Influences of Surface Substitutional Ti Atom on Hydrogen Adsorption, Dissociation, and Diffusion Behaviors on the α‑U(001) Surface

The hydrogen adsorption, dissociation, and diffusion behaviors on both clean and Ti-doped α-U(001) surfaces are systematically studied with density functional theory method. Through detailed potential energy surface calculations, we find that the dissociation at the bridge sites is energetically more favorable, where the H₂ molecule dissociates without any energy barrier and the dissociated hydrogen atoms move into two neighboring 3-fold sites. Once a substitutional Ti atom exists on the α-U(001) surface, the hydrogen molecule similarly dissociates without any energy barriers. However, the diffusion of the dissociated hydrogen atoms is dramatically changed after introduction of a surface substitutional Ti atom. The into-bulk penetration of a hydrogen atom through a defect-free surface is endothermic and needs to overcome an energy barrier of 0.8–0.9 eV. In contrast, the penetration to the subsurface sites near the doped Ti atom is exothermic, and the activation barrier decreases by 0.3–0.4 eV. Our results indicate that surface doped titanium atoms in the outermost layer may behave like hydrogen trapping sites for α-U

New Insights into the Crystal Structures of Plutonium Hydrides from First-Principles Calculations

One of the important research contents on hydrogen corrosion of plutonium is the determination of the complex crystal structures of plutonium hydrides and the bonding interactions between plutonium and hydrogen. However, it is very difficult to carry out the structural characterization of plutonium hydrides because of their high activity, high toxicity, and radioactivity. In this work, the crystal structures, lattice vibrations, and bonding properties of plutonium hydrides under ambient pressure are investigated by means of the density functional theory + <i>U</i> approach. Results show that PuH₃ exhibits many competition phase structures. After considering spin polarization, strong correlation (<i>U</i>), and spin–orbit coupling effects on the total energy and lattice dynamics stability, it is found that PuH₃ at ambient pressure is more likely to be hexagonal <i>P</i>6₃<i>cm</i> or trigonal <i>P</i>3<i>c</i>1 structure, instead of the usual supposed structures of hexagonal <i>P</i>6₃/<i>mmc</i> (LaF₃-type) and face-centered cubic (BiF₃-type). The calculated electronic structures clearly indicate that <i>P</i>6₃<i>cm</i> (<i>P</i>3<i>c</i>1) PuH₃ is a semiconductor with a small band gap about 0.87 eV (0.85 eV). The Pu–H bonds in Pu hydrides are dominated by the ionic interactions

Ammonia Activation by Ce Atom: Matrix-Isolation FTIR and Theoretical Studies

The activation of ammonia by cerium atom has been investigated in solid argon using infrared spectroscopy and density functional theoretical calculations. The results reveal that the spontaneous formation of CeNH₃ complex on annealing is the initial step in the reactions of cerium atoms with ammonia. The CeNH₃ complexes rearrange to generate the inserted HCeNH₂ molecules on irradiation. A “triplet–singlet” spin conversion occurs along the reaction path in which HCeNH₂ (³A″) isomerizes into H₂CeNH (¹A′). The H₂CeNH molecules finally decompose to yield HCeN + H₂ upon photolysis. The periodic trend and differences for the M + NH₃ (M = Ti, Zr, Hf, Ce, Th) systems are discussed on the basis of the present and previous works. DFT calculations predict that the most stable ground state for HHfNH₂ and HThNH₂ is singlet due to the stronger relativistic effects in Hf and Th atoms, while that for HTiNH₂, HZrNH₂, and HCeNH₂ is triplet. Besides, the H₂-elimination process is different for Ce and M (Ti, Zr, Hf, Th) cases