High symmetry surface

Kwon, Ki-Young Wong, Kin L. Pawin, Greg et. al. Unidirectional Adsorbate Motion on a High-Symmetry Surface Walking Molecules Can Stay the Course, Phys. Rev. Lett. 95, 166101 (2005) [Issue 16-14 October 2005]... 

Fig. 1. STM images resolving (a) the hexagonal atomic structure of the close-packed fcc(lll) surface and (b) the anisotropic fcc(llO) surface of Ag. The surface unit cells and high symmetry directions are marked, (c) Schematic one-dimensional potential energy surface experienced by a simple individual adsorbate along a high-symmetry surface direction (Em migration energy barrier ICf, bonding energy a surface lattice constant).

For solids, crystal face, there is a unique value of asv (which is a scalar) while ySv depends also on the orientation along the face. Moreover, ctsv is always a positive quantity (breaking bonds needs work to be done) while ysv can be either positive or negative (Nolfi and Johnson 1972). For high symmetry surfaces, the surface tension is related to the surface energy by the equation (Shuttleworth 1950) ... 

Hirano and Shinjo [22] calculated the condition for static friction between high-symmetry surfaces of fee and bcc metals. Many of their results are consistent with the conclusion that the elastic interactions within the bulk dominate the... 

In the second step, full optimizations were performed with the relaxation of all atoms in the system, where the oxygen atom was again positioned initially at all high-symmetry surface sites. Thereby, the contribution of particle relaxation to the adsorption energy could be quantified. The potential energy of adsorption after either of these steps was calculated with respect to the energy of the free O2 molecule ... 

In the discussion above we have tried to show that the theoretical calculations performed make it possible to describe the basic peculiarities of the electronic surface states appropriate to high-symmetry surfaces of binary d-metal carbides and nitrides with the Bl-type structure. However, some problems connected with specific features of interatomic bonding and the energy spectrum of external boundaries of refractory phases (which occur, for example, in multicomponent amorphous refractory films) are only at an early stage of development and need further theoretical and experimental investigations. 

Figure 9.1 High symmetry adsorption sites for O and O2 on Pt(l 11). Large gray circles represent Pt atoms, and small open circles represent O atoms, t, b, h, and f stand for top, bridge, hep, and fee, respectively. The surface unit cell is dehneated. (Reproduced with permission from Xu et al. [2004].)...

In aqueous solutions, the high negative surface charge of a zeolite must be neutralized by binding counterions, such as Na+, K+, and Ca +. The distribution of zeolite pore size can be modified, such that small molecules are included in the pores those too large to diffuse into the pores are excluded. Structure types are named by a three-letter lUPAC code, based in part on the name of the zeolite first used to identify the specific type. They are also classified by pore size, framework density, and/or symmetry. 

Figure 4.16 Illustration of potential high symmetry binding sites of H (filled circles) on Cu(100). Atoms in topmost layer of the Cu(100) surface are shown as large open circles.

We can avoid this symmetry-induced trap by deliberately breaking the symmetry of our atom s coordinates. One easy way to do this is to repeat our calculations after moving the H atom a small amount (say, 0.2 A) in some arbitrary direction that does not coincide with one of the symmetry directions on the surface. What we find, if we run calculations in which we start the H atom at a point about 0.2 A away from each of the high-symmetry sites mentioned above is that the H atom relaxes to the fourfold hollow site even if it is started quite near the top and bridge sites. This shows that the top and bridge sites are not minima for this system. 

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