INDEX metal close-packing

Figure Bl.21.1. Atomic hard-ball models of low-Miller-index bulk-temiinated surfaces of simple metals with face-centred close-packed (fee), hexagonal close-packed (licp) and body-centred cubic (bcc) lattices (a) fee (lll)-(l X 1) (b)fcc(lO -(l X l) (c)fcc(110)-(l X 1) (d)hcp(0001)-(l x 1) (e) hcp(l0-10)-(l X 1), usually written as hcp(l010)-(l x 1) (f) bcc(l 10)-(1 x ]) (g) bcc(100)-(l x 1) and (li) bcc(l 11)-(1 x 1). The atomic spheres are drawn with radii that are smaller than touching-sphere radii, in order to give better depth views. The arrows are unit cell vectors. These figures were produced by the software program BALSAC [35]-...

Once the values of hkl are found, then the arrangement of atoms on these surfaces is easily obtained, and Figure 1.2 shows the commonest low-index form of these surfaces. If the common surfaces of the fee structure are examined, it will be seen that the surface structure changes quite remarkably. The (111) surface is clearly a close-packed structure but the (100) surface has a square arrangement of metal atoms and the fee (110) surface, which shows grooves running parallel to the c-axis, is even more remarkable. The coordination of the surface atoms clearly is also very different, with the coordination evidently 9 in the (111) surface, 8 in the (100) surface and a remarkable 6 in the (110) surface, as compared to 12 in the bulk. 

Figure 1,2 Atomic arrangement on various clean metal surfaces. In each of the sketches (a) to (h) the upper and lower diagrams represent top and side views, respectively. Atoms drawn with dashed lines lie behind the plane of those drawn with thick lines, Atoms in unrelaxed positions (i.e. in the positions they occupy in the bulk) are shown as dotted lines. From G.A. Somorjai, Chemistry in Two Dimensions, Cornell University Press, London, 1981, p. 133, For the Miller index convention in hexagonal close-packed structures, see also G.A. Somorjai loc. cit, Used by permission of Cornell University Press,...

The metal substrates used in the LEED experiments have either face centered cubic (fee), body centered cubic (bcc) or hexagonal closed packed (hep) crystal structures. For the cubic metals the (111), (100) and (110) planes are the low Miller index surfaces and they have threefold, fourfold and twofold rotational symmetry, respectively. 

It should be noted that results on the major faces of iron Indicate that a close-packed [110] direction of the oxide is parallel to the second most closely packed [100] direction of the metal. The relationship, however, does not hold for oxide on some of the high index faces as shown by Bardolle s work. Bardolle (53) has proposed the general rule that FeO forms on the metal surface in such a way as to make the <111> directions of great atomic density of the metal correspond as closely as possible with the <110> directions of great density of iron Ions in the oxide. K is not possible, however, to predict the orientation of oxide on any particular face from this rule alone. 

Figure Bl.21.1. Atomic hard-ball models of low-Miller-index bulk-terminated surfaces of simple metals with face-centred close-packed (fee), hexagonal close-packed (hep) and body-centred cubic (bcc) lattices (a) fee... 

The adsorption of anions on metal electrodes has been one of the major topics in surface electrochemistry. Specific adsorption of anions occurs when the anion loses aU or part of its solvation shell and forms a direct chemical bond with the substrate. In this situation the surface coverage by anions can be high and the adlayer tends to form a close-packed structure that depends critically on the surface atomic geometry of the underlying substrate and the balance between the anion-metal and anion-anion interaction energies. The structures of halide anions adsorbed onto Au(Jtkl), Ag(hkl), and Pt(hkl) low-index surfaces have been the most widely studied systems by SXS, and a comprehensive review of ordered anion adlayers on metal electrodes is given by Magnussen [57]. 

Contents Close-Packed Crystals. - Ionic Crystals. - Molecular Crystals. - Valence Crystals. -Metals. - Surfaces. - Cooperative Effects. - Appendices. - Author Index. - Subject Index. 

Iron is a metal with a body-centered cubic lattice. The three low-index planes used in the present study are shown in Fig. 1. The Fe(IIO) plane is the closest-packed iron surface with only six-coordinated (C ) atoms exposed. The Fe(lOO) plane is less closely packed and only has four-coordinated (C4) atoms in its surface. The Fe(l 11) plane was the most open surface studied and has both C4 and seven-coordinated (C7) atoms exposed. Three approximately circular iron specimens were cut from a high-purity (4iV) single-crystal rod and diamond polished, the final dimensions being ap-ivoximately 6 mm diameter and 1 mm thickness. 

Figure 33 shows the surface structures of several stepped surfaces of high Miller index. These surfaces are stable in a stepped/terrace configuration [26]. On the clean surface of a close-packed metal, the steps are usually of one atom in height, periodically distributed, and are separated by terraces of roughly equal width. Typical diffraction patterns of such surfaces are shown in figure 34. The formation of doublets or triplets indicates the appearance of new periodicities from which the stepped structure of these high Miller index surfaces can be obtained. 

On the (110) face (the more open face of the three low-index faces on fee metals) the adsorbed H atoms are alternatively adsorbed on each side of the close-packed [110] rows, in the pseudo-threefold fee sites close to the bridge sites [82]. They may jump to one subsurface site O2 at the second layer level, under the bridge sites. On the (110) faces of Ni and Pd, above a coverage of one, hydrogen induces reconstruction into a (llO)-(l x 2) structure, probably of the pairing-row type [8,15,82]. This opens the surface more and allows accommodation of H on the second metal layer, leading to a coverage of 1.5 H per first layer metal atom, and makes it easier to reach subsurface sites [8,83,84],... 

Typical XRD patterns (Fig2) show that nanowires are relatively well crystallized. Samples are poly-phasic and the pattern is indexed as a mixture of the hexagonal close packed (hep) lattice (69%) and the face centered cubic structure (fee) (31%) as determined by Match software. The presence of a small percentage of CoO phase is visible in some of our spectra. This latter is a well-known phenomenon of surface oxidation in metal nanowires [14]. 

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