Hydrogen in actinides: electronic and lattice properties

Abstract Hydrides of actinides, their magnetic, electronic, transport, and thermodynamic properties are discussed within a general framework of H impact on bonding, characterized by volume expansion, affecting mainly the 5 f states, and a charge transfer towards H, which influences mostly the 6 d and 7 s states. These general mechanisms have diverse impact on individual actinides, depending on the degree of localization of their 5 f states. Hydrogenation of uranium yields UH 2 and UH 3 , binary hydrides that are strongly magnetic due to the 5 f band narrowing and reduction of the 5 f -6 d hybridization. Pu hydrides become magnetic as well, mainly as a result of the stabilization of the magnetic 5 f 5 state and elimination of the admixture of the non-magnetic 5 f 6 component. Ab-initio computational analyses, which for example suggest that the ferromagnetism of β -UH 3 is rather intricate involving two non-collinear sublattices, are corroborated by spectroscopic studies of sputter-deposited thin films, yielding a clean surface and offering a variability of compositions. It is found that valence-band photoelectron spectra cannot be compared directly with the 5 f n ground-state density of states. Being affected by electron correlations in the excited final states, they rather reflect the atomic 5 f n −1 multiplets. Similar tendencies can be identified also in hydrides of binary and ternary intermetallic compounds. H absorption can be used as a tool for fine tuning of electronic structure around a quantum critical point. A new direction is represented by actinide polyhydrides with a potential for high-temperature superconductivity.