Crystal Structure and Magnetic Properties of Uranium Hydride UH2 Stabilized as a Thin Film

Hydrogen in actinides: electronic and lattice properties

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 5fstates, and a charge transfer towards H, which influences mostly the 6dand 7sstates. These general mechanisms have diverse impact on individual actinides, depending on the degree of localization of their 5fstates. Hydrogenation of uranium yields UH₂and UH₃, binary hydrides that are strongly magnetic due to the 5fband narrowing and reduction of the 5f-6dhybridization. Pu hydrides become magnetic as well, mainly as a result of the stabilization of the magnetic 5f⁵state and elimination of the admixture of the non-magnetic 5f⁶component.Ab-initiocomputational analyses, which for example suggest that the ferromagnetism ofβ-UH₃is 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 5fⁿground-state density of states. Being affected by electron correlations in the excited final states, they rather reflect the atomic 5f^n-1multiplets. 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.

Advances in actinide thin films: synthesis, properties, and future directions

Actinide-based compounds exhibit unique physics due to the presence of 5f electrons, and serve in many cases as important technological materials. Targeted thin film synthesis of actinide materials has been successful in generating high-purity specimens in which to study individual physical phenomena. These films have enabled the study of the unique electron configuration, strong mass renormalization, and nuclear decay in actinide metals and compounds. The growth of these films, as well as their thermophysical, magnetic, and topological properties, have been studied in a range of chemistries, albeit far fewer than most classes of thin film systems. This relative scarcity is the result of limited source material availability and safety constraints associated with the handling of radioactive materials. Here, we review recent work on the synthesis and characterization of actinide-based thin films in detail, describing both synthesis methods and modeling techniques for these materials. We review reports on pyrometallurgical, solution-based, and vapor deposition methods. We highlight the current state-of-the-art in order to construct a path forward to higher quality actinide thin films and heterostructure devices.

Pressure-induced evolution of crystal and electronic structure of neptunium hydrides

An extensive exploration of high-pressure phase diagrams of NpH_x (x = 1-10) compounds was performed by using swarm-intelligence-based CALYPSO structure searches. We propose five stable hydrogen-rich clathrate phases (P4/nmm-NpH₅, Cmcm-NpH₇, Fm3̄m-NpH₈, P6₃/mmc-NpH₉, and Fm3̄m-NpH₁₀) that are composed of unusual H cages with stoichiometries H₂₀, H₂₄, H₂₉, and H₃₂ in which the H atoms are weakly covalently bonded to one another, with neptunium atoms occupying centers of the cages. The electronic structure analyses show that these predicted hydrogen-rich structures are all metallic phases, and Np-H and H-H bonds are formed by ionic and covalent bond interactions, respectively. The charge transfer from the Np atom plays an important role in the stability of the proposed structures. All hydrogen-rich clathrate structures show superconductivity behavior in their respective stability pressure range. Our work is an important step in understanding the phase stability and bonding behavior of NpH_x under extreme conditions and provides a valuable reference for experimental synthesis and identification of cage-like neptunium hydrides.