Facetting transitions

Eq. (22) is meant only as an example to illustrate that an unstable surface which spontaneously facets is composed of attracting steps. We do not address the underlying cause of the attraction. Note, in this model, f would be a function of temperature which would vanish at the faceting transition temperature and become negative above this temperature corresponding to a repulsion between steps. 

Surface roughening-faceting transitions have attracted considerable interest in recent years. The most intensively studied surface from this point of view has been the (110) surface of pure Pb . In order to understand roughening-faceting transitions, it is useful to begin by considering the factors which determine whether facets are present on the equilibrium form of a crystal. 

Cu(l-10) An adsorption process is apparent between -0.290 and -0.40 V (Fig. 1) which consumes 0.050 mC/cm2. As shown in Fig. 8 the peaks shift -0.061 V/decade with chloride concentration which reflects the Esin-Markov effect. At potentials below the -0.310 V STM reveals the (110) terraces to be elongated with steps faceted in the <100> direction, i.e. orthogonal to the close packed <110> of the metal lattice. As the potential is moved toward more positive potentials a faceting transition occurs where the chloride covered terraces undergo a reconstruction, as shown Fig. 9. 

It is interesting to consider the packing density of halide on the (110) surface in comparison to the van der Waals diameter of the respect halide ions. The nearest neighbor distance of a compact (3x2) structure corresponds to 0.383 nm while the van der Waals diameter of bromide and chloride are reported to lie in the range of 0.39 nm and 0.36 nm respectively [14]. Thus, the (3x2) bromide structure corresponds to saturation coverage based on a close packed layer while in the case of chloride an increase in the coverage and compression beyond the (3x2) structure is possible and this results in either reconstruction of the (110) terrace or perhaps a faceting transition to a (n 10) orientation. Further work is necessary to clarify this issue. 

Fig. 8. The dependence of the peak potential of the faceting transition on chloride concentration. Assuming the transition occurs at a fixed charge, the slope -0.061 V/dec is proportional to the Esin-Markov coefficient.

Fig. 9. STM image revealing the faceting transition between (110) terraces and a (nlO) like structure as the potential is swept between A.) -0.395 V and B.) -0.194 V. (74 nm x 74 nm)...

Copper single crystal electrodes with orientations of (100) and (110) were imaged by atomic force microscopy during copper deposition in acid sulfate solution with and without chloride. At low overpotentials, facets appear only in the presence of chloride. The roughening and faceting transitions observed as potential was varied, and the stabilization of facets and terrace edges by chloride are analyzed in thermodynamic terms. 

Not surprisingly, ordered metal adlayers exhibit many of the characteristics already noted for anion adsorption such as elec-trocompression. Moire patterns, faceting transitions, as well as additional effects such as two-dimensional alloying [4-26]. Of course, for many ordered upd systems, structural assignment of the imaged... 

In another process for the synthesis of PPS, as well as other poly(arylene sulfide)s and poly(arylene oxide)s, a pentamethylcyclopentadienylmthenium(I) TT-complex is used to activate -dichlorobenzene toward displacement by a variety of nucleophilic comonomers (92). Important facets of this approach, which allow the polymerization to proceed under mild conditions, are the tremendous activation afforded by the TT-coordinated transition-metal group and the improved solubiUty of the resultant organometaUic derivative of PPS. Decomplexation of the organometaUic derivative polymers may, however, be compHcated by precipitation of the polymer after partial decomplexation. 

One of the frmdamental structural facets of organic chemistry, which has been explained most satisfactorily in MO terms, is the existence of a small barrier to rotation about single bonds. In ethane, for example, it is known that the staggered conformation is about 3kcal/mol more stable than the ecl sed conformation so that the eclipsed conformation represents a transition state for transformation of one staggered conformation into another by rotation. 

D. J. Gates. Surface angle transitions and facets in a solvable TSK model of steady crystal growth. J Cryst Growth 180 36, 1997. 

Numerous quantum mechanic calculations have been carried out to better understand the bonding of nitrogen oxide on transition metal surfaces. For instance, the group of Sautet et al have reported a comparative density-functional theory (DFT) study of the chemisorption and dissociation of NO molecules on the close-packed (111), the more open (100), and the stepped (511) surfaces of palladium and rhodium to estimate both energetics and kinetics of the reaction pathways [75], The structure sensitivity of the adsorption was found to correlate well with catalytic activity, as estimated from the calculated dissociation rate constants at 300 K. The latter were found to agree with numerous experimental observations, with (111) facets rather inactive towards NO dissociation and stepped surfaces far more active, and to follow the sequence Rh(100) > terraces in Rh(511) > steps in Rh(511) > steps in Pd(511) > Rh(lll) > Pd(100) > terraces in Pd (511) > Pd (111). The effect of the steps on activity was found to be clearly favorable on the Pd(511) surface but unfavorable on the Rh(511) surface, perhaps explaining the difference in activity between the two metals. The influence of... 

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