New Insights into Phase Separation of Cerium Hydrides under Pressure.

We report theoretical calculations of the static ground-state structures and pressure-induced phase transformations of four cerium hydrides: CeH, CeH2, CeH2.5, and CeH3. Under pressure, the experimental CaF2-type structure of CeH2 is likely to disproportionate to face-centered cubic (fcc) Ce and a cubic Pm3̅n (ß-UH3 type) structure of CeH3 above 6 GPa. At further increasing pressures, fcc Ce will transform to a tetragonal I4/mmm structure above 12 GPa, while CeH3 moves through the following sequence of phases: Pm3®n (ß-UH3 type) → Pm3̅n (A15 type) → R3®m; the corresponding transition pressures are calculated to be 10 and 70 GPa, respectively. The tetragonal I41/amd structure of CeH2.5 has the similar decomposition as that of CeH2. Finding this previously unreported pressure-induced decomposition of CeH2 will pave the way for investigations on the nature of hydrogen-metal interactions.

Predicted structures and superconductivity of LiYHn (n = 5-10) under high pressure.

The structures of LiYHn (n = 5-10) compounds in the pressure range of 0-300 GPa have been extensively explored using the CALYPSO structure prediction method based on the particle swarm optimization algorithm and first-principles calculation. Four stable structures (P21/m LiYH6, C2/c LiYH8, P1̄ LiYH9, R3̄m LiYH10) and three metastable phases (Pnma LiYH6, P1̄ LiYH8, Immm LiYH9) were predicted. They all exhibit metallic and superconducting behavior in their respective stable pressure ranges, and the predicted superconducting transition temperature Tc is within 22-109 K when the pressure is greater than 100 GPa. It was found that after doping Li into YHn (n = 6, 9, 10), the H2 units in the system increased, the electron-phonon coupling interaction weakened, and Tc decreased when the structural characteristics, electronic density of states distribution, and superconductivity of LiYHn and YHn (n = 6, 8, 9, 10) were compared. Systems that have a high density of H_s states and a low number of Y_d states at the Fermi level have stronger electron-phonon coupling (EPC) interactions and higher Tc.

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

An extensive exploration of high-pressure phase diagrams of NpHx (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 NpHx under extreme conditions and provides a valuable reference for experimental synthesis and identification of cage-like neptunium hydrides.

Activating Titanium Metal with H2 Plasma for the Hydrogen Evolution Reaction.

Developing a high-performance nonprecious metal electrocatalyst for water splitting is a strong demand for the large-scale application of electrochemical H2 production. In this work, we design a facile and scalable strategy to activate titanium metal for the hydrogen evolution reaction (HER) in alkaline media through incorporating hydrogen into the α-Ti crystal lattice by H2 plasma bombardment. Benefiting from the accelerated charge transfer and enlarged electrochemical surface area after H2 plasma treatment, the H-incorporated Ti shows remarkably enhanced HER activity with a much lower overpotential at -10 mA cm-2 by 276 mV when compared to the pristine Ti. It is revealed that the retention of the incorporated H(D) atoms in the Ti crystal lattice during HER accounts for the durable feature of the catalyst. Density functional theory calculations demonstrate the effectiveness of hydrogen incorporation in tuning the adsorption energy of reaction species via charge redistribution. Our work offers a novel route to activate titanium or other metals by H incorporation through a controllable H2 plasma treatment to tune the electronic structure for water splitting reactions.

Quantitative measurement of hydrogen isotopes in titanium using laser-induced breakdown spectroscopy.

Laser-induced breakdown spectroscopy (LIBS) has been considered a promising technology for nuclear safeguard inspection, especially for isotope content ratio determination, since it can be easily designed for portable, fast, and in situ measurement. However, it was a challenge to determine hydrogen isotopes in metal samples due to the unfavorable spectral interference, the poor calibration of the hydrogen content, and the small difference between the atomic emission intensity of hydrogen isotopes at around 656.28 nm. This paper presents the determination of hydrogen isotope contents ratio using LIBS under partially baseline-resolved conditions. The results show that by introducing a proper buffer atmosphere for the LIBS measurement, the resolution of the hydrogen and deuterium emissions could be improved, but still not enabled, by a baseline resolution with a moderate resolution spectrometer. However, with the method of integral intensity correction, the accurate quantitative measurement of hydrogen and deuterium contents in a metal matrix could be achieved. This work provided the possibilities for the further development of LIBS in hydrogen isotopes in in situ measurement for nuclear safeguards.

