Kink site position 110 direction

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. 

Ad-atoms can be deposited into a kink site position either directly or via a step position. Otherwise, the atom in a kink site position can be separated from the kink site position to a step position or to an ad-atom position as shown in Figure 7.13. 

There is, in principle, the direct transfer between kink site position and electrolyte. This process, which must be taken into account at larger overpotentials, will not be discussed further. 

B) The direct dissolution, transfer of a charged ion from the kink site position to the electrolyte. In this case the rate equation for the direct anodic dissolution of kink site atoms would be... 

At higher anodic potentials the direct dissolution from the kink site positions will become the rate-determining step described by the four equations ... 

In the discussion of atomistic aspects of electrodepKJsition of metals in Section 6.8 it was shown that in electrodeposition the transfer of a metal ion M"+ from the solution into the ionic metal lattice in the electrodeposition process may proceed via one of two mechanisms (1) a direct mechanism in which ion transfer takes place on a kink site of a step edge or on any site on the step edge (any growth site) or (2) the terrace-site ion mechanism. In the terrace-site transfer mechanism a metal ion is transferred from the solution (OHP) to the flat face of the terrace region. At this position the metal ion is in an adion state and is weakly bound to the crystal lattice. From this position it diffuses onto the surface, seeking a position with lower potential energy. The final position is a kink site. 

Step-Edge Ion-Transfer Mechanism. The step-edge site ion transfer, or direct transfer mechanism, is illustrated in Figure 6.14. It shows that in this mechanism ion transfer from the solution (OHP) takes place on a kink site of a step edge or on any other site on the step edge. In both cases the result of the ion transfer is a M adion in the metal crystal lattice. In the first case, a direct transfer to the kink site, the M adion is in the half-crystal position, where it is bonded to the crystal lattice... 

Figure 4.3. Electrochemical growth of a stepped crystal surface (a schematic representation). (1) ion in the electrolyte, (2) adatom on a flat terrace, (3) atom at a kink site (half crystal position), (4) vacancy, (5) atoms in a two-atomic cluster, (l->2- 3) - surface difliision mechanism, (l->3) - direct attachment mechanism. (a,b,c) - distribution of the adatoms concoitration...

The attachment frequency to the nucleus of the critical size, (Oa,c depends on the form of the nucleus and on the adopted mechanism of deposition (direct transfer, surface diffusion, etc.). Zo is the number of sites per unit area where nucleation can start. For a homogeneous surface, Zo is given by the number of adsorption sites for adatoms and is of the order of 10 cm". The factor fc contains the vibrational frequencies of the cluster atom in the positions i with respect to the frequency of vibration of the kink atoms and can be assumed to be of the order of unity. It has to be remembered, however, that the inclusion of an entropy term, particularly when dissociation energies are used for the calculation of AGc, makes its value largely uncertain. It has been emphasized already that configurations other than equilibrium (Wulff-Kaischew) ones can contribute to the overall nucleation rate. Kaischew and Stoyanov have shown that, at low overpotentials, this contribution becomes appreciable. 

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