Hydrogen Transportation Behaviour of V–Ni Solid Solution: A First-Principles Investigation

Hydrogen embrittlement causes deterioration of materials used in metal–hydrogen systems. Alloying is a good option for overcoming this issue. In the present work, first-principles calculations were performed to systematically study the effects of adding Ni on the stability, dissolution, trapping, and diffusion behaviour of interstitial/vacancy H atoms of pure V. The results of lattice dynamics and solution energy analyses showed that the V–Ni solid solutions are dynamically and thermodynamically stable, and adding Ni to pure V can reduce the structural stability of various VHx phases and enhance their resistance to H embrittlement. H atoms preferentially occupy the characteristic tetrahedral interstitial site (TIS) and the octahedral interstitial site (OIS), which are composed by different metal atoms, and rapidly diffuse along both the energetically favourable TIS → TIS and OIS → OIS paths. The trapping energy of monovacancy H atoms revealed that Ni addition could help minimise the H trapping ability of the vacancies and suppress the retention of H in V. Monovacancy defects block the diffusion of H atoms more than the interstitials, as determined from the calculated H-diffusion barrier energy data, whereas Ni doping contributes negligibly toward improving the H-diffusion coefficient.

First-Principles Investigation of Atomic Hydrogen Adsorption and Diffusion on/into Mo-doped Nb (100) Surface

To investigate Mo doping effects on the hydrogen permeation performance of Nb membranes, we study the most likely process of atomic hydrogen adsorption and diffusion on/into Mo-doped Nb (100) surface/subsurface (in the Nb12Mo4 case) via first-principles calculations. Our results reveal that the (100) surface is the most stable Mo-doped Nb surface with the smallest surface energy (2.75 J/m2). Hollow sites (HSs) in the Mo-doped Nb (100) surface are H-adsorption-favorable mainly due to their large adsorption energy (−4.27 eV), and the H-diffusion path should preferentially be HS→TIS (tetrahedral interstitial site) over HS→OIS (octahedral interstitial site) because of the correspondingly lower H-diffusion energy barrier. With respect to a pure Nb (100) surface, the Mo-doped Nb (100) surface has a smaller energy barrier along the HS→TIS pathway (0.31 eV).

Une surstructure de α-Ge, type diamant, induite par un dopage d'antimoine

Single crystals of antimony-doped germanium, Ge1–xSbx+0.01(x≃ 0.0625), were grown by chemical transport reaction. The alloy crystallizes as a superstructure of diamond-type α-Ge. All atoms in the asymmetric unit lie on special positions and are characterized by strong covalent bonds. The antimony atoms substitute for one germanium atom at full occupancy at Wyckoff position 4a(site symmetry -43m), and are also at an adjacent tetrahedral interstitial site with partially occupation (16%) at position 4c(or 4d) (site symmetry -43m). The structural model does not show close Sb...Sb contacts, and suggests that the interstitial antimony atoms move between the two adjacent tetrahedral sites.

First-Principles Study on Various Point Defects Formed by Hydrogen and Helium Atoms in Tungsten

The different point defects formed by two hydrogen atoms or two helium atoms in tungsten were investigated through first-principles calculation. The energetically favorable site for a hydrogen atom is tetrahedral interstitial site while substitutional site is the most preferred site for a helium atom. The formation energies of two hydrogen or helium atoms are determined by their positions, and they are not simply 2 times the formation energy of a single hydrogen or helium atom’s defect. After relaxation, two adjacent hydrogen atoms are away from each other while helium atoms are close to each other. The reasons for the interaction between two hydrogen or helium atoms are also discussed.

Effect of the alloying element titanium on the stability and trapping of hydrogen in pure vanadium: A first-principles study

With a first-principles method based on density functional theory, the effect of the alloying element titanium ( Ti ) on the thermodynamic stability and electronic structure of hydrogen ( H ) in pure vanadium ( V ) is investigated. The interactions between H and the vacancy and the defect solution energies in a dilute V – Ti binary alloy are calculated. The results show that: (i) a single H atom prefers to reside in a tetrahedral interstitial site in dilute V – Ti binary alloy systems; (ii) H atoms tend to bond at the vacancy sites; a mono-vacancy is shown to be capable of trapping three H atoms; and (iii) the presence of Ti in pure V can increase the H trapping energy and reduce the H trapping capability of the vacancy defects. This indicates that doping with Ti to form dilute V – Ti binary alloys can inhibit the solution for H , and thus suppress the retention of H . These results provide useful insight into V -based alloys as a candidate structural material in fusion reactors.

