Across the first third of the actinide series 5f electrons exhibit behavior intermediate between the delocalized regime often found in d-block transition-metal series and the almost exclusively localized 4f electrons. This complex electronic behavior of actinides has been well illustrated in the chalcogenides, where early actinides, notably Th/U, tend to act like larger tetravalent transition metals whereas the heavier actinides such as Pu behave like a trivalent lanthanide. A small number of Np chalcogenide compounds have been reported. They are either isostructural with Th/U analogues within which Np is predominantly tetravalent or isostructural with Pu/lanthanide analogues within which Np is trivalent. Based on these limited data, the 5f electrons of Np in chalcogenide environments appear intermediate between the itinerant and localized regimes, which makes Np the key to understanding the behavior of actinide chalcogenides. Evidence for this intermediate role is provided in our recent studies on Np2Se5 and NpSe2.  Np2Se5 displays a deviation from the Th analogues in An�Q and Q�Q interactions despite of their same compositions and structure-types. The newly discovered NpSe2 adopts a structure different from those of AnSe2 (An = Th�U)or AnSe2 (An = Pu�Cf), with distinct An�Q and Q�Q interactions. Structural, spectroscopic and magnetic results are described to illustrate some unusual properties pointing to a Np valence intermediate between +3 and +4. This work is supported by the U.S. DOE, OBES, Chemical Sciences, Geosciences, and Biosciences Division, Heavy Elements Program, under contract DE-AC02-06CH11357.

Neptunium: The key to understanding the behavior of actinide chalcogenides

247th National Meeting (2014)

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