Insight into structural stability and helium diffusion behavior of Fe-Cr alloys from first-principles.

We have performed the first-principles method to study the structural stability and helium diffusion behavior of Fe-Cr alloys. The calculated bulk modulus of 284.935 GPa in the non-magnetic (NM) state is in good agreement with others. We have obtained solid evidence that the alloy structures meet the mechanical stability criteria and lattice dynamics conditions in the anti-ferromagnetism (AFM) and non-magnetic (NM) states. Compared with bulk γ-Fe, a slightly larger Young's modulus indicates that the doping of Cr helps to enhance the stiffness of the material and the ability to resist the reversible deformation of shear stress, but the ductility decreased slightly. Our results revealed that the addition of interstitial He atom promotes the expansion and deformation of the lattice, and further enlarges the cell volume. The presence of Cr in the alloy structures promotes the migration of a single helium atom between octahedral interstitials, and at the same time, inhibits the diffusion of helium atoms between tetrahedral interstitials to a large extent, which seem to be trapped in tetrahedral interstitials and cannot escape. The electronic properties show that the alloy materials exhibit obvious metallicity, and the doping of Cr generates an impurity state at lower energy, which is mainly formed by the s, p of Fe and s, p shell electrons of Cr. The charge density difference graphs corroborate that there is bonding interactions between Fe and Cr atoms. Bader charge analysis shows that a stronger polar covalent bond is formed between Fe and Cr in the non-magnetic (NM) state than in the anti-ferromagnetism (AFM) state. Our results provide useful information for understanding the initial growth of helium bubbles in experiments.

Metallization and positive pressure dependency of bandgap in solid neon.

The metallization of neon remains a controversial problem as there is no consensus in theoretical simulations and no experimental verification. In this work, the insulator-to-metal transition in fcc solid neon at high pressure was revisited with a coupling of the all-electron full-potential linear augmented-plane wave (FP-LAPW) method and the GW correction to avoid the potential unreliability of pseudopotential under high pressure and correct the inaccurate energy gaps caused by local density or generalized gradient approximation of the exchange-correlation. This FP-LAPW + GW calculation predicts that the bandgap closes at a density of 88.3 g/cm3 and a pressure of 208.4 TPa. Moreover, the reported positive pressure dependency of energy gap (increases with increasing density) for solid neon in 1.5-10.0 g/cm3 was confirmed with our FP-LAPW calculations, and the underlying mechanism was first revealed based upon analysis of the charge density distribution and the electron localization function. The results of this research will provide a valuable reference for future high pressure experiments and shed new insight into the planetary interiors.

Phase Segregation, Transition, or New Phase Formation of Plutonium Dioxide: The Roles of Transition Metals.

As impurities are virtually impossible to exclude from Pu oxides in realistic environments, understanding the roles of impurities is crucial for the applications and designs of Pu oxides. Here we perform a systematic first-principles DFT + U calculation to find the trends of transition-metal (TM) behaviors in PuO2 in terms of energetics, atomic properties, oxidation states, and electronic structures. The results show that group IV-B elements Ti, Zr, and Hf are energetically and electronically favorable in PuO2 and render the possibilities of forming Pu-TM-O ternary phases. In contrast, the remaining TMs tend to destabilize PuO2 and whether phase segregation or transition occurs largely depends on the redox conditions: oxidation one induces segregation, whereas reduction one facilitates the transition from PuO2 to Pu2O3. On the basis of the correlations between the properties of TMs and their relative stabilities in PuO2, we conclude that the degree of electron match between TMs and Pu plays the decisive role in the stability, as established for the cases of tetravalent elements, whereas some electron-mismatched but energetically stable TMs such as III-B and V-B elements could drive the valence transition of Pu, resulting in the phase instability of PuO2.