Modified embedded-atom method potential for cadmium

Huntington–Elcock–McCombie (HEM) mechanism involving six consecutive and correlated jumps, a triple-defect mechanism (TDM) involving three correlated jumps and an anti-structure bridge (ASB) mechanism invoking the migration of an anti-structure atom are the three mechanisms currently in vogue to explain the self- and solute diffusion in intermetallic compounds. Among them, HEM and TDM are cyclic in nature. The HEM and TDM constitute the theme of the present article. The concept of random walk is applied to them and appropriate expressions for the diffusion coefficient are derived. These equations are then employed to estimate activation energies for self-diffusion via HEM and TDM processes and compared with the available experimental data on activation energy for self-diffusion in intermetallic compounds. The resulting activation energies do not favour HEM and TDM for the self-diffusion in intermetallic compounds. A comparison of the sum of experimentally determined activation energies for vacancy formation and migration with the activation energies for self-diffusion determined from radioactive tracer method favours the conventional monovacancy-mediated process for self-diffusion in intermetallic compounds.

Article Atomic defects and diffusion in intermetallic compounds with D03 structure: an ab-initio study was published on October 1, 2004 in the journal International Journal of Materials Research (volume 95, issue 10).

Theoretical aspects of diffusion in intermetallic compounds are discussed. Order-disorder compounds as well as compounds highly ordered at any temperature are considered. Theory is compared to the experiment.

Diffusion is a process that is fundamental in the art and science of materials. The knowledge of diffusion behaviour, therefore, is essential for the production of materials or for their use in practical applications. In the first part of this paper, a brief review is given on the historical developments of the quantitative study of diffusion: the establishment of the diffusion law by A. Fick, the first quantitative measurement of solid-state diffusion (Au in Pb) by W. C. Roberts-Austen and the demonstration of the self-diffusion in Pb using a natural radioactive isotope by G. Hevesy. In the second part, the development and the present status of our understanding of several problems: correlation effects in diffusion, fast diffusion in metals and the mechanism of diffusion in intermetallic compounds will be reviewed.

Background. We investigated diffusion coefficient parameters for a vacancy diffusion mechanism in the presence of two point defects (vacancies and impurity atoms), taking into account the temperature factor, and conducted by means of computer simulations based on density functional theory (DFT). Objective. Development of theoretical concepts of the mechanisms of diffusion at the atomic and subatomic levels, including the temperature dependence of the vacancy formation energy, the migration energy, and the diffusion activation energy in the metallic systems Ag – Mo and Mo – Ag. Methods. The calculations were performed in VASP, using the full-potential projector augmented wave (PAW) method, and a PBE-sol generalized gradient approximation . Optimization of the structure geometry was carried out by relaxation of the ions' positions in a super cell made by 64 atoms. A specific feature of the calculations was the application of experimental values for the lattice parameters of the core element matrix at relevant temperatures. Results. The peculiarities of calculating the vacancy formation energy, the migration energy, and the diffusion activation energy from first principles are presented. It is shown that thermal excitation has a significant impact on the vacancy formation energy, the migration energy, and the diffusion activation energy at high temperatures. Also, the possibility of a compensation effect has been found, namely, the simultaneous changing of the various free energy contributions to the energy of vacancy formation in metallic systems Ag – Mo and Mo – Ag. Evaluation of the contributions of free vibration energy, electronic thermal excitation energy, and energy of pair interactions depending on temperature helps to clarify the picture of the effect of material’s thermal expansion. Conclusions. The estimated vacancy formation energy, the migration energy, and the diffusion activation energy are in good agreement with previously reported theoretical and experimental data. The presence of the mutual compensation of different contributions to the vacancy formation energy in the metallic complex systems has been confirmed. Confirming the existence and the characteristics of a compensation effect for the system Mo – Ag requires more research.

The effective formation energies of vacancies and antistructure atoms in the D03 phase of FexAl1–x, FexSi1–x and NixSb1–x are calculated by means of the ab-initio electron theory and statistical mechanics. It is shown how the results of these calculations can be combined with available experimental data mainly from tracer diffusion studies, positron-annihilation experiments, quasielastic neutron scattering, and quasielastic Mossbauer studies to promote the understanding of the diffusion mechanisms in these materials.

Vacancy formation energies in metals: A comparison of MetaGGA with LDA and GGA exchange–correlation functionals

In this paper we examine the behaviour of the vacancy wind factors in the intrinsic diffusivities and the chemical or interdiffusivity in highly ordered stoichiometric AB intermetallic compounds. We show that the six-jump cycle gives quite different limits for these factors from those given by the well-known Manning expressions. Remarkably, there is non-trivial behaviour of these factors even when the vacancy flux is zero. The findings are all verified by computer simulations of a four-frequency model in the square planar lattice and an Ising model in the B2 structure. The consequences of the findings for the interpretation of experimental data are commented on.

