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

Abstract

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 LIII-edge X-ray Absorption Near Edge Structure (XANES) measurements show a clear shift in edge position to higher energies for Np2Se5 compared to Np3Se5 (Np(3+)2Np(4+)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) μB/Np, given by a best fit of susceptibilities to a modified Curie-Weiss law over the temperature range 50-320 K.

NpSe2: a Binary Chalcogenide Containing Modulated Selenide Chains and Ambiguous‐Valent Metal

“…Overall, the peak positions and their shapes indicate valence that are intermediate between pure 3+ and 4+. The interpretation of ambiguous valence for Np in NpSe 2 is consistent with our X‐ray absorption near‐edge structure (Figure S4) and with a previously reported study …”

“…Whereas the XANES and XES spectra from our NpSe 2 sample are similar to those of Np 2 Se 5 , the magnetic susceptibility results show significant differences. Although they both can be fit with a modified Curie–Weiss law (Supporting Information) over the temperature range of 50–320 K, the Np effective moment for NpSe 2 (Figure S6) is significantly larger (2.7(2) μ Β ) than previously reported for Np 2 Se 5 (1.9 μ Β ) . Although the effective moment of NpSe 2 is closer to the free‐ion value of 2.68 μ Β for Np 3+ than to 3.62 μ Β for Np 4+ , a direct determination of valence state is not possible from these values alone because of potential crystal‐field effects, which would reduce the moment from the free‐ion value.…”

“…Our recent reinvestigation of Np 2 Se 5 reveals that Np chalcogenides deviate from other actinide analogues, such as Th 2 Se 5 , in metal–chalcogen and chalcogen–chalcogen interactions despite having the same stoichiometry and substructure‐type . Here we describe the new binary neptunium selenide, NpSe 2 , which possesses M–Q and Q–Q connectivities that differ from any other metal dichalcogenides, including Th–U and Pu–Cf .…”

“…Np 3 Se 5 is mixed‐valent with crystallographically unique Np 3+ and Np 4+ sites in a 2:1 ratio . Np 2 Se 5 is included for comparison as its structure consists of unmodulated linear Se‐chain and ambiguous‐valent Np sites . The main peak in the Np 3 Se 5 reference spectrum is assigned the position of the Np 3+ emission and the peak in the NpO 2 to the position for a Np 4+ component.…”

“…However, given the that the bonding interactions between chain Se atoms are weaker than the single Se−Se bond, the average valence of Se(2) and Se(3) in NpSe 2 is less than 0 and greater than −2. It is not feasible to perform empirical bond‐valence sum calculations to estimate the valence of Np, as there are only a limited number of reliable Np−Se distances reported . Nonetheless, based on charge‐balance considerations, the average valence of Np in NpSe 2 is less than +4.…”