Metal face-centered cubic structure

Structural Properties at Low Temperatures It is most convenient to classify metals by their lattice symmetiy for low temperature mechanical properties considerations. The face-centered-cubic (fee) metals and their alloys are most often used in the construc tion of cryogenic equipment. Al, Cu Ni, their alloys, and the austenitic stainless steels of the 18-8 type are fee and do not exhibit an impact duc tile-to-brittle transition at low temperatures. As a general nile, the mechanical properties of these metals with the exception of 2024-T4 aluminum, improve