Metals close-packed hexagonal

Table VII.—Interatomic Distances and Atomic1 Diameters for some Close-Packed Hexagonal Metals...

The Co-doped close-packed hexagonal metals Y, Hf, Lu, Ti, Zr, Tl, Re, Zn, and Cd all show a quadrupole splitting in the Mossbauer resonance which correlates to the axial ratio c/a of the host metal [78]. 

Table 2.3 Percentage interlayer relaxations, Ad. for several (unreconstructed) close-packed hexagonal metal surfaces, as obtained from DFT (LDA and PBE) calculations and LEED analyses. All data is from Ref [67],...

Properties. Thallium is grayish white, heavy, and soft. When freshly cut, it has a metallic luster that quickly dulls to a bluish gray tinge like that of lead. A heavy oxide cmst forms on the metal surface when in contact with air for several days. The metal has a close-packed hexagonal lattice below 230°C, at which point it is transformed to a body-centered cubic lattice. At high pressures, thallium transforms to a face-centered cubic form. The triple point between the three phases is at 110°C and 3000 MPa (30 kbar). The physical properties of thallium are summarized in Table 1. 

A summary of physical and chemical constants for beryUium is compUed ia Table 1 (3—7). One of the more important characteristics of beryUium is its pronounced anisotropy resulting from the close-packed hexagonal crystal stmcture. This factor must be considered for any property that is known or suspected to be stmcture sensitive. As an example, the thermal expansion coefficient at 273 K of siagle-crystal beryUium was measured (8) as 10.6 x 10 paraUel to the i -axis and 7.7 x 10 paraUel to the i -axis. The actual expansion of polycrystalline metal then becomes a function of the degree of preferred orientation present and the direction of measurement ia wrought beryUium. 

The hexagonal-close-packed (hep) metals generally exhibit mechanical properties intermediate between those of the fee and bcc metals. For example Zn encounters a ductile-to-brittle transition whereas Zr and pure Ti do not. The latter and their alloys with a hep structure remain reasonably ductile at low temperatures and have been used for many applications where weight reduction and reduced heat leakage through the material have been important. However, small impurities of O, N, H, and C can have a detrimental effect on the low temperature ductihty properties of Ti and its alloys. 

We begin by looking at the smallest scale of controllable structural feature - the way in which the atoms in the metals are packed together to give either a crystalline or a glassy (amorphous) structure. Table 2.2 lists the crystal structures of the pure metals at room temperature. In nearly every case the metal atoms pack into the simple crystal structures of face-centred cubic (f.c.c.), body-centred cubic (b.c.c.) or close-packed hexagonal (c.p.h.). 

The electron transport properties described earlier markedly differ when the particles are organized on the substrate. When particles are isolated on the substrate, the well-known Coulomb blockade behavior is observed. When particles are arranged in a close-packed hexagonal network, the electron tunneling transport between two adjacent particles competes with that of particle-substrate. This is enhanced when the number of layers made of particles increases and they form a FCC structure. Then ohmic behavior dominates, with the number of neighbor particles increasing. In the FCC structure, a direct electron tunneling process from the tip to the substrate occurs via an electrical percolation process. Hence a micro-crystal made of nanoparticles acts as a metal. 

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. 

Soft, lustrous metal silver-like appearance close-packed hexagonal crystal system density 8.78 g/cm paramagnetic magnetic moment 11.2 Bohr magnetons melts at 1,472°C vaporizes at 2,694°C electrical resistivity 195 microhm-cm at 25°C Young s modulus 6.71xl0n dynes/cm2 Poisson s ratio 0.255 thermal neutron cross section 64 barns insoluble in water soluble in acids (with reactions). 

Silvery-white metal close-packed hexagonal structure density 1.74 g/cm at 20°C, 1.57 g/cm3 at 650 C (hquid melt) melts at 650 C vaporizes at 1,090°C vapor pressure 5 torr at 678 C and 20 torr at 763 C electrical resistivity 4.46 microhm-cm at 20 C, 28.0 microhm-cm at 650 C (hquid melt) surface tension 563 dynes/cm at 681 C modulus of elasticity 6.5x10 Ib/sq in Poisson s ratio 0.35 thermal neutron absorption cross section 0.059 bam soluble in dilute acids. 

Silvery gray lustrous metal or bluish black amorphous powder close-packed hexagonal lattice transforms to a body-centered cubic structure at 865°C density 6.506 g/cm melts at about 1,852°C vaporizes at 4,377°C elec-... 

The chemisorption case is exemplified by oxygen and sulfur on metals, the physisorption case by krypton and xenon on metals and graphite. Intermediate cases do exist for example, undissociated CO on metals is not physisorbed but chemisorbed and nevertheless it seems in many cases to be able to produce close-packed hexagonal overlayers. Also, some metal surfaces [for example, Pt(lOO), Ir(lOO), Au(100)] reconstruct into different lattices, exhibiting the effect of adsorbate-adsorbate interactions (here the adsorbate is just another metal atom of the same species as in the substrate). 

The density of beryllium is 1.847 g/cm3 based upon average values of lattice parameters at 255C (a — 22.856 nm and c — 35.832 nm). Beryllium products generally have a density around 1.850 g/cm3 or higher because of impurities, such as aluminum and other metals, and beryllium oxide. The crystal structure is close-packed hexagonal. The alpha-form of beryllium transforms to a body-centered cubic structure at a temperature very close to the melung point. 

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