Multiple Helium Atoms Diffusion in Tungsten: A Molecular Dynamic Simulation

The diffusion process of multiple He atoms in W is simulated by a molecular dynamics (MD) method with the W-H-He analytic bond-order potential. The diffusivities of different number of helium (He) atoms in W are determined by the mean squared displacement (MSD) method at different temperatures. The diffusivity-temperature (D-T) relationship is fitted to the Arrhenius equation to obtain the pre-factor and the diffusion barrier. Under the temperature of 1200K He atoms diffuse together, and above 1200K they separate from each other. When the number of He atoms is greater than three, all He atoms oscillate at the tetrahedral interstitial site (TIS) instead of diffusing under 400K. In the temperature range of 400-1200K, the diffusion barriers of He atoms, the number of which is from two to five, are 0.098, 0.170, 0.125 and 0.112eV, respectively. Contrasting with one He atom (0.058eV), the higher diffusion barriers reflect a greater difficulty in diffusion of multiple He atoms in W. In addition, when the number of He atoms is over five, vacancies are formed in W, and He atoms occupy the vacancies.

Electrically Active and Electrically Inactive 3d Transition Metal Centers in Si

Systematic first-principles calculations on transition metal (TM) impurities of the 3d series in Si have been performed. The equilibrium sites, migration energies, electrically-active gap levels, charge and spin states are predicted. While the properties of the isolated interstitials are experimentally well-known, much less experimental information is available about the consequences of their interactions with vacancy-like defects. We discuss here the properties of isolated interstitial Ti, Fe, and Ni, their interactions with vacancies and divacancies, the properties of the resulting substitutional impurities, and of the TM-divacancy {V-TM-V} complexes. In equilibrium, interstitial Ti, Fe, and Ni do not become substitutional, but a number of processing steps commonly used in PV manufacturing introduce highly mobile vacancies into the bulk. These vacancies strongly interact with interstitial TMs. At the substitutional site, Ti, Fe, and Ni have very different electrical properties than at the tetrahedral interstitial site. In particular, the electrical activity (and stable spin state) of Ti and Fe are greatly reduced, suggesting that the passivation by vacancies plays an unrecognized role during a variety of high-temperature processes.

First-Principles Calculations Of Diffusion Of Chlorine Atoms In GaAs

The properties of chlorine atoms in crystalline GaAs, such as stable configurations, migration paths, charge-state effects, and interaction with dopant atoms are theoretically investigated. The calculations are based on the local density functional theory using first-principles pseudopotentials in a supercell geometry. We determine the stable charge state of an isolated Cl atom as a function of the Fermi energy. When the Fermi level is situated at the top of the valence band of GaAs, the Cl atom occupies preferentially the bond-center site of a Ga-As bond in the positive charge state. The Cl atom diffuses through the GaAs crystal via a path in the region of high electron density, with a fairly large energy barrier. When the Fermi level is at the bottom of the conduction band, the lowest-energy configuration of the Cl atom is the tetrahedral interstitial site in the negative charge state and the bond center site is very slightly higher in energy. In Si-doped GaAs, the C1 atom occupies the tetrahedral interstitial site with the substitutional Si donor atom as a nearest neighbor, forming a neutral Cl-Si complex. The Cl-Si complex is weak and easily dissociates into the isolated C1 and Si atoms in GaAs. A comparison will be made between the behavior of Cl and F atoms in GaAs.

Atomic Hydrogen in GaN

ABSTRACTBased on extensive first-principles total-energy calculations we study the electronic structure, atomic geometry and energetics of atomic hydrogen in cubic GaN. All charge states of hydrogen (H+, H0, H-) are examined. For H- the gallium tetrahedral interstitial site is energetically most stable. All other sites are much higher in energy, indicating a high diffusion barrier for H- in GaN. H+ favors positions on a sphere with a radius of ≈ 1 Å and a nitrogen atom in the center. Among these positions the nitrogen antibonding site is energetically most stable. An unexpectedly large negative-U effect (U = —2.5eV) indicates that H0 is unstable.

Hard sphere modeling of the effect of slip on interstitial sites in the B2 structure

Geometrical modeling of the B2 structure indicates that the tetrahedral interstitial site is always the largest both before and after an a/2〈111〉 translation on {110}, such as occurs during the slip of a partial dislocation in some B2 compounds. The tetrahedral site within the APB which trails a gliding a/2〈111〉 dislocation is larger than in the unslipped lattice, suggesting that interstitial atoms will segregate there. Also, some interstitial sites in a B2 lattice are larger than those in a bcc lattice of the same lattice parameter, suggesting that interstitials may have greater solubility in B2 compounds.