Electronic structure and isomer shifts of neptunium compounds

Abstract

The electronic structures of $\ensuremath{\alpha}\mathrm{Np}$ metal and 28 Np compounds are calculated with the generalized gradient approximation to density-functional theory, implemented with the full-potential linear-muffin-tin-orbital method. The calculations are compared to experimental isomer shifts providing a calibration of the ${}^{237}\mathrm{Np}$ isomeric transition with a value of $\ensuremath{\Delta}〈{r}^{2}〉=(\ensuremath{-}40.1\ifmmode\pm\else\textpm\fi{}1.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3} {\mathrm{fm}}^{2}$ for the difference in nuclear radius between the excited isomeric level and the ground state. The isomer shift is primarily determined by the chemical environment. Decreasing the volume, either by external or chemical pressure, causes an $\stackrel{\ensuremath{\rightarrow}}{f}s+d$ charge transfer on Np, which leads to a higher electron contact density. The possible f-electron localization in Np compounds is discussed using self-interaction corrections, and it is concluded that f-electron localization has only a minor influence on the isomer shift.