In the present paper, it is shown by some experimental examples how a recently discoverd, new interstitial relaxation mechanism , based on log-range intercrystalline diffusion, affects the damping spectraof H-absorbing intermetallic compounds. Its potential to develop new materials with adjustable damping properties is discussed

Defects and diffusion in intermetallic compounds

The reactions of ruthenium and osmium thin films with their silicon substrates lead to the formation of the following phases: the isomorphous Ru2Si3 and Os2Si3, and OsSi2. Ru2Si3 forms by a diffusion controlled mechanism from approximately 450 to 525 °C; the activation energy is 1.8 eV. Os2Si3 grows by the same process and with the same activation energy as Ru2Si3, but at slightly higher temperatures (for equivalent rates). OsSi2 grows at about 750 °C by a nucleation controlled mechanism. With an alloy of ruthenium containing 33 at.% rhodium, one obtains RuSi, which forms by a diffusion controlled mechanism with an activation energy of 2.4 eV from 400 to 475 °C, and Ru2Si3. In the case of the ruthenium alloy, the formation of Ru2Si3 is not diffusion controlled; the controlling process is akin to a nucleation mechanism. Ru2Si3 and Os2Si3 form by silicon motion. This is also true of OsSi2, but the high formation temperature results in some metal motion also during the formation of that phase. The diffusion processes are discussed on the basis of simple ideas about diffusion in intermetallic compounds. The nucleation processes are shown to occur for the phases OsSi2 and Ru2Si3 (with rhodium), as in other nucleated phases, when the enthalpy contribution to the driving force is close to zero. The observation leads one to anticipate that for most (perhaps all) platinum-like metals the silicides with the highest silicon content should be unstable at low temperatures.

ABSTRACTA class of experimental techniques known as hyperfine methods can be used to measure electric field gradients (EFGs) through the hyperfine interaction experienced by tracer nuclei. When EFGs fluctuate at rates comparable to the inverse characteristic timescale of the hyperfine method, there is a loss of signal coherence that can be used to determine EFG fluctuation rates. This has been used to measure, for example, EFG fluctuations accompanying atomic jumps of radiotracers using perturbed angular correlation spectroscopy (PAC). Nominally, there is a one-to-one correspondence between EFG fluctuation and tracer jump, but when tracer jumps are mediated by a vacancy diffusion mechanism, a subset of multiple tracer-vacancy exchanges will not affect spectra when they occur much faster than the hyperfine timescale, leading to an underestimate of underlying tracer jump rate if this correlated random-walk effect is not taken into consideration. The present work calculates the factor by which EFG fluctuation rate differs from tracer jump rate based on a time-dependent, random-walk analysis of tracer displacement probabilities in a vacancy encounter model for the special case of self-diffusion in the L12 crystal structure.

Nuclear relaxation caused by diffusion of 111In/Cd probe atoms was measured in four phases having the tetragonal FeGa3 structure (tP16) using perturbed angular correlation spectroscopy (PAC) and used to gain insight into diffusion processes in phases having more than one diffusion sublattice. The three indide phases studied in this work have two inequivalent and interpenetrating In-sublattices, labeled In1 and In2, and nuclear quadrupole interactions were resolved for probes on each sublattice. The phases are line-compounds with narrow field-widths. Diffusional relaxations, fitted using an exponential damping ansatz, were measured at the two opposing boundary compositions as a function of temperature. “High” and “low” relaxation regimes were observed that are attributed to In-poorer and In-richer compositions, under the reasonable assumption that the atomic motion occurs via an indium-vacancy diffusion mechanism. Relaxation was observed to be greater for tracer atoms starting on In2 sites in the indides immediately following decay of 111In into 111Cd, which is attributed to a preference of daughter Cd-tracer atoms and/or indium vacancies to occupy In1 sites. Activation enthalpies for relaxation are compared with enthalpies for self-diffusion in indium metal.

Modified embedded-atom interatomic potential for Fe-Ni, Cr-Ni and Fe-Cr-Ni systems

The body-centered cubic transition metal tungsten is frequently used as a pressure calibration material at high temperatures and pressures due to its outstanding mechanical and thermal properties. In this study, molecular dynamics simulations were performed to investigate the behavior of tungsten under harsh temperature and pressure conditions and the impact of fundamental defects, particularly vacancies, and voids, on its physical, structural, and mechanical properties through their correlation with elastic constants. The study also covers mechanical stability, elastic properties, brittleness and ductility, and hardness. The simulations utilized two different embedded atom methods and one modified embedded atom method interatomic potentials. The results show that the fundamental structural characteristics and properties of pure tungsten crystal, including lattice constant, density, cohesive and vacancy formation energies, elastic constants, and moduli in the ground state for all three potentials, are in good agreement with previous experimental and theoretical calculations and results. The calculated results demonstrate that the elastic constants-related properties for defective structures also have the same trend as the perfect crystal. The presence of defects in the crystal causes a decrease in properties at all temperatures and pressures, directly correlated to the fraction of crystal defects. As the percentage of vacancies increases, a further reduction in the elastic constants is observed. Likewise, these findings reveal that the presence of scattered vacancies in the crystal structure causes a more significant decrease in the substance's properties than a void in the center of the crystal (with the same percentage). The presence of any vacancy weakens the interatomic bonds of the atoms around the vacancy, while the existence of a void in the center has less effect on the interatomic bonds of atoms further away from the center of the crystal.