Hexagonal closed packed

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 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. 

Crystalline copper and magnesium have face-centred-cubic and close-packed-hexagonal structures respectively. 

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. 

Figure 26. Constant current mode STM image of isolated (A), self-organized in close-packed hexagonal network (C) and in fee structure (E) of silver nanoclusters deposited on Au(l 11) substrate (scan size (A) 17.1 x 17.1 nm, f/t=—IV, /t=ltiA, (C) 136 X 136 nm, f/t = — 2.5 V, /t = 0.8 tiA, (E) 143 x 143 nm, = —2.2 V, /, = 0.72 nA). I U) curves and their derivatives in the inserts of isolated (B), self-organized in close-packed hexagonal network (D) and in fee structure (F) of silver nanoclusters deposited on Au(l 11) substrate. (Reprinted with permission from Ref. [58], 2000, Wiley-VCH.)...

A simple structure which can be described in terms of superposition of (even if far away, not close-packed) hexagonal layers is that of graphite Chyt C%u The hexagonal net is also called graphitic net. 

Their normal crystal structure, at ambient conditions, corresponds to the body-centred cubic cI2-W-type structure. At very low temperatures, the close-packed hexagonal hP2-Mg-type structure has been observed for Li and Na, while for Rb and Cs the face-centred cubic close-packed cF4-Cu-type structure is known at high pressure. No polymorphic transformation has been reported for potassium. 

TiAl (also called Ti-Al a2 phase) has a close-packed hexagonal structure (hP8-Ni3Sn-type DO19). It has a specific modulus and stress rupture resistance comparable to that of super-alloys but (as with several other intermetallics) the complete absence of room-temperature plasticity poses severe mechanical-structural problems. Significant improvements have been obtained by the addition of Nb (typically in the range 10-12 at.%) or ofNb + V + Mo. 

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. 

Occurs as a close-packed hexagonal alpha-form and a hody-centered cubic beta modification melting point 2,233°C vaporizes at 4,602°C electrical resistivity 35.5 microhm-cm at 20°C magnetic susceptibility 0.42xlCL6 emu/g at 25°C thermal neutron absorption cross section 105 barns/atom work function 3.5 eV modulus of elasticity 20x10 psi tensile strength 58,000 psi at 25°C insoluble in water, dilute mineral acids and nitric acid at all concentrations soluble in hydrofluoric acid, concentrated sulfuric acid and aqua regia. 

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. 

Titanium has two allotropic modifications (1) alpha form and (2) beta modification. The alpha form has a close-packed hexagonal crystal structure density 4.54 g/cm3 at 20°C and stable up to 882°C. It converts very slowly to a body-centered cubic beta form at 882°C. The density of the beta form is 4.40 g/cm3 at 900°C (estimated). The other physical properties are as follows ... 

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-... 

On Cu(lll) different structures were proposed. The bulk deposited cadmium forms a close-packed hexagonal lattice in perchlorate solutions, growing according to modified Stranski-Krastanov mechanism [292]. 

Transmission electron microscopy (TEM) has been an underutilized yet valuable too in particle size characterization of MC particles in LB films. Monolayer films of trioctylphosphine oxide-capped CdSe (18), spread as a monolayer on an aqueous subphase, were transferred to a TEM grid. A close-packed hexagonal arrangement of 5.3-nm (cr —4%) crystallites was found. TEM images were also obtained for HMP-stabilized CdS incorporated in BeH/octadecylamine films (79) and for CdS formed under an amine-based surfactant monolayer and transferred to a TEM grid (14). In one study, direct viewing of CdS and CdSe particles made from Cd2+-FA films on TEM grids was not possible due to poor phase contrast between the particles and the film (30). Diffraction patterns were observed, however, that were consistent with crystalline (3-CdS or CdSe. Approximately spherical particles of CdSe could... 

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