Step dislocations

For the weathering of trace minerals from the solid matrix, the dissolution occurs selectively on spots where the mineral is exposed to the surface. These mineral surfaces are usually not smooth, but show dislocations (screw, jump, step dislocations) and point defects (vacant sites, interstitial sites) (Fig. 23 left). Dissolved ions are immediately transported from the surface into solution, so that no gradient can develop. Since the total concentrations of trace minerals in the solution are low, no equilibrium can be reached. In the following the dissolution of trace minerals is called surface-controlled. 

Fig. 23 Comparison between surface-controlled reactions (left 1= interstitial sites, 2= vacant sites, 3= screw dislocation, 4= jump dislocation, 5=step dislocation) and diffusion-controlled processes (right)...

The kinetics of 2D Me-S surface alloy and 3D Me-S bulk alloy formation also strongly depend on surface inhomogeneities of S such as atomic disorder, kink sites, monatomic steps, dislocations, grain boundaries, etc. (cf. Chapter 1). Therefore, the... 

Cation vacancies and interstitials, (111) twins and stacking faults, grain boundaries, microstrains, misfit dislocation network at C03O4/C0O interface Dislocations and (100) stacking faults intergrowth of e and P phases. Cations vacancies and superstructure (110) stacking faults and twins Clusters of point defects (110) twins surface steps, dislocations, spinel microinclusions, planar defects stabilized by impurities. 

Metal deposition and dissolution (34) In the electrodeposition of solid metals such as silver and zinc, the cation is transported across the electrochemical interface to sites on the electrode surface (Figure 6-4). The positive charge of the cation is offset by electrons from the metal, and the adsorbed species becomes an adatom. These species have surface mobility and migrate along the electrode surface to an imperfection such as a step dislocation, where they enter into the crystal lattice. In the absence of sufficient step dislocations to accommodate the rate of deposition, the adatom surface concentration increases until two- or three-dimensional nucleation occurs. The rate of such nucleation and surface migration strongly influences the morphology of the electrocrystalhzation process. The reverse of this process is involved with electrodissolution of crystalline electro-deposits. 

The establishment of an exact quantitative relationship between the thermodynamic potential, (p0, or the potential of the adsorption layer (the Stern layer) potential, (pd, and the electrokinetic potential, , is an important and at present unsolved problem. Depending on the thickness of the layer with increased viscosity near the solid surface, the electrokinetic potential may either approach the value of the Stem layer potential or be lower than the latter. In some cases (e.g. for quartz), as shown in studies by D.A. Fridrikhsberg and M.P. Sidorova [10,11], the difference between the electrokinetic and thermodynamic potentials may be related to the hydration (swelling) of the solid surface and the formation of a gel-like layer resistant to deformation, within which a partial potential drop takes place. The difference between (pdand C, may also be related to microscopic surface roughness of the solids, i.e. to the presence of growth steps, dislocations and other defects (see Chapter IV). 

Summary. The potential of in-situ scanning probe techniques for the local investigation of surface properties and reactions at "nonideal" electrodes is presented in a typical example in the field of metal underpotential deposition, the essential role of the step dislocations for the local progress of adsorbate formation and also for the longterm adsorbate stability is shown and discussed for the adsorption of Pb and TI monolayers at stepped Ag(l 11) electrodes. 

Fig. 4.3 The structure of a step dislocation. The deformation can be graphically represented as due to the insertion of an additional lattice plane into the upper part of the crystal. In the neighbourhood of the dislocation, there are expanded and compressed regions. Due to the strong dependence of the intermolecular potential on distance, the excitation states react very sensitively to the presence of these regions.

A step dislocation can be understood as a slippage of part of the crystal relative to the rest. The boundary between the slipped and the unslipped regions is the dislocation line (Fig. 4.3). A step dislocation can also be considered to be an extra lattice plane inserted into the undisturbed crystal. The dislocation line is a line along this step (the edge of the plane). The neighbourhood of a dislocation line is strained. In the case of spiral dislocations, a section of the crystal lattice is rotated by one lattice constant through a slippage (Fig. 4.4). 

Dislocations, and the associated internal strains, are thermodynamically unstable as we have mentioned, and to a certain extent can be eliminated by annealing. They have a strong influence on the mobility of charge carriers and of excitons in the crystal. In any case, they disturb the periodicity of the lattice and act as scattering centres. They also modify the distribution and the concentration of impurity molecules in the crystal. It thus becomes clear with the example of step dislocations (Fig. 4.3) that the lattice above the slippage plane is compressed by the insertion of an additional lattice plane, while it is expanded below the slippage plane. Smaller molecules than those of the host can then occupy lattice sites in the upper region. 

Figure 1.12 Step dislocation. An additional lattice plane is partially inserted into the ideal lattice (according to KitteP ).

Figure 1.13 Construction of the Burgers vector, b to characterize a step dislocation. S, starting point, E, end point of circumvention of the step dislocation.

Figure 1.15 Small angle grain boundary. The defect can be considered as a two-dimensional array of step dislocations tilting the two crystals by an angle 0. The defect is characterized by the distance d between the step dislocations and by the tilting angle = old (a lattice constant according to KitteP ).

Figure 2.15 Progressive growth of two step dislocations of opposite sign (A and B).

As shown by metallographic studies, slip bands are formed by emergence of slip steps or intrusion-extrusion mechanism (Fig. 4.74). The intrusion-extrusion pairs are larger in the presence of a corrosive media. These observations suggest that the emergent slip bands are attacked by a corrosive medium which causes local stress intensification leading to premature failure. At preferentially attacked slip band steps, dislocations are unlocked and it becomes easier for the metal to be deformed (Fig. 4.75). This has been observed for carbon steels in aqueous media. The shp bands produce numerous sites for crack initiation. The density of slip bands is increased by corrosion. 

Fig. 5.14 Gliding of step dislocation in the x direction along a glide plane spanned by the line of dislocation (s z axis, i.e. plane of the paper) and the Biu gers vector (b x axis) driven by mechanical stress (see arrows). FVom Ref. [132].

Fig. 5.17 When two dislocations on nonparallel glide planes meet, steps are produced. Depending on whether the Burgers vectors of the step dislocations I and II are perpendicular (a) or parallel (b) to each other, jog or kink sites are produced (that is, steps from glide plane to glide plane or steps in the glide pltme). From Ref. [132].

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