Hexagonal close packed structure

Beryllium is a light metal (s.g. 1 -85) with a hexagonal close-packed structure (axial ratio 1 568). The most notable of its mechanical properties is its low ductility at room temperature. Deformation at room temperature is restricted to slip on the basal plane, which takes place only to a very limited extent. Consequently, at room temperature beryllium is by normal standards a brittle metal, exhibiting only about 2 to 4% tensile elongation. Mechanical deformation increases this by the development of preferred orientation, but only in the direction of working and at the expense of ductility in other directions. Ductility also increases very markedly at temperatures above about 300°C with alternative slip on the 1010 prismatic planes. In consequence, all mechanical working of beryllium is carried out at elevated temperatures. It has not yet been resolved whether the brittleness of beryllium is fundamental or results from small amounts of impurities. Beryllium is a very poor solvent for other metals and, to date, it has not been possible to overcome the brittleness problem by alloying. 

A freshly prepared flame-annealed Au(100) surface has been found to be reconstmcted188,487,534,538 and the surface atoms exhibit a hexagonal close-packed structure to yield the (hex)-stmcture. One-directional long-range corrugation of 1.45 nm periodicity and 0.05 nm height has been found on the Au( 100) surface.188,488 When the reconstruction is lifted due to specific adsorption of SO - anions at more positive , the surface changes to a (1 x 1) structure.538... 

The differing malleabilities of metals can be traced to their crystal structures. The crystal structure of a metal typically has slip planes, which are planes of atoms that under stress may slip or slide relative to one another. The slip planes of a ccp structure are the close-packed planes, and careful inspection of a unit cell shows that there are eight sets of slip planes in different directions. As a result, metals with cubic close-packed structures, such as copper, are malleable they can be easily bent, flattened, or pounded into shape. In contrast, a hexagonal close-packed structure has only one set of slip planes, and metals with hexagonal close packing, such as zinc or cadmium, tend to be relatively brittle. 

Side and expanded views of hexagonal and cubic close-packed crystal types. In the hexagonal close-packed structure, spheres on both sides of any plane are in the same positions, and the third layer is directly above the first. In the cubic close-packed structure, layers take up three different positions, and the fourth layer is directly above the first. 

In either of these close-packed structures, each sphere has 12 nearest neighbors 6 in the same plane, 3 in the dimples above, and 3 in the dimples below. The expanded views in Figure 11-30 show the different arrangements of the hexagonal and cubic close-packed crystalline types, hi the hexagonal close-packed structure, notice that the third layer lies directly above the first, the fourth above the second, and so on. The layers can be labeled ABAB. 

Moreover, it was found that incorporation of nanoparticles about 8 nm in diameter in a-Si H led to improved properties, the most important one being enhanced stability against light soaking and thermal annealing [387]. A later study revealed a typical crystallite size of 2-3 nm. with a hexagonal close-packed structure [388]. Diamond structures can also be observed [389]. Hence the name polymorphous silicon is justified. 

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,...

Interstitial atoms in clusters. As the size of clusters increases (and also that of their central cavity) the insertion of atoms becomes easier and easier. In particular for 12-atom clusters having a cubo-octahedral structure, the insertion of an atom having the same radius as that of the peripheral atoms is possible. Notice that this arrangement can be compared with those of the metallic cubic and hexagonal, close-packed structures. 

Titanium, zirconium and hafnium in normal conditions crystallize in the hexagonal close-packed structure (a modification) with a c/a slightly smaller than the ideal one c/a = 1.587 (Ti), 1.593 (Zr) and 1.581 (Hf). At high temperature they have the bcc W-type structure ((3 modification). High-pressure transformations are known (Tables 5.21-5.23). 

Physical properties of the element are anticipated or calculated. Sdvery metal having two aUotropic forms (i) alpha form that should have a double hexagonal closed-packed structure and (ii) a face-centered cubic type beta form density 14.78 g/cm (alpha form), and 13.25 g/cm (beta form) melting point 985°C soluble in dilute mineral acids. 

Silvery-white metal hexagonal close-packed structure density 9.84 g/cm melts at 1,663°C vaporizes at 3,402°C electrical resistivity 59 microhm-cm slightly paramagnetic thermal neutron cross section 108 barns soluble in acids. 

Pale yellow metal attains a green oxide coating on exposure to air exhibits two crystalline modifications (1) an alpha form, that has a hexagonal close-packed structure, a density of 6.773 g/cm and a molar volume 20.82 cc/mol, and (2) a beta form that has an open body-centered cubic structure having a density of 6.64 g/cm and a molar volume of 21.20 cc/mol. The alpha form transforms to beta at 792°C. 

Silvery white metal soft and hght turns shghtly yeUow when exposed to air density 2.99 g/cm exhibits two aUotropic modifications a hexagonal close-packed structure stable up to 1,335° transforms to body-centered cubic form above 1,335°C, having a density 3.19 g/cm melts at 1,541°C vaporizes at 2,831°C electrical resistivity 56.2x10 ohm-cm thermal neutron absorption cross section 24 1 bams decomposes in water. 

Silvery-gray metal slowly tarnishes in moist air crystallizes in hexagonal close-packed structure density 11.49 g/cm (calculated) melts at 2,172°C vaporizes at 4,265°C Young s (elastic) modulus 3.76 x 10 kg/cm Poisson s ratio 0.293 thermal neutron absorption cross-section 22 barns superconductor below 11°K insoluble in water and hydrochloric acid dissolves in nitric acid, concentrated sulfuric acid and aqua regia. 

Silvery-white lustrous metal hexagonal close-packed structure density 9.321 g/cm3 at 25° melts at 1,545°C vaporizes at 1,947°C electrical resistivity 79 microhm-cm compressibility 2.6x10 cm /kg effective magnetic moment 7.62 Bohr magneton insoluble in water dissolves in concentrated acids. 

Bluish-white lustrous metal brittle at room temperature malleable between 100 to 150°C hexagonal close-packed structure density 7.14 g/cm melts at 419.6°C vaporizes at 907°C vapor pressure 1 torr at 487°C, 5 torr at 558°C and 60 torr at 700°C good conductor of electricity, electrical resistivity 5.46 microhm-cm at 0°C and 6.01 microhm-cm at 25°C surface tension 768 dynes/cm at 600°C viscosity 3.17 and 2.24 centipoise at 450 and 600°C, respectively diamagnetic magnetic susceptibility 0.139x10 cgs units in polycrystalline form thermal neutron absorption cross-section 1.1 barns. 

The host crystal of chrysoberyl has a hexagonal-close-packed structure. The space group is orthorhombic Pnma with four molecules per unit cell. The AP ions are octahedrally coordinated by the oxygen ions and occur in two not equivalent crystal field sites in the lattice. The AP" sites lying in the mirror-... 

Azzaroni etal. [163] have used STM to study electrochemical reactivity of thiourea toward Au(lll). Sequential STM imaging has shown that thiourea adsorbs as striped arrays that evolve to the hexagonal close-packed structure when surface charge density is decreased. The transient hep structure undergoes electrooxidation to formamidine disulfide, which slowly yields adsorbed sulfur. Adsorption of thiourea on the pc-Au electrode from KCIO4 solutions has also been studied [164]. The film pressure and the Gibbs surface... 

Reconstruction of Au(lOO) surface has been reported by Kolb and Schneider [341]. The Au(lOO) surface reconstructs to the hexagonal close-packed structure... 

In our study, we utilize a pure vibrational excited state t = 1 as the target of coherent control. The Hamiltonian describing the vibrational subspace of a para-hydrogen crystal with hexagonal close packed structure is given as... 

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