Syntheses, structures, and magnetic properties of Np₃S ₅ and Np₃Se₅

Black prisms of Np₃Q₅ (Q = S, Se) have been synthesized by the stoichiometric reactions between Np and Q at 1173 K in a CsCl flux. The structures of these compounds were characterized by single-crystal X-ray diffraction methods. The Np₃Q₅ compounds are isostructural with U₃Q₅. The structure of Np ₃Q₅ is constructed from layers of Np(1)Q₈ distorted bicapped trigonal prisms that share faces with each other on bc planes. Each Np(1 )Q₈ layer further shares Q(2) edges with two adjacent identical neighbors to form a three-dimensional framework. The space inside each channel within this framework is filled by one single edge-sharing Np(2)Q₇ distorted 7-octahedron chain running along the b axis. Magnetic susceptibility measurements show that Np₃S₅ and Np₃Se₅ have antiferromagnetic orderings at 35(1 ) and 36(1) K, respectively. Above the magnetic ordering temperatures, both Np ₃S₅ and Np₃Se₅ behave as typical Curie-Weiss paramagnets. The effective moments obtained from the fit of the magnetic data to a modified Curie-Weiss law over the temperature range 70 to 300 K are 2.7(2) μ_B (Np₃S5) and 2.9(2) μ_B (Np₃Se₅).