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Uranium-based materials are valuable assets in the energy, medical, and military industries. However, understanding their sensitivity to hydrogen embrittlement is particularly challenging due to the toxicity of uranium and the computationally expensive nature of quantum-based methods generally required to study such processes. In this regard, we have developed a Chebyshev Interaction Model for Efficient Simulation (ChIMES) that can be employed to compute energies and forces of U and UH3 bulk structures with vacancies and hydrogen interstitials with accuracy similar to that of Density Functional Theory (DFT) while yielding linear scaling and orders of magnitude improvement in computational efficiency. We show that the bulk structural parameters, uranium and hydrogen vacancy formation energies, and diffusion barriers predicted by the ChIMES potential are in strong agreement with the reference DFT data. We then use ChIMES to conduct molecular dynamics simulations of the temperature-dependent diffusion of a hydrogen interstitial and determine the corresponding diffusion activation energy. Our model has particular significance in studies of actinides and other high-Z materials, where there is a strong need for computationally efficient methods to bridge length and time scales between experiments and quantum theory. [ABSTRACT FROM AUTHOR]

A layered sodium-ion battery cathode, O3/P3/P2-type NaNi1/3Mn1/3Fe1/3O2, has been systematically investigated by first-principles density functional theory to explore the detailed structural and Na-ion diffusion behavior during desodiation. Our results suggest that the (NaO6) spacing is greatest in the P3 phase and lowest in the O3 phase, with the P2 phase exhibiting intermediate spacing. During desodiation, the intermediate stages have a greater (NaO6) spacing than the initial and final stages. The great (NaO6) spacing facilitates the formation of the P3 phase, resulting in the structural evolution of NaxNi1/3Mn1/3Fe1/3O2 from the O3 to the P3 phase at x ≈ 0.59, finally reaching the O3 structure again at x ≈ 0.12. The electronic structure clearly proves that both Ni and Fe are active in O3/P3/P2-type NaxNi1/3Mn1/3Fe1/3O2. Ni2+ is oxidized to Ni3+ as Na content decreases from x = 1 to x = 0.66, then further oxidized to Ni4+ at x = 0.33, and finally, Fe3+ → Fe4+ oxidation occurs at x = 0. In the Na ion diffusion behavior, the order of the barrier is O3 (0.82 eV) > P2 (0.53 eV) > P3 (0.35 eV) at the initial stage, whereas it is O3 (0.53 eV) > P3 (0.21 eV) > P2 (0.16 eV) at a highly desodiated stage. The former can be traced back to the (NaO6) spacing, but the latter is related to the different Na sites. Our results thus provide a factor of the structural evolution and Na ion diffusion barrier by considering (NaO6) width and Na site changes during desodiation. [ABSTRACT FROM AUTHOR]

The effects of seven alloying elements (Co, Cr, Mo, Re, Ru, Ta, and W) on the Ni-vacancy diffusion behavior and the rupture strength of γ-Ni/γ′-Ni3Al interfaces are studied using density functional theory calculations. Our results reveal that all seven solutes prefer to occupy Al sites close to or Ni sites far from the interface. These solutes except for Co could significantly increase the barrier of Ni-vacancy diffusion within the γ/γ′ interface region. The retarding effects are similar when solutes are located at their favored Al and Ni sites, which follows the sequence of Ta > W > Mo > Re > Ru > Cr > Co. Besides, it is found that the presence of solute atoms could always increase the rupture strength of their neighboring interface but reduce that of their next neighboring interface. The best strengthening effects on the neighboring interface are achieved by Re and W. In terms of the entire interface region, alloying at the favored Al site brings a better strengthening effect than that at the favored Ni site. The charge density difference analysis demonstrates that the charge accumulation level at the interface explains the variable strengthening effects among different alloying elements. [ABSTRACT FROM AUTHOR]

High temperature creep deformation of hcp-Mg alloys is dominated by dislocation climb driven by out-of-plane (OOP) vacancy migration. Past experiments and atomistic simulations have indicated that Zn addition reduces vacancy migration tendencies and improves creep resistance. Here, we have compared in-plane (IP) and out-of-plane (OOP) vacancy migration mechanisms in binary Mg–X (Ca, Y, and Gd) and ternary Mg–X (Ca, Y, and Gd)–Zn alloys using density functional theory based first principles computations. Irrespective of Zn addition, the migration barrier for OOP diffusion was consistently higher than IP in our prototype binary and ternary alloys. The presence of Zn in ternary systems, however, substantially increases the OOP activation barrier relative to binary alloys. The higher OOP barrier in Mg–X–Zn was attributed to favorable local relaxation, enhanced charge localization, higher interplanar bond stiffness, and greater s orbital electron occupancy in the peak saddle state. Combined, these factors restrict non-conservative dislocation climb by impeding out-of-plane vacancy movement and improve the creep resistance of ternary Mg–X (Ca, Y, and Gd)–Zn alloys. [ABSTRACT FROM AUTHOR]

