Energetics and diffusion of point defects in Au/Ag metals:A molecular dynamics study

To reveal the potential aging mechanism for self-irradiation in Pu–Ga alloy, we choose Au–Ag alloy as its substitutional material in terms of its mass density and lattice structure. As a first step for understanding the microscopic behavior of point defects in Au–Ag alloy, we perform a molecular dynamics(MD) simulation on energetics and diffusion of point defects in Au and Ag metal. Our results indicate that the octahedral self-interstitial atom(SIA) is more stable than the tetrahedral SIA. The stability sequence of point defects for He atom in Au/Ag is: substitutional site octahedral interstitial site tetrahedral interstitial site. The He–V cluster(Hen Vm, V denotes vacancy) is the most stable at n = m. For the mono-vacancy diffusion, the MD calculation shows that the first nearest neighbour(1 NN) site is the most favorable site on the basis of the nudged elastic band(NEB) calculation, which is in agreement with previous experimental data. There are two peaks for the second nearest neighbour(2 NN) and the third nearest neighbour(3 NN) diffusion curve in octahedral interstitial site for He atom, indicating that the 2 NN and 3 NN diffusion for octahedral SIA would undergo an intermediate defect structure similar to the 1 NN site. The 3 NN diffusion for the tetrahedral SIA and He atom would undergo an intermediate site in analogy to its initial structure. For diffusion of point defects, the vacancy, SIA, He atom and He–V cluster may have an analogous effect on the diffusion velocity in Ag.     

Radiation defects in graphite

This article discusses the nature of radiation defects in graphite, reviewing past and recent developments in understanding their structure, interactions and effect on physical properties. The principal focus is on behaviour at the atomic and microstructural level, with an interest both in understanding graphite moderator damage in nuclear reactors and building a foundation for the range of emerging technological applications of defect-engineered graphitic materials. It is hoped that this article will both clarify the picture that has emerged over the last 50 years and provide a useful background to ongoing efforts.     

Energetics of native point defects in cubic silicon carbide

In this work we present a detailed investigation of native point defects energetics in cubic SiC, using state-of-the-art first principles computational method. We find that, the carbon vacancy is the dominant defect in p-type SiC, regardless the growth conditions. Silicon and carbon antisites are the most common defects in n-type material in Si-rich and C-rich conditions respectively. Interstitial defects and silicon vacancy are less favorite from the energetic point of view. The silicon vacancy tends to transform into a carbon vacancy-antisite complex and the carbon interstitial atom prefers to pair to a carbon antisite. The dumbbell structure is the lowest-energy configuration for the isolated carbon interstitial defect, and the tetrahedral interstitial silicon is a stable structure in p-type and intrinsic conditions, while in n-type material the dumbbell configuration is the stable one. Our results suggest that, in samples grown in Si-rich stoichiometric conditions, native defects are a source of n-doping and of compositional unbalance of nominally intrinsic SiC, in accord with experimental evidence.Received: 9 January 2004, Published online: 28 May 2004PACS: 61.72.Ji Point defects (vacancies, interstitials, color centers, etc.) and defect clusters - 68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc. - 74.62.Dh Effects of crystal defects, doping and substitution     

Energetics and kinetics of direct phase conversion from graphite to diamond

The atomistic mechanism for direct conversion of graphite to diamond is a long-standing problem in condensed matter physics. The newly identified cold-compressed graphite phases of M, W and O carbon provide a crucial link to understand the graphite-to-diamond phase transformation. We demonstrate by ab initio calculations that pressure has a dual role in lowering the conversion barrier and enhancing the production stability during the first-stage cold-compressed phase conversion of graphite toward the intermediate metastable M, W and O carbon phases. However, it has little effect on the relative enthalpy and high conversion barrier during the second-stage conversion process toward the diamond polytypes, showing a temperature dominated conversion process. These results may give explanation regarding the necessity of high pressure and high temperature during the graphite-todiamond reaction.     

First-principles simulations of boron diffusion in graphite

Boron strongly modifies electronic and diffusion properties of graphite. We report the first ab initio study of boron interaction with the point defects in graphite, which includes structures, thermodynamics, and diffusion. A number of possible diffusion mechanisms of boron in graphite are suggested. We conclude that boron diffuses in graphite by a kick-out mechanism. This mechanism explains the common activation energy, but large magnitude difference, for the rate of boron diffusion parallel and perpendicular to the basal plane.     

Point defects and diffusion in NiO

A defect model for NiO is developed and is fit to the electrical-conductivity data [26], the deviation-from-stoichiometry data [7], and the cation-self-diffusion data [14, 17]. This model involves neutral, singly charged, and doubly charged nickel vacancies and charge-compensating electron holes. Both singly and doubly charged cation vacancies are required to explain the data; neutral cation vacancies (if present) are not required by the present data. However, the jump frequencies of the two types of charged cation vacancies are generally not equal; the doubly charged cation vacancy moves with the smaller activation enthalpy. The defect data are quantitatively consistent with the chemical-diffusion data [26] and with a correlation factor?v = 0.75.     

