Reinvestigation of Np₂Se₅: A Clear Divergence from Th₂S₅ and Th₂Se₅ in Chalcogen-Chalcogen and Metal-Chalcogen Interactions

Single crystals of Np₂Se₅ have been prepared through the reactions of Np and Se at 1223 K in an Sb₂Se₃ flux. The structure of Np₂Se₅, which has been characterized by single-crystal X-ray diffraction methods, crystallizes in the tetragonal space group P4₂/nmc. The crystallographic unit cell includes one unique Np and two Se positions. Se(1) atoms form one-dimensional infinite chains along the a and b axes with alternating intermediate Se-Se distances of 2.6489 (8) and 2.7999 (8) Å, whereas Se(2) is a discrete Se^2- anion. Each Np is coordinated to 10 Se atoms and every NpSe₁₀ polyhedron shares faces, edges, or vertices with 14 other identical metal polyhedra to form a complex three-dimensional structure. Np L_III-edge X-ray Absorption Near Edge Structure (XANES) measurements show a clear shift in edge position to higher energies for Np₂Se₅ compared to Np ₃Se₅ (Np³⁺₂Np⁴⁺Se ^2-₅). Magnetic susceptibility measurements indicate that Np₂Se₅ undergoes a ferromagnetic-type ordering below 18(1) K. Above the transition temperature, Np₂Se₅ behaves as a paramagnet with an effective moment of 1.98(5) μ_B/Np, given by a best fit of susceptibilities to a modified Curie-Weiss law over the temperature range 50-320 K.