5f-Electron structure in light actinides from Mössbauer studies

Abstract

The 5f-electron shell in light actinides exhibits properties often quite different from those of their 4f-counterparts in the lanthanides. The main reason is the wider radial extent of the 5f-electrons, which can give rise to a reasonable overlap with other orbitals. The consequence is the formation of 5f-bands and the possibility of significant hybridization. Also, spin-orbit coupling and crystalline field splitting may well be of the same magnitude and relativistic effects play oa more dominant role. The specific electronic structure properties of the elemental metals, of intermetallics and of non-conducting compounds of the light actinides have been investigated thoroughly by Mössbauer spectroscopy, mainly on²³⁸U and²³⁷Np. Chemical compounds can be formed with widely different (formal) charge states of the actinide ion. They often have a strong covalent bonding character. Features of such bonds have been studied by systematic investigations of the hyperfine coupling parameters and pertinent results are discussed. The possibility of forming 5f-bands of different width and hybridization in metals and intermetallics leads to a broad spectrum of magnetic properties. In particular, we find both localized and itinerant electron magnets. The system of cubic Laves phases NpX₂ renders itself best for a detailed study of magnetic behavior. Mössbauer measurements under applied high pressure demonstrate that the actinide-actinide separation is one crucial parameter controlling the amount of 5f-delocalization. In addition, it is shown that spin and charge dynamical processes play an important role. These findings are further augumented by new compressibility data on NpAl₂ and NpOs₂.