Half crystal position

Solid metal electrodes with a crystalline structure are different. The crystal faces forming the surface of these electrodes are not ideal planes but always contain steps (Fig. 5.24). Although equilibrium thermal roughening corresponds to temperatures relatively close to the melting point, steps are a common phenomenon, even at room temperature. A kink half-crystal position—Fig. 5.24c) is formed at the point where one step ends and the... 

Fig. 5.24 Crystalline metal face (a) ad-atom, (b) ad-atom cluster, (c) a kink (half-crystal position), (d) atom at the step, (e) atom in the kink, (f) vacancy, (g) vacancy cluster. (According to E. Budevski)... 

It was mentioned on page 306 (see Fig. 5.24) that, even at room temperature, a crystal plane contains steps and kinks (half-crystal positions). Kinks occur quite often—about one in ten atoms on a step is in the half-crystal position. Ad-atoms are also present in a certain concentration on the surface of the crystal as they are uncharged species, their equilibrium concentration is independent of the electrode potential. The half-crystal position is of basic importance for the kinetics of metal deposition on an identical metal substrate. Two mechanisms can be present in the incorporation of atoms in steps, and thus for step propagation ... 

The anodic dissolution of metals on surfaces without defects occurs in the half-crystal positions. Similarly to nucleation, the dissolution of metals involves the formation of empty nuclei (atomic vacancies). Screw dislocations have the same significance. Dissolution often leads to the formation of continuous crystal faces with lower Miller indices on the metal. This process, termed facetting, forms the basis of metallographic etching. 

The lattice energy at a half-crystal position (kink site) is defined as the attachment energy and the energy released in forming a slice containing more than two PBCs is denoted by E j These are related to the lattice energy as follows ... 

Habib, and surface potential determination, 893 Habib llockris model of water, 899 I Iabib Bockris isotherm, 943,949 Half-crystal position, electrodeposition,... 

The half-crystal position is the same as the position called a kink site, and is shown in Fig. 7.134. This term is due to Kossel (1927) and Stranski (1928). The half-crystal position occurs in the way shown in Fig. 7.134. The energy of binding of an atom in the kink position is just half of the total energy of an atom in the bulk of the ciystal. 

Fig. 7.134. The half-crystal (or kink) position. The figure shows the binding energy of an atom in a kink position and how the name half-crystal position has been derived. A represents the missing part of the bulk crystal above the surface plane, fithe missing half of the surface plane, and Cthe missing half of the atomic row along the step in front of the kink atom. Together they add the halfcrystal to a bulk crystal. (Reprinted from E. Budevski, G. Staikov, and W. J. Lorenz, Electrochemical Phase Formation and Growth, 18, copyright 1996, John Wiley Sons. Reproduced by permission of John Wiley Sons, Ltd.)...

Since the heat of evaporation of cesium atoms from the second atom layer is 17,500 cal., according to Taylor and Langmuir (54), and since evaporation occurs from the half crystal position c we find... 

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... 

See also crystallization overpotential (polarization), - nucleation and growth kinetics, - equilibrium forms of crystals and droplets, - half-crystal position, - Kaischew, - metal deposition, - supersaturation, - Stranski, - Zeldovich. 

Half-crystal position (kink position) — The term was introduced into the theory of crystal growth simultaneously and independently by W. Kossel [i] and - St ran-ski [ii,iii], who were the first to realize the necessity of a close consideration of the elementary acts of attachment and detachment of single particles (atoms, ions, or molecules) to and from a crystal surface. 

An atom in the half-crystal position (also called kink position) (Fig. 1), is bonded (i) with a semi-infinite crystal block, (ii) with a semi-infinite crystal lattice plane, and (iii) with a semi-infinite crystal row. Attaching or detaching one atom to and from the half-crystal position a new half-crystal position is created and this is what makes this position a repeatable step in the successive building or disintegration of the bulk crystals. 

Half-crystal position (kink position) — Figure. The Kossel s cubic crystal (a) and the atom in the half-crystal position (b)... 

Kink position - half-crystal position Klemensiewicz, Zygmunt Aleksander... 

The dissolution process begins with an atom in a half crystal position at the surface. In the case of silver, it can be shown that two steps can be experimentally distinguished (3,4) (1) moving of half crystal atoms into adsorption positions on the surface and (2) transfer of silver ions from adsorption positions through the inner double layer into the solution. 

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. 

The second remarkable feature of an atom in a kink position is that it is bonded to the crystal with exactly one half of the bonds of one bulk atom. This is schematically illustrated in Fig. 2.9 for a crystal with a simple cubic lattice. The parts A, B, and C represent exactly the missing half of the infinitely large crystal. After adding A, B, and C, the half ciystal position is changed to a bulk position. For a crystal with a hep or fee (bulk) lattice, a kink atom or an atom in a half crystal position has 6 first neighbors, that is, half of the coordination number of a bulk atom. 

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... 

To my knowledge, Ivan Stranski never contributed directly to the field of electrochemical science. However, his definitions of the fundamental concepts half-crystal position and separation work (py2 of single atom therefrom [8, 9], as well as the definition of the concept of mean separation work of the atoms from a crystal face in contact with a vapor phase [16-18], were used by R. Kaischev [19], who in 1946 proposed definitions of these quantities applicable also to the more complex case of crystals in contact with an ionic electrolyte solution (see also [12]). 

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