Place-exchange

The other major defects in solids occupy much more volume in the lattice of a crystal and are refeiTed to as line defects. There are two types of line defects, the edge and screw defects which are also known as dislocations. These play an important part, primarily, in the plastic non-Hookeian extension of metals under a tensile stress. This process causes the translation of dislocations in the direction of the plastic extension. Dislocations become mobile in solids at elevated temperamres due to the diffusive place exchange of atoms with vacancies at the core, a process described as dislocation climb. The direction of climb is such that the vacancies move along any stress gradient, such as that around an inclusion of oxide in a metal, or when a metal is placed under compression. 

The process requires the interchange of atoms on the atomic lattice from a state where all sites of one type, e.g. the face centres, are occupied by one species, and the cube corner sites by the other species in a face-centred lattice. Since atomic re-aiTangement cannot occur by dhect place-exchange, vacant sites must play a role in the re-distribution, and die rate of the process is controlled by the self-diffusion coefficients. Experimental measurements of the... 

Figure 1.4 Proposed steps in the chemisorption of OH on/in Pt, starting with arrays of OH groups over the uppermost metal atom layer, increasing the coordination number of the adsorbed OH by place exchange, and next generating a mixed, metal/oxygen overlayer while further oxidizing to form O atoms. From Conway et al. [1990].

Figure 9.6 Visual representation of the platinum oxide growth mechanism, (a) Interaction of H2O molecules with the Pt electrode occurring in the 0.27 V < < 0.85 V range, (b) Discharge of 5 ML of H2O molecules and formation of 5 ML of chemisorbed oxygen (Ochem)- (c) Discharge of the second ML of H2O molecules the process is accompanied by the development of repulsive interactions between (Pt-Pt) -Ofi m surface species that stimulate an interfacial place exchange of Ochem and Pt surface atoms, (d) Quasi-3D surface PtO lattice, comprising Pt and moieties, that forms through the place-exchange process. (Reproduced with permission...

During the second process, place exchange occurs resulting in the oxide atom moving below the plane of the surface ... 

The rate of the second electron transfer, and the subsequent place exchange, are not significant until c. a monolayer of adsorbed OH has been formed. 

The authors were also the first to postulate the existence of a place-exchange mechanism at Pt, suggesting that the path of oxide formation is ... 

Notice the reversible step (3.29) and that the place-exchange is rate-determining. The inference was that at potentials > 0.95 V the oxygen-containing film takes on the characteristics of a true place-exchanged, or phase , oxide. As is discussed below, the conclusions of Reddy and colleagues (1968) on the properties of the oxide film at potentials > 1.0 V vs. RHE are essentially correct. 

As a result of the accumulated evidence, including that discussed above, Conway and colleagues (1973) postulated the mechanism represented by equations (3.35) to (3.37), (3.30) and (3.31) above, with place-exchange occurring after the step represented by equation (3.35). 

The two predominant features in Figure 3.24 are attributable to the 4f orbitals of the Pt electrode. The two peaks were deconvoluted as shown into a main peak and a smaller satellite peak. At potentials > 0.7 V vs. SCE, a peak at 77.1 eV was observed which was attributed to PtO. On the basis of these results, those of Kim et ai (1971), and the coulometric and ellipsometric data discussed above, Augustynski and Balsenc (1979) proposed that the signal attributed to the Pt 4f orbitals shifted via formation of PtO was only observed after the formation of the phase oxide, since it is only after this place exchange that the chemical environment of the Pt atoms is modified... 

A mechanism which proceeds through surface reconstruction of the substrate has been identified for Ni deposition on Au(lll) [120, 121]. The process begins with place exchange of nickel into a particular position in the reconstructed Au(lll) surface, followed by deposition of Ni islands on top of the imbedded atom. At higher overpotentials, nucleation occurs instead at step edges, so that control of the potential allows control of the nucleation process and the distribution of Ni in the early stages of growth. In this instance, the nucleation process has been captured by STM on the atomic scale. 

It aay be assuaed that oxygen adsorption as a result of place exchange aechanisa leaves after desorption soae platinua atoas in a position of adatoas at the (ill) ordered surface. 

Oxide formation on noble metal electrodes usually proceeds by two-dimensional adsorption followed by place exchange between the metal and oxide species to form a three-dimensional overlayer. Subsequent reduction... 

Figure 6.1 General schematic representation of pol3mier-mediated assembly of nanoparticles (a) functionalization of nanoparticles through place-exchange method, (b) incorporation of complementary functional group to pol3miers, and (c) self-assembly of nanoparticles through electrostatic or hydrogen bonding interactions.

Hostetler MJ, Templeton AC, Murray RW. Dynamics of place-exchange reactions on monolayer-protected gold cluster molecules. Langmuir 1999 15 3782-3789. 

Exchange Reaction. A process whereby atoms of the same element in two different molecules or in two different positions in the same molecule transfer places. Exchange reactions... 

For place exchange, no change in the relative intensity of the lines would be... 

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