Kink site position 100 surface

Deposition of Me on a 3D native bulk phase proceeds by incorporation of atoms in kink site" positions as a final step of the overall reaction (1.1) (cf. Section 2.1). In contrast. Me dissolution can also take place at all sites where lattice atoms are more loosely bound to the crystal than kink atoms. Usually dissolution starts, in addition to the kink sites, at crystal edges and corners, or at surface defects and inhomogeneities. Therefore, the Me deposition-dissolution processes are not necessarily symmetric. At the Nernstian equilibrium potential, equality... 

Among the different surface atom positions illustrated on Fig. 2.8, the kink site position, or the half crystal position, as introduced independently by Kossel [2.12] and Stranski [2.13], has a special significance for the definition of the equilibrium conditions (vapor pressure, equilibrium concentration, equilibrium electrode potential, etc.) of the infinitely large (bulk) crystal. 

Results are presented for stepped surfaces with terrace orientations (100) and (111) on Pt and (001) on Ru. In fact, these planes, together with various others, form the surface of a field emitter tip which can be regarded as a catalyst particle with a diameter of about 20 to 200 nm. Although our probe hole measurements sample only a few atomic sites (up to about 200) the detailed crystallography of the probed area, i.e. terrace widths and step site symmetries, is not known because the concomitant removal of substrate atoms by field evaporation (from kink site positions) during the measurements causes continuous alterations of the morphology. 

The adsorbed metal ion is called an adatom. From the ad-atom position, the cation is transported by surface diffusion to a position at which it is already part of the growing metal structure (Step 3, Eq. 2). This position is called the kink site position (HalbkristaUage Mei/2). Step 3 is accompanied by transfer of z - X) electrons. 

On a real surface the atoms undergo reorientation processes, which lead to changes in the distribution of energies between different bonds. The bond of an atom in a kink site position (the calculation will be described in the next chapter) can differ from the ideal value. 

In the process of sublimation the atom is separated from a special position on the surface, the kink site position (half crystal position). In the ideal kink site position the... 

Values of energies of atoms in ideal kink site positions and bond energies between two metal atoms were summarized for transition and main group metals in Table 2.3. On the real surface, the bond energies in kink site positions and the bond energies between two neighbors can differ from the ideal model values because of relaxation processes on the surface. 

An alternative view on the UPD process provides the concept of substrate supported low-dimensional phase formation from Staikov, Budevski and LorenzThis concept explains unexpected stabilities of underpotential deposits. In this concept the UPD layer is considered to be a two-dimensional phase stabilized by the bond between substrate and UPD metal. A linear phase is the adsorption of atoms on steps and a phase of zero dimension is a group of atoms around a kink site position or around a surface dislocation. This concept is used to explain the positively shifted potentials in the cyclic voltammogram of UPD. Higher stabUily of the low-dimensional phase is described by introducing activities of atoms in the substrate-supported phase of lower dimension OMe,od> Me,id> d The deposition potential is given by the equation... 

In the following section, a statistical description of the surface processes will be presented. This is based on the definition of a residence time of an atom in a kink site position. 

Figure 7.13 shows the (100) surface of a metal crystallizing in the face-centered cubic lattice. Three planes of atoms and a step running in the [110] direction are shown. On the step a kink site position is shown. 

If is much smaller than [ksp], the number of kink site positions decrease, because the renewal of the kink site positions by accumulation of metal ions is so slow that more and more kink site positions disappear because of the natural surface reconstruction. If is much larger than [ksp], however, the number of kink positions will increase because new surface structures can develop. Thus one can say for stationary conditions that N,should approach the value = 1. The density of kink site positions [fop] finally therefore approach the value... 

The calculation of [fop], takes into account the nature of the metal (by r) and the electrochemical conditions (by if). The value of ig reflects the different experimental parameters like electrolyte composition, and additives, etc. Therefore, the values of the density of kink site positions provide an image of the surface dynamic at equilibrium conditions. While the absolute values might be questionable, the comparative nature of the described procedures allows at least a comparison of the dynamics of experimental systems. The values for Ag reflect the density of kink site positions for three experimental conditions. The largest value of 5 X lO cm corresponds to one of the highest experimental exchange current densities observed. In this case, [fopJo approaches the surface density of atoms on... 

With increasing step density the softness increases. Furthermore, the softness increases with the surface density of kink site positions, or, quite generally, with increasing surface roughness. On polycrystalline surfaces, the same rules apply and the softness increases with an increasing amount of surface roughness. 

Additives can control the growth rate by blocking surface processes, e.g., surface diffusion or the transfer in and out of kink site positions. Therefore, interaction of additives with the different crystallographic structures is a dominating factor in the development of crystallographic shape and structure. 

SURFACE SELECTIVITY IN ALLOY DEPOSITION 8.5.1 Kink site positions of alloys... 

Substituting the surface densities of kink site positions by equilibrium constants and ion concentrations in the electrolyte bath one obtains... 

The surface density of the principal four kink site positions (in square brackets) is related to the ion concentration of the two components Co and Fe in the electrolyte, described by and Cp, by the equations... 

The concept of kink site positions and the selectivity function of these positions can be extended to ternary systems. The variety of kink site positions on the surface grows with the number of components. This is shown for a close packed cubic structure if only taking into account first neighbors. Then the following kink site positions are possible on the surface. 

It was approximated that in the treatment of a binary alloy, the character of a kink site position is determined by the previously deposited atom and not so much by the environment of the atom. If this is extended to a ternary alloy one can describe the processes on the surface by nine main reactions... 

The following mechanism is based on an alloy of two comp)onents A and B. In an approximate description on the surface four different kinds of kink site positions exist, AA, AB, BA, and BB. Three steps for each component describe the dissolution via ad-atoms. 

Surface density of kink site positions Frequency shift Complex frequency shift Gibbs energy of alloy A j ... 

Fig. 9. Schematic of a two-dimensional cross section of an AgBr emulsion grain showing the surface and formation of various point defects A, processes forming negative kink sites and interstitial silver ions B, positive kink site and C, process forming a silver ion vacancy at a lattice position and positive kink...

Terrace Ion-Transfer Mechanism, In the terrace siteion-transfer mechanism a metal ion is transferred from the solution (OHP) to the flat face of the terrace region (Fig. 6.15). At this position the metal ion is in the adion (adsorbed-like) state, having most of its water of hydration. It is weakly bound to the crystal lattice. From this position it diffuses on the surface, seeking a position of lower energy. The final position is a kink site. 

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