Structural Chemistry of the Neptunium–Germanium Binary System

The structural chemistry of the complete binary neptunium–germanide system has been investigated by means of X-ray diffraction. In addition to the well-known NpGe3 (AuCu3 type), at least five new binary phases have been found. The crystal structure of NpGe2-x (defect ThSi2 type), NpGe2-x (defect AlB2 type), NpGe (USi type), Np5Ge4 (Ti5Ga4 type), and Np5Ge3 (W5Si3 type) have been refined from single-crystal and powder X-ray diffraction data. X-ray powder analyses by Rietveld-type profile refinement revealed a small homogeneity range of the three first phases, as usually observed with the rare-earth or uranium isotypic compounds, whereas Np5Ge4 and Np5Ge3 seem to be line compounds as the unit cell parameters were independent of the starting composition. The formation and the homogeneity range of these new phases are discussed, and the structural evolution is compared with the analogous uranium binary system.