Face-centered cubic, fee

Except for helium, all of the elements in Group 18 free2e into a face-centered cubic (fee) crystal stmeture at normal pressure. Both helium isotopes assume this stmeture only at high pressures. The formation of a high pressure phase of soHd xenon having electrical conductivity comparable to a metal has been reported at 33 GPa (330 kbar) and 32 K, and similar transformations by a band-overlap process have been predicted at 15 GPa (150 kbar) for radon and at 60 GPa (600 kbar) for krypton (51). 

The stmcture of Pmssian Blue and its analogues consists of a three-dimensional polymeric network of Fe —CN—Fe linkages. Single-crystal x-ray and neutron diffraction studies of insoluble Pmssian Blue estabUsh that the stmcture is based on a rock salt-like face-centered cubic (fee) arrangement with Fe centers occupying one type of site and [Fe(CN)3] units randomly occupying three-quarters of the complementary sites (5). The cyanides bridge the two types of sites. The vacant [Fe(CN)3] sites are occupied by some of the water molecules. Other waters are zeoHtic, ie, interstitial, and occupy the centers of octants of the unit cell. The stmcture contains three different iron coordination environments, Fe C, Fe N, and Fe N4(H20), in a 3 1 3 ratio. 

Iron (qv) exists in three aHotropic modifications, each of which is stable over a certain range of temperatures. When pure iron free2es at 1538°C, the body-centered cubic (bcc) 5-modification forms, and is stable to 1394°C. Between 1394 and 912°C, the face-centered cubic (fee) y-modification exists. At 912°C, bcc a-iron forms and prevails at all lower temperatures. These various aHotropic forms of iron have different capacities for dissolving carbon. y-Iron can contain up to 2% carbon, whereas a-iron can contain a maximum of only about 0.02% C. This difference in solubHity of carbon in iron is responsible for the unique heat-treating capabilities of steel The soHd solutions of carbon and other elements in y-iron and a-iron are caHed austenite and ferrite, respectively. 

Silver chloride crystals are face-centered cubic (fee), having a distance of 0.28 nm between each ion in the lattice. Silver chloride, the most ionic of the halides, melts at 455°C and boils at 1550°C. Silver chloride is very ductile and can be roUed into large sheets. Individual crystals weighing up to 22 kg have been prepared (10). 

The electronic stmcture of cobalt is [Ar] 3i/4A. At room temperature the crystalline stmcture of the a (or s) form, is close-packed hexagonal (cph) and lattice parameters are a = 0.2501 nm and c = 0.4066 nm. Above approximately 417°C, a face-centered cubic (fee) aHotrope, the y (or P) form, having a lattice parameter a = 0.3544 nm, becomes the stable crystalline form. The mechanism of the aHotropic transformation has been well described (5,10—12). Cobalt is magnetic up to 1123°C and at room temperature the magnetic moment is parallel to the ( -direction. Physical properties are Hsted in Table 2. 

Structural Properties at Low Temperatures It is most convenient to classify metals by their lattice symmetiy for low temperature mechanical properties considerations. The face-centered-cubic (fee) metals and their alloys are most often used in the construc tion of cryogenic equipment. Al, Cu Ni, their alloys, and the austenitic stainless steels of the 18-8 type are fee and do not exhibit an impact duc tile-to-brittle transition at low temperatures. As a general nile, the mechanical properties of these metals with the exception of 2024-T4 aluminum, improve as the temperature is reduced. Since annealing of these metals and alloys can affect both the ultimate and yield strengths, care must be exercised under these conditions. 

It is well known that the 0 of a metal depends on the surface crystallographic orientation.6,65,66 In particular, it is well established that 0 increases with the surface atomic density as a consequence of an increase in the surface potential M. More specifically, for metals crystallizing in the face-centered cubic (fee) system, 0 increases in the sequence (110) <(100) <(111) for those crystallizing in the body-centered cubic (bcc) system, in the sequence (111) < (100) <(110) and for the hexagonal close-packed (hep) system, (1120) < (1010) < (0001). 

