Numerical simulation of interstitial electrotransport in metals

Numerical simulation results are presented for the time evolution of the interstitial concentration profile in a metal subject to an applied potential difference. Simulation results are presented for five simple theoretical models of the interstitial electrotransport: a dilute solution or ideal gas model, two non-interacting lattice gas models, and two interacting lattice gas models. The last four involve concentration-dependent electrical mobilities and diffusion coefficients. Both uniform and step function initial concentration profiles are considered. The results indicate both qualitative and quantitative differences in the time evolution of the concentration profiles, which should permit experimental discrimination among the different models. The results also suggest that a combination of measurements involving both uniform and step function initial profiles would permit a more complete characterization of the system than measurements involving only one type of initial profile.     

Hydrogen binding to fixed interstitial impurities in metals

We have calculated the binding energy of hydrogen to the inert impurity atoms He, Ne, Ar, Kr, and Xe and also to the chemically active impurities C, O, and N in fixed interstitial configurations in Ni using quasiatom (effective medium) theory including lattice relaxation. We find that hydrogen is only weakly bound to any of these defects, that the chemically active defects do not produce a chemical bond with the hydrogen, and that the (～0.1 to 0.6 eV) binding is due to the strain field surrounding the defect.     

Diffusion and solubility of some interstitial impurities in deformed metals

Summary The problem of impurity diffusion accompanying segregation phase nucleation on dislocations has been studied using the approximation of the local equilibrium with respect to the impurity distribution between the volume solution and dislocation regions. It has been shown that the known experimental data on diffusion and solubility of some interstitial impurities in cold-worked b.c.c. and f.c.c. metals and alloys can be described in the framework of the dislocation trap model. The characteristics of the impurity segregation regions near dislocations have been obtained from the treatment of the diffusion and solubility data for the systems. On the basis of the crystallographic and thermodynamic considerations the possibility of the existence of such segregation phase regions along dislocations in the systems in question has been shown.     

Local moment formation at interstitial impurity sites in metals

We have calculated the electronic structure and related properties around an interstitial impurity in several metallic hosts. This was done using the real-space linear muffin tin orbital scheme (RS-LMTO-ASA), a first-principles, self-consistent approach implemented directly in real space. We show that interstitial Fe does not develop a local moment in trivalent and tetravalent Sc, Y, Ti and Zr hosts. In divalent Ba, Ca, Sr and Yb we find that the appearance of local moment is extremely dependent on nearest neighbor relaxation, while in alkali metals such as K, we expect the interstitial Fe impurity to be magnetic. We show that these trends can be qualitatively understood using simple ideas based on the Wolff model and the Stoner criterium. We also consider Fe impurities in Gd, a trivalent magnetic host. We find an unusually large induced local moment at the interstitial Fe site and discuss the origin of this effect. Finally, we compare our results with TDPAD and in beam Mössbauer experiments in these systems.     

The non-Arrhenius migration of interstitial defects in bcc transition metals

Thermally activated migration of defects drives microstructural evolution of materials under irradiation. In the case of vacancies, the activation energy for migration is many times the absolute temperature, and the dependence of the diffusion coefficient on temperature is well approximated by the Arrhenius law. On the other hand the activation energy for the migration of self-interstitial defects, and particularly self-interstitial atom clusters, is very low. In this case a trajectory of a defect performing Brownian motion at or above room temperature does not follow the Arrhenius-like pattern of migration involving infrequent hops separated by the relatively long intervals of time during which a defect resides at a certain point in the crystal lattice. This article reviews recent atomistic simulations of migration of individual interstitial defects, as well as clusters of interstitial defects, and rationalizes the results of simulations on the basis of solutions of the multistring Frenkel–Kontorova model. The treatment developed in the paper shows that the origin of the non-Arrhenius migration of interstitial defects and interstitial defect clusters is associated with the interaction between a defect and the classical field of thermal phonons. To cite this article: S.L. Dudarev, C. R. Physique 9 (2008).     

Isotope effects in electron scattering by light interstitial impurities in metals

We study the effects of zero-point motion on the scattering of electrons by light interstitial impurities in metals. Expressions are obtained for the electron self-energy at the Fermi level. Significant isotope effects can occur when the scatterer and the electron wave-function in the pure metal are of different parity.     

Clusterization of light interstitial impurities and the 1/f noise in metals

The effect of clusterization of light interstitial impurities on the 1/f noise spectrum in metals has been studied by numerical simulation. It is shown that an increase in the intensity of the 1/f noise may serve as an indication of clusterization, and that the shape of the activation energy spectrum can provide information on the character of impurity ordering in the clusters formed.     

