordered Lanthanide/Actinide Chalcogenide with a Novel Structure Type
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The compound La2U2Se9 was obtained in high yield from the stoichiometric reaction of the elements in an Sb2Se3 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. La2U2Se9 displays an antiferromagnetic transition at TN = 5 K; above 50 K, the paramagnetic behavior can be fit to the Curie−Weiss law, yielding a μeff of 3.10(1) μB/U. The low-temperature specific heat of La2U2Se9 exhibits no anomalous behavior near the Néel 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 La2U2Se9.