The most important metals for catalysis are those of the groups VIII and I-B of the periodic system. Three crystal structures are important, face-centered cubic (fee Ni, Cu, Rh, Pd, Ag, Ir, Pt, Au), hexagonally dose-packed (hep Co, Ru, Os) and body-centered cubic (bcc Fe). Figure 5.1 shows the unit cell for each of these structures. Note that the unit cells contain 4, 2, and 6 atoms for the fee, bcc, and hep structure, respectively. Many other structures, however, exist when considering more complex materials such as oxides, sulfides etc, which we shall not treat here. Before discussing the surfaces that the metals expose, we mention a few general properties. 

Figure 5.1. Unit cells of the face-centered cubic (fee), body-centered cubic (bcc), and hexagonally closed packed (hep) lattices.

Nanoclusters tend to form facets in order to minimize surface energy. Since the surface energy of a face-centered cubic (fee) metal follows the order of (111)<(100) <(1 1 0), most nanoclusters are bounded by (1 1 1) and (1 00) surfaces, Figure 2a and b. In many cases, the ratio of the growth rate along the (100) versus (111)... 

We prepared thin film Pt alloy electrodes by Ar-sputtering Pt and the second metal targets simultaneously onto a disk substrate at room temperature (thickness approximately 200 nm). The resulting alloy composition was determined by gravimetry and X-ray fluorescent analysis (EDX). Grazing incidence (i7= 1°) X-ray diffraction patterns of these alloys indicated the formation of a solid solution with a face-centered cubic (fee) crystal stmeture. 

The term crystal structure in essence covers all of the descriptive information, such as the crystal system, the space lattice, the symmetry class, the space group and the lattice parameters pertaining to the crystal under reference. Most metals are found to have relatively simple crystal structures body centered cubic (bcc), face centered cubic (fee) and hexagonal close packed (eph) structures. The majority of the metals exhibit one of these three crystal structures at room temperature. However, some metals do exhibit more complex crystal structures. 

Consequently, we favor a method that shows a view of organic molecules on a surface drawn as close as possible to their relative sizes. Two such surfaces, a face centered cubic (fee) (111) and an fee (100), are shown in Fig. 1.9, in which M, 2M, and 3M represent plane, edge, and corner sites, respectively, according to the nomenclature invented by Siegel and associates.21 These are... 

FIGURE 1.9 (111) and (100) faces of a face-centered cubic (fee) regular cubooctrahedron... 

Fig. 1. Face-centered cubic (fee) octahedron, m = 9 (to is the number of atoms along an edge).

The XRD for the sample of Ni-metallized on graphite (4.6wt%) is provided in Fig. 2c, however the strongest peaks of Ni metal are overlapping with those peak reflections from the MAG-10 graphite. But, the X-ray diffraction in Fig. 2a revealed the Ni particles were pure nickel crystalline with a face-centered cubic (fee) structure. 

In addition to the two structures already discussed, another arrangement of atoms in a cubic unit cell is possible. Atoms of a metal are identical, so the ratio of atomic sizes is 1.000, which allows a coordination number of 12. One structure that has a coordination number of 12 is known as face-centered cubic fee), and it has one atom on each comer of the cube and an atom on each of the six faces of the cube. The atoms on the faces are shared by two cubes, so one-half of each atom belongs in each cube. With there... 

To illustrate this, take the situation in a very common and relatively simple metal structure, that of copper. A crystal of copper adopts the face-centered cubic (fee) structure (Fig. 2.8). In all crystals with this structure slip takes place on one of the equivalent 111 planes, in one of the compatible <110> directions. The shortest vector describing this runs from an atom at the comer of the unit cell to one at a face center (Fig. 3.10). A dislocation having Burgers vector equal to this displacement, i <110>, is thus a unit dislocation in the structure. 

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