Close packed metal systems

In close packed metal systems there are two sites available, see Fig 6.18 the constricted tetrahedral sites or the more spacious octahedral sites. However, even after only partly filling the lowest energy sites, the... 

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). 

It is not possible to establish directly the value of the M-M distance corresponding to bond order of 1. Methods so easily applied to organic systems cannot be so readily applied here. First, the metallic radius for 12-coordinate metal is an average value, and second, as mentioned, the M-M distance (average) established for close-packed metals generally corresponds to a bond-order value of less than 1. At best only, the distance taken to correspond to a bond order of 1 is a crude approximation. Clearly, such arguments are enforced in any attempt to establish which correspond to bond orders of 2 or more. 

Grayish metal hexagonal close-packed crystal system, lattice constant, a=2.286 A and c=3.584 A density 1.85 g/cm permeable to x-rays highly ductile modulus to weight ratio very high, elastic modulus 44.5 x 10 at 25°C (for hot-pressed block and sheet) melting point 1,287°C vaporizes at 2,471°C sound transmission velocity 12,600 m/sec reflectivity (white hght) 55% thermal neutron absorption cross-section 0.0090 barns/atom electrode potential, Be/Be2+(aq) 1.85 V electrical resistivity 3.36 x 10-i° ohm.m (at 20°C). 

Bluish-white lustrous soft metal closed-packed hexagonal system density 8.69 g/cm3 Brinnel hardness 21 melts at 321.1°C vaporizes at 767°C vapor pressure 5 torr at 455°C electrical resistivity 6.8 microhm-cm at 0°C insoluble in water. 

Silvery white metal soft and malleable hexagonal closed pack crystal system transforms to face-centered cubic crystals at 310°C which further transforms to a body-centered cubic allotropic modification at 868°C density 6.166 g/cm3 Brinnel hardness (as cast) 37 melts at 918°C vaporizes at 3,464°C vapor pressure 1 torr at 2,192°C electrical resistivity 56.8 x 10 ohm-cm at 25°C Young s modulus 3.84 x lO- dynes/cm Poisson s ratio 0.288 thermal neutron cross section 8.9 bams. 

A Model System for Hydrogen Interaction with a Close Packed Metal Lattice... 

Of the 12 slip systems possessed by the CCP stmcture, five are independent, which satisfies the von Mises criterion. For this reason, and because of the multitude of active slip systems in polycrystalline CCP metals, they are the most ductile. Hexagonal close-packed metals contain just one close-packed layer, the (0 0 0 1) basal plane, and three distinct close-packed directions in this plane [I I 2 0], [2 I I 0], [I 2 I 0] as shown in Figure lO.Vh. Thus, there are only three easy glide primary slip systems in HCP metals, and only two of these are independent. Hence, HCP metals tend to have low... 

In practice, asymmetric O Is peaks are often found for oxide systems, and these find their explanation in the coexistence of a surface spectrum and a bulk spectrum. The contribution of the surface spectrum for polar systems and in the presence of water strongly bound via hydrogen bridges (and thus not pumped away in a normal XPS system without thermal desorption) may then be larger than the estimated 15% and could amount to a clearly visible structure. The contribution of the -yl species, however, is frequently not easily detectable, as its abundance is strictly limited to a maximum of one monolayer, being sub-stoichiometric with respect to a close packed metal layer as it saturates two dangling coordinations per oxygen atom. 

In the original implementation of the embedded-atom method, the electron densities contributed by each atom were assumed to be spherically symmetric. This led to generally reasonable results for close-packed metals. There have also been attempts to extend this method to other systems such as silicon without introducing one-electron terms by incorporating an angular dependence into the electron densities p(r) contributed by the atoms. Whereas this... 

The structural representations shown in Fig. 3.2 are useful for visualizing the slip-planes that may exist in various metal crystal systems. For example, from the face-centered cubic (fee) lattice shown in Fig. 3.2a, it is easy to see how planes of atoms might slide rather easily over one another from the upper left to the lower right. This lattice also has several other such slip planes. While it is not readily apparent from the figure, a three-dimensional model of these crystal systems would show that the body-centered cubic (bcc) lattice offers the least number of slip planes of the three shown, and the hexagonal-close-packed (hep) system falls in between the fee and bcc systems. 

Since the number of slip systems is not usually a function of temperature, the ductility of face-centered cubic metals is relatively insensitive to a decrease in temperature. Metals of other crystal lattice types tend to become brittle at low temperatures. Crystal structure and ductility are related because the face-centered cubic lattice has more slip systems than the other crystal structures. In addition, the slip planes of body-centered cubic and hexagonal close-packed crystals tend to change at low temperature, which is not the case for face-centered cubic metals. Therefore, copper, nickel, all of the copper-nickel alloys, aluminum and its alloys, and the austenitic stainless steels that contain more than approximately 7% nickel, all face-centered cubic, remain ductile down to the low temperatures, if they are ductile at room temperature. Iron, carbon and low-alloy steels, molybdenum, and niobium, all body-centered cubic, become brittle at low temperatures. The hexagonal close-packed metals occupy an intermediate place between fee and bcc behavior. Zinc undergoes a transition to brittle behavior in tension, zirconium and pure titanium remain ductile. 

Slip Systems for Face-Centered Cubic, Body-Centered Cubic, and Hexagonal Close-Packed Metals... 

At potentials positive to the bulk metal deposition, a metal monolayer-or in some cases a bilayer-of one metal can be electrodeposited on another metal surface this phenomenon is referred to as underiDotential deposition (upd) in the literature. Many investigations of several different metal adsorbate/substrate systems have been published to date. In general, two different classes of surface stmetures can be classified (a) simple superstmetures with small packing densities and (b) close-packed (bulklike) or even compressed stmetures, which are observed for deposition of the heavy metal ions Tl, Hg and Pb on Ag, Au, Cu or Pt (see, e.g., [63, 64, 65, 66, 62, 68, 69 and 70]). In case (a), the metal adsorbate is very often stabilized by coadsorbed anions typical representatives of this type are Cu/Au (111) (e.g. [44, 45, 21, 22 and 25]) or Cu/Pt(l 11) (e.g. [46, 74, 75, and 26 ]) It has to be mentioned that the two dimensional ordering of the Cu adatoms is significantly affected by the presence of coadsorbed anions, for example, for the upd of Cu on Au(l 11), the onset of underiDotential deposition shifts to more positive potentials from 80"to Br and CE [72]. 

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