Tendency to nearly cubic local lattice structure around interstitial hydrogen in the alkali metals

The behaviour of the lattice response in the bcc alkali metals and the change in sign of the impurity-host interaction between first and second neighbour distances are shown to cooperate to build up a relaxed atomic environment which, for the D_4h ‘octahedral’ interstitial position, is close to a regular octahedral coordination. The calculation is done by using the self-consistent electron density for a proton in jellium to take into account non-linear screening, and the relaxed crystal structure is found by the method of lattice statics. The displacements around the hydrogen impurity are found to be many time larger than those predicted earlier by use of the spherical solid model.     

Electric field gradient for interstitial positive muons in fcc metals: An AB Initio calculation of size effect

The electric field gradients (EFGs) resulting from interstitial point defects in fcc metals have been investigated. The defect induced charge density, used to evaluate the valence effect EFG, is calculated self-consistently in the density functional formalism. An ab initio calculation of the size effect EFG is carried out for a positive muon at an octahedral site in the fcc lattice in the elastic continuum model. The components of the strain field tensor are evaluated assuming the lattice of dressed point ions interacting through the screened Coulomb potential. No adjustable parameter has been used. The theoretical results are in good agreement with the experimental values within experimental uncertainties. It is emphasized that both the strain and conduction electron contributions are equally important in the estimation of the electric field gradient.     

Phase transitions in metals on metals

Phase transitions in two-dimensional metal layers on metal surfaces are discussed, with emphasis on systems with attractive lateral interactions on densely packed surfaces. The experimental tools which give information on these transitions are described briefly and the results obtained with them are illustrated by examples of various metals on W and Mo surfaces.     

Electrostriction in dielectrics and metals

We examine electrostriction in ellipsoidal samples of dielectrics and metals. We show that electrostriction in metals is independent of the external shape of the sample, whereas in dielectrics electrostriction is a quadratic function of the shape parameter. State Teachers University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 53–58, November, 1998.     

Pseudopotential in metals and alloys

In this communication, the pseudopotential investigation of, the various properties of non-transition metals and alloys, is discussed. Various one parametric model pseudopotentials, derived from well known spherical functions s_l(x), are employed in the calculations. Many recurrence relations of the s_l(x) function have been described. The effects of exchange and correlation on conduction electrons are also considered separately by using different dielectric screenings in various properties. The ion-ion interaction, force constants, phonon spectrum, temperature coefficient of Knight shift and electronic transport coefficients of certain metals and alloys are evaluated. The results are compared with available experimental values. Generally good agreement is achieved. The screening charge density of certain metals in low and high density region are also determined.     

Evidence for a relation between interstitial long-range migration in h.c.p. metals and their c/a-ratio

The temperature for the long-range migration of an interstitial atom in h.c.p. metals is - when normalized to the melting temperature - observed to be a linear function of the c/a-ratio. This can be understood in terms of the relative size of open atomic lenses in the lattice. The special cases of Zn and Cd (extremal c/a-values) are tentatively explained by a crowdion mechanism for the creation and annealing of Frenkel defects.     

Monovacancy and interstitial migration in ion-implanted silicon

The migration of monovacancies (V0) and self-interstitials (I) has been observed in ion-implanted low-doped float-zone silicon by variable-energy positron annihilation spectroscopy. V0 and I were created by the in situ implantation of approximately 20 keV helium ions below 50 K. Monitoring the time evolution of the vacancy response during isothermal heating enabled the measurement of activation energies for I and V(0) [corrected] migration of 0.078(7) and 0.46(28) eV, respectively. In highly As-doped Si, partial V annihilation occurs via free I migration, with a second stage of annealing, probably associated with V-As complexes, above room temperature.     

Defects in metals

Recent study of defects in metals using Mössbauer spectroscopy is presented. Vacancy and interstitial atoms are focussed as defects in metals. General remarks for the study of defects are presented first and then, as examples of the study,¹¹⁹Sb and⁵⁷Co Mössbauer measurements to monitor the trapping and detrapping process of quenched-in vacancies in Au are discussed with the aid of the complementary techniques which are resistivity measurement, computer simulation and positron lifetime measurement. Detailed results from these complementary techniques are described. Recent theoretical calculations for defect-associated states are also presented briefly.     

Magnetostriction in metals

After a presentation of the theoretical background and experimental methods the data available for the paramagnetostriction of transition metals and alloys are surveyed and discussed, special consideration being given to the roles of exchange enhancement and orbital susceptibility. Attention is then turned to oscillatory magnetostriction (related to the de Haas-van Alphen effect) and the information it provides about the Fermi surfaces of metals and degenerate semiconductors.