Body-centered cubic system

The nohle gases and most metals crystallize in either the hep or the ccp structure as would be expected for neutral atoms. The alkali metals, barium, and a few transition metals crystallize In the body-centered cubic system, though the reasons for this choice are unknown. 

Physical Properties. Hafnium is a hard, heavy, somewhat ductile metal having an appearance slightly darker than that of stainless steel. The color of hafnium sponge metal is a dull powder gray. Physical properties of hafnium are summarized in Table 1. These data are for commercially pure hafnium which may contain from 0.2 to 3% zirconium. Although a number of radioactive isotopes have been artificially produced, naturally occurring hafnium consists of six stable isotopes (Table 2). Hafnium crystallizes in a body-centered cubic system which transforms to a hexagonal close-packed system below 2033 K. 

Molybdenum hexafluoride [7783-77-9] MoF, is a volatile liquid at room temperature. It is very moisture sensitive, hydrolysing immediately upon contact with water to produce HF and molybdenum oxyfluorides. MoF should therefore be handled in a closed system or in a vacuum line located in a chemical hood. The crystals possess a body-centered cubic stmcture that changes to orthorhombic below —96° C (1,2). The known physical properties are Hsted in Table 1. 

Tin exists in two ahotropic forms white tin (P) and gray tin (a). White tin, the form which is most familiar, crystallizes in the body-centered tetragonal system. Gray tin has a diamond cubic stmcture and may be formed when very high purity tin is exposed to temperatures well below zero. The ahotropic transformation is retarded if the tin contains smah amounts of bismuth, antimony, or lead. The spontaneous appearance of gray tin is a rare occurrence because the initiation of transformation requires, in some cases, years of exposure at —40° C. Inoculation with a-tin particles accelerates the transformation. 

At high pressures, solid II can be converted (slowly) to solid III. Solid III has a body-centered cubic crystal structure. Line bd is the equilibrium line between solid II and solid III, while line be is the melting line for solid III.P A triple point is present between solid II, solid III, and liquid at point b. Two other triple points are present in this system, but they are at too low a pressure to show on the phase diagram. One involves solid II, liquid, and vapor while the other has solid I, solid II, and vapor in equilibrium. 

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). 

Let us first consider, as an example, the copper-zinc system of alloys.1 The ordinary yellow brass of commerce is restricted in composition to the first (copper-rich) phase of the system. This phase, which has the face-centered cubic structure characteristic of copper, is followed successively, as the zinc content is increased, by the /3-phase (body-centered cubic),... 

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. 

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. 

An A-B diblock copolymer is a polymer consisting of a sequence of A-type monomers chemically joined to a sequence of B-type monomers. Even a small amount of incompatibility (difference in interactions) between monomers A and monomers B can induce phase transitions. However, A-homopolymer and B-homopolymer are chemically joined in a diblock therefore a system of diblocks cannot undergo a macroscopic phase separation. Instead a number of order-disorder phase transitions take place in the system between the isotropic phase and spatially ordered phases in which A-rich and B-rich domains, of the size of a diblock copolymer, are periodically arranged in lamellar, hexagonal, body-centered cubic (bcc), and the double gyroid structures. The covalent bond joining the blocks rests at the interface between A-rich and B-rich domains. 

Although the bond-orientational metrics defined above have proven useful for identifying numerous space-filling crystalline morphologies43 like face-centered cubic, body-centered cubic, simple cubic, and hexagonally close-packed lattices, they are inadequate for detecting order in systems that organize... 

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. 

Fig. 4. EDS spectroscopy results of E-particles from a high bum-up LWR fuel superimposed on the isothermal section of ternary phase diagram from Kleykamp (1985) at 1700 CC. These analyses show that there is distinct heterogeneity in the composition of metallic particles in the fuels. Hence, spot analysis of an individual e-parlicle may not provide direct evidence of corrosion. The metallic system is dominated by the hexagonal close packing (e) that occupies most of the phase space. The tr-space and the body centered cubic

Phase analysis and texture of the metal particles. Iron powders are constituted of the a-Fe phase with a body-centered cubic (bcc) lattice, whereas Fe-Co powders appear as a mixture of three phases that are quite similar to those of pure metals (bcc for a-Fe and a mixture of hep and fee for cobalt) (6). In the Fe.Nil(m system, a single fee phase is observed over the whole available composition range U s 25) with a linear dependence of the lattice parameter versus z, which shows the existence of a fee solid solution as already evidenced for the Co.rNiu)o-. system (33). The XRD patterns of the Fe [CovNi(1()o -,v)] i - powders depend on the composition An fee phase is always observed either as a single phase or as the main phase a second hep phase with weak and broad lines appears for a cobalt content x > 35 a third body-centered cubic (hcc) phase can be evidenced when x > 80. 

Crystal Systems. The cubic crystal system is composed of three space lattices, or unit cells, one of which we have already studied simple cubic (SC), body-centered cubic (BCC), anA face-centered cubic (FCC). The conditions for a crystal to be considered part of the cubic system are that the lattice parameters be the same (so there is really only one lattice parameter, a) and that the interaxial angles all be 90°. 

This type of constraint will be absent in amorphous materials because any of the Nc components can be added (or removed) anywhere in the material without exchanging with any other components. The dNi will also be independent for interstitial solutes in crystalline materials that lie in the interstices between larger substitutional atoms, as, for example, carbon atoms in body-centered cubic (b.c.c.) Fe, as illustrated in Fig. 8.8. In such a system, carbon atoms can be added or removed independently in a dilute solution. 

The transition metal carbides do have a notable drawback relative to engineering applications low ductility at room temperature. Below 1070 K, these materials fail in a brittle manner, while above this temperature they become ductile and deform plastically on multiple slip systems much like fee (face-centered-cubic) metals. This transition from brittle to ductile behavior is analogous to that of bee (body-centered-cubic) metals such as iron, and arises from the combination of the bee metals strongly temperature-dependent yield stress (oy) and relatively temperature-insensitive fracture stress.1 Brittle fracture is promoted below the ductile-to-brittle transition temperature because the stress required to fracture is lower than that required to move dislocations, oy. The opposite is true, however, above the transition temperature. 

The Cu-Zn system (brass) is complex as shown in Figure 9.1. The a phase is a ccp solid solution of Zn in Cu. The (3-brass is body-centered cubic, the composition corresponding to CuZn. Each phase exists over a range of Cu/Zn ratios corresponding to a solid solution with Zn or Cu added to the compound. The y-brass, CupZns, has a complex cubic structure and e-brass, CuZn3, has an hep structure. Hume-Rothery found that many intermetallic compounds have structures similar to (3-, y-, and e-brass at the same electron-to-atom ratio as the corresponding brass compounds. Some examples of these so-called electron... 

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