The major challenge in the application of Cu2S, as a solar absorber for photovoltaic, is its excessive Cu-vacancy formation tendency within the crystal structure that makes it a degenerate semiconductor. A recent study on Ag alloyed Cu2S acanthite phase has shown a reduced Cu-vacancy formation and a high Cu diffusion barrier. Though low chalcocite is the experimentally known ground state phase at room temperature, its low crystallinity makes it computationally expensive for complex doping modeling. In this work, we first establish the structural correspondence between a newly predicted acanthite and experimentally known low chalcocite phases of Cu2S. This study shows that the acanthite crystal structure of Cu2S can be used to model complex doping in the low chalcocite. The simulated pair distribution functions and diffusions of Cu at room temperature in acanthite and low chalcocite phases of Cu2S show that they have similar structural behavior. Pristine band structures and density of state plots of these two phases also show similarity. Next, we report density functional theory based first-principles investigations of Cu2−xSnxS systems for x = 0 to 0.31 in the acanthite phase for thermodynamic, electronic, and optical properties. We have found that Sn doping coupled with Cu vacancies creates charge-neutral defects and results in higher photoabsorption in the visible light spectra. It was found from these detailed studies that a low concentration of Sn doping is preferable. Then, Sn doping and Cu-vacancy related defects in low chalcocite Cu2S are also studied. Ab initio molecular dynamics simulations show that these compound defects in Cu2S do not negatively affect Cu diffusion inside the crystal. We propose a possible route to synthesize Sn doped acanthite like Cu2S. [ABSTRACT FROM AUTHOR]

Sintering is one of the main causes of degradation of nanocatalysts. With a view to studying the process of sintering, and its relative likelihood across elements, we have used ab initio density functional theory to compute the pathways and energy barriers Ed for the diffusion of small clusters Ptn on MgO(001), n = 1–4. We compare with the corresponding results for Aun, Agn, and Pdn. In general, diffusion barriers, but also sintering energies, are highest for Pt, resulting in opposing trends from kinetics and thermodynamics. We find smooth and positive correlations between Ed and Eb, Eb and Ecoh, Ecoh and Tm, and thus, between Ed and Tm, where Eb is the binding energy of the cluster on MgO, and Ecoh and Tm are the cohesive energy and melting temperature, respectively, of the corresponding bulk metal. These trends are present for diffusion of the monomers, trimers, and tetramers, but not the dimers; this can be explained by the topography of the energy landscape separating the global minimum from the transition state. The temperature T0 at which metal clusters on a given support become mobile is given by α + βTm, where α and β are constants that depend on the support. We also present similar results for self-diffusion of monomers of the four metals on the (111) surfaces. Such scaling relations could be used to rapidly estimate diffusion barriers, and hence the growth and sintering behavior, of potential catalytic metal clusters. [ABSTRACT FROM AUTHOR]

In this work, the feasibility of a monolayer Be2C as the anode material for lithium-ion battery (LiB) was investigated using the density functional theory. Our study reveals that the adsorption of Li atoms changes the electronic conductivity of a monolayer Be2C from semiconducting to metallic. This resulted in a low Li diffusion barrier of 0.11 eV, which is highly needed for the fast charge and discharge processes of the LiB. Additionally, the predicted open-circuit voltage was 0.33 V, and the calculated maximum theoretical capacity was impressively high (1785 mAh/g). Our findings suggest that the monolayer Be2C is a promising anode material for high-performance LiB. [ABSTRACT FROM AUTHOR]

The atomic-scale mechanism of self-diffusion on coinage metal surfaces has been investigated by density functional theory calculations before. However, the detailed role of the co-adsorbed anions and of the electrochemical interface is not yet fully understood. We present diffusion paths of a Au adatom on c(2 × 2)-Cl and c ( 2 × 2 2 ) R 4 5 ◦ -Cl Au(001) surfaces. In the case of the fully Cl covered Au(001) surfaces, the diffusion energy barriers are ≥0.44 eV. In the presence of a neighboring Cl vacancy, the diffusion barriers are drastically reduced to about half of the value for the fully Cl covered surface. Thus, Au self-diffusion is promoted by Cl vacancies. A first estimate for the effect of water on the diffusion energy barrier of the Cl vacancy is derived by calculating the interaction of configurations along the diffusion path with a water layer. [ABSTRACT FROM AUTHOR]

Understanding how defects in solids interact with external electric fields is important for technological applications such as memristor devices. Using Density Functional Theory (DFT) calculations, we have studied the influence of an external electric field on the formation energies and diffusion barriers of surface and subsurface oxygen vacancies at the (101) surface of anatase TiO2. DFT in the generalized gradient approximation as well as DFT+U methods with different U values have been utilized, with the electric field treated self-consistently by adding a sawtooth-like potential to the bare ionic potential. Our results show that the direction and strength of the applied field can have a significant influence on the relative stabilities of surface and subsurface defects, whereas the effect on the subsurface-to-surface defect migration is found to be relatively minor. [ABSTRACT FROM AUTHOR]

We calculate the diffusion barrier of fluorine adatoms on doped graphene in the diluted limit using Density Functional Theory. We found that the barrier Δ strongly depends on the magnitude and character of the graphene's doping (δn): it increases for hole doping (δn<0) and decreases for electron doping (δn>0). Near the neutrality point the functional dependence can be approximately by Δ=Δ0 - αδn, where α ⋍ 6×10-12 meV cm². This effect leads to significant changes of the diffusion constant with doping even at room temperature and could also affect the low temperature diffusion dynamics due to the presence of substrate induced charge puddles. In addition, this might open up the possibility to engineer the F dynamics on graphene by using local gates. [ABSTRACT FROM AUTHOR]

Models of intercalated Li and its diffusion in Si-Graphene interfaces are investigated using density functional theory. Results suggest that the presence of interfaces alters the energetics of Li binding and diffusion significantly compared to bare Si or Graphene surfaces. Our results show that cavities along reconstructed Si surface provide diffusion paths for Li. Diffusion barriers calculated along these cavities are significantly lower than penetration barriers to bulk Si. Interaction with Si surface results in graphene defects, creating Li diffusion paths that are confined along the cavities but have still lower barrier than in bulk Si. [ABSTRACT FROM AUTHOR]