Diffusion mechanism interstitial

One type of diffusion mechanism is known as the interstitial mechanism because it involves movement of a lattice member from one interstitial position to another. When diffusion involves the motion of a particle from a regular lattice site into a vacancy, the vacancy then is located where the site was vacated by the moving species. Therefore, the vacancy moves in the opposite direction to that of the moving lattice member. This type of diffusion is referred to as the vacancy mechanism. In some instances, it is possible for a lattice member to vacate a lattice site and for that site to be filled simultaneously by another unit. In effect, there is a "rotation" of two lattice members, so this mechanism is referred to as the rotation mechanism of diffusion. 

Fig. 25. Schematic diagram indicating a possible hydrogen diffusion mechanism, by which (a) the hydrogen leaves a Si—H bond for an interstitial position, (b) inserts into a strained Si—Si bond, (c) returns to an interstitial position, and (d) passivates a pre-existing dangling bond (Kakalios and Jackson, 1988). 

In the case of interstitials—self-interstitials, impurities, or dopants—two diffusion mechanisms can be envisaged. In the simplest case, an interstitial can jump to a neighboring interstitial position (Fig. 5.8a). This is called interstitial diffusion and is sometimes referred to as direct diffusion to distinguish it from vacancy diffusion (indirect diffusion). 

Figure 5.11 Diffusion mechanisms (a) exchange (e) and ring (r) diffusion (b) kick-out diffusion, leading to (c) a substitutional defect and a self-interstitial.

When ionic conductivity is by way of interstitials, both conductivity and diffusion can occur by random motion, so that the correlation factor and HR are both equal to 1. In general, the correlation factor for a diffusion mechanism will differ from 1, and in such a case D can be described by the following relationship ... 

Atomic hydrogen formed as an intermediate in the hydrogen evolution reaction is adsorbed to the surface of the membrane. Molecular hydrogen is formed by one of at least two mechanisms, but parallel to this, a fraction of the atomic hydrogen is absorbed by the metal, eventually leading to an equilibrium between adsorbed and absorbed atomic hydrogen. In the absorbed state, the hydrogen atoms are able to diffuse as interstitials in the metal lattice. 

Figure 4.40 Illustration of diffusion mechanisms in alloys and ionic solids (a) interchange (exchange) (b) ring rotation (rare) (c) interstitial migration and (d) vacancy migration. From W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceramics. Copyright 1976 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

Another system obeying Fick s law is one involving the diffusion of small interstitial solute atoms (component 1) among the interstices of a host crystal in the presence of an interstitial-atom concentration gradient. The large solvent atoms (component 2) essentially remain in their substitutional sites and diffuse much more slowly than do the highly mobile solute atoms, which diffuse by the interstitial diffusion mechanism (described in Section 8.1.4). The solvent atoms may therefore be considered to be immobile. The system is isothermal, the diffusion is not network constrained, and a local C-frame coordinate system can be employed as in Section 3.1.3. Equation 2.21 then reduces to... 

Figure 8.7 Interstitial mechanism for diffusion of interstitial atoms. The smaller...

On the basis of thermodynamic considerations, some of the lattice sites in the crystal are vacant, and the number of vacant lattice sites generally is a function of temperature. The movement of a lattice atom into an adjacent vacant site is called vacancy diffusion. In addition to occupying lattice sites, atoms can reside in interstitial sites, the spaces between the lattice sites. These interstitial atoms can readily move to adjacent interstitial sites without displacing the lattice atoms. This process is called interstitial diffusion. The interstitial atoms may be impurity atoms or atoms of the host lattice, but in either case, interstitial atoms are generally present only in very dilute amounts. However, these atoms can be highly mobile, and in certain cases, interstitial diffusion is the dominant diffusion mechanism. 

Molecular reorientations at Bjerrum fault sites are responsible for the dielectric properties of ice. A second type of fault (proton jumps from one molecule to a neighbor) accounts for the electrical conductivity of ice, but cannot account for the high dielectric constant of ice. Further discussion of such ice faults is provided by Franks (1973), Franks and Reid (1973), Onsager and Runnels (1969), and Geil et al. (2005), who note that interstitial migration is a likely self-diffusion mechanism. 

As explained in Chapter 5, the transport mechanism in dense crystalline materials is generally made up of incessant displacements of mobile atoms because of the so-called vacancy or interstitial mechanisms. In this sense, the solution-diffusion mechanism is the most commonly used physical model to describe gas transport through dense membranes. The solution-diffusion separation mechanism is based on both solubility and mobility of one species in an effective solid barrier [23-25], This mechanism can be described as follows first, a gas molecule is adsorbed, and in some cases dissociated, on the surface of one side of the membrane, it then dissolves in the membrane material, and thereafter diffuses through the membrane. Finally, in some cases it is associated and desorbs, and in other cases, it only desorbs on the other side of the membrane. For example, for hydrogen transport through a dense metal such as Pd, the H2 molecule has to split up after adsorption, and, thereafter, recombine after diffusing through the membrane on the other side (see Section 5.6.1). 

Figure 3 Defects and associated diffusion mechanism 1 and 2, diffusion mechanism by direct exchange 3, diffusion through vacancy 4, direct interstitial mechanism 5, indirect interstitial or caterpillar mechanism 6, Frenkel defect 7, indirect exchange 8, Schottky defect. (Ref 9. Reproduced by permission of Cambridge University Press)...

Be NMR has been used to detect slow atomic motion of beryllium in Zr-Ti-Cu-Ni-Be metallic glasses. The results, obtained by a spin alignment echo technique, are consistent with Be diffusion occuring by a mechanism involving thermal fluctuations of the spread-out free volume rather than by vacancy-assisted or interstitial diffusion mechanisms (Tang et al. 1998). 

In this subsection we examine the mechanism of the very fast diffusion. In the bulk medium the vacancies and interstitial site play a primary role in accelerating the diffusion. However, these diffusion mechanisms are not relevant in microclusters. It is well known that the vacancies created inside the cluster are immediately pushed to the surface. Indeed in our simulation the creation of vacancies inside the cluster is a very rare event even at the temperature close to the melting temperature. Moreover, we cannot find any evidence that the interstitial deformation takes place inside the cluster, and therefore neither of them is responsible for the rapid diffusion into the cluster. The key feature of the cluster that distinguishes the cluster from the bulk medium is that it is surrounded by the surface beyond which no atoms exist. In other words, the outside of the cluster is occupied by vacancies. As a result, the atoms on the surface move very actively along the surface. Such an active movement along the surface will be responsible for the rapid diffusion in the radial direction of the cluster. We focus our attention to the details of the active diffusive motion along the surface of the cluster, and we present a direct evidence that the surface activity controls the radial diffusion. A direct measure of the surface motion is the diffusion rate of the surface atoms... 

To relate Djon to defect diffusivities, however, more information about the system is needed. For starters, it is imperative to know the diffusion mechanism — if diffusion is by vacancies, their concentration or mole fraction A has to be known to relate the two since, according to Eq. (7.14), Dvac — Ds/A. Needless to say, if the number of defects is not known, the two cannot be related. If, however, the tracer diffuses by an interstitial... 

The temperature dependence of the diffusion coefficient will depend on the diffusion mechanism. If diffusion occurs interstitially, the temperature dependence of D will include only the migration energy term, Ai/, since the probability of the site adjacent to an interstitial atom being vacant is 1.0. 

---