Problem of neptunium valence in chalcogenides

An attempt to determine the population density of 5f atomic orbitals Pf in neptunium chalcogenides is undertaken and Np valence changes depending on the coordination numbers, internuclear distances and structural compounds on the basis of Moessbauer spectroscopy on 237Np nuclei are analyzed. The Moessbauer experiments have shown that isomer shifts (IS) in Np chalcogenides lie within the IS variation interval for NpF3 and NpF4. It is shown that in a series of Np compounds the ionic character of chemical bonds decreases while going from S atom to Te: NpS3, Np3S5, Np2S3, NpSe3, Np3Se5, Np2Se3, NpTe3. Following the same sequence the population density of 5f-AO atom increases and its oxidation degree decreases. The decrease of the ionic character of Np-X bond results in gradual changes of the population density and a valence jump