Energetics of atomic hydrogen diffusion on Si(100)

We present first principles calculations of the potential energy surface for the diffusion of a single hydrogen atom on Si(100)2 × 1. Surface relaxation is found to be very important for the energetics of diffusion. A strong anisotropy is predicted for hydrogen motion: H should diffuse mainly along dimer rows, where activation energies are ~ 1.3 eV, while the barrier for row-to-row hopping is ~ 0.5 eV higher. Our results indicate that diffusion can be considered a fast process compared to H₂ recombinative desorption.     

Computer simulation investigation of diffusion selectivity in graphite slit pores

Equilibrium molecular dynamics simulations are reported of oxygen and nitrogen molecules confined in graphite slit pores. Self- and collective diffusion coefficients have been calculated as a function of pore width, temperature and density for each pure component in the pore space. The aim of this study was to elucidate the mechanism by which oxygen and nitrogen are kinetically separated when air is passed over an adsorbent bed consisting of molecular sieving carbon in the commercial production of oxygen. It was found that a critical pore width exists for each species at which there is a sharp drop in the rate of diffusion (both self- and collective diffusion) of each fluid. The critical pore width is one for which the individual molecules are prevented from rotating freely about one of their axes. The greater length of a nitrogen molecule means that the critical pore width is higher for this species than for oxygen. Consequently, oxygen molecules diffuse substantially faster than nitrogen molecules in the vicinity of the nitrogen critical pore width. From an analysis of correlation functions and their corresponding power spectra it is shown that the restricted rotations, which occur at or below the critical pore width, cause a decoupling of translational and rotational modes, with the net result being a lowering of translational diffusion. The nitrogen critical pore width lies within the range of the mean pore size of most commercial molecular sieving carbons, and so this mechanism may help to explain the high oxygen selectivities reported in the literature.     

Luminescence mechanisms of oxygen-related defects in AlN

Spectral characteristics of native oxygen-related defects existing in the crystalline lattice of AlN were studied. Features of photoluminescence observed under exposure to ultraviolet light together with those of the photostimulated luminescence testify the recombination character of luminescence. The mechanism of luminescence of oxygen-related defects is proposed.     

Real-time imaging of K atoms on graphite: interactions and diffusion

Scanning tunneling microscopy (STM) at liquid helium temperature is used to image potassium adsorbed on graphite at low coverage (≈0.02 monolayer). Single atoms appear as protrusions on STM topographs. A statistical analysis of the position of the atoms demonstrates repulsion between adsorbates, which is quantified by comparison with molecular dynamics simulations. This gives access to the dipole moment of a single adsorbate, found to be 10.5±1 D. Time-lapse imaging shows that long-range order is broken by thermally activated diffusion, with a 30 meV barrier to hopping between graphite lattice sites.     

Low energy excitations in graphite: The role of dimensionality and lattice defects

In this paper, we present a high resolution angle resolved photoemission spectroscopy (ARPES) study of the electronic properties of graphite. We found that the nature of the low energy excitations in graphite is particularly sensitive to interlayer coupling as well as lattice disorder. As a consequence of the interlayer coupling, we observed for the first time the splitting of the π bands by ≈0.7 eV near the Brillouin zone corner K. At low binding energy, we observed signatures of massless Dirac fermions with linear dispersion (as in the case of graphene), coexisting with quasiparticles characterized by parabolic dispersion and finite effective mass. We also report the first ARPES signatures of electron-phonon interaction in graphite: a kink in the dispersion and a sudden increase in the scattering rate. Moreover, the lattice disorder strongly affects the low energy excitations, giving rise to new localized states near the Fermi level. These results provide new insights on the unusual nature of the electronic and transport properties of graphite.     

Effects of defects on the electronic structure of ion-irradiated graphite

The effects of N⁺ and Ne⁺ ions impinging on a graphite target are studied by ultraviolet photoelectron spectroscopy. Changes in the valence band of the N⁺-irradiated graphite surface are found to be inherently different from the Ne⁺-ion-induced structural modification. They reveal a build-up of additional -defect states at the top of the band, and confirm what appears to be a distinct character of the influence of nitrogen on an amorphous carbon matrix.     

Muon diffusion and trapping by defects in electron-irradiated Nb and Ta

The transverse spin relaxation of positive muons ( ⁺) has been measured on Nb and Ta after irradiation with 3 MeV electrons. In high-purity Nb the ⁺ diffusivity derived from the trapping at irradiation-induced defects above 100 K is explained in terms of adiabatic hopping. At lower temperatures there is evidence for the dominating processes to be fewphonon incoherent tunnelling and coherent hopping. Annealing results in the formation of new defects capable of trapping the ⁺. In Ta at least two types of irradiation-induced defects capable of trapping ⁺ survive up to annealing temperatures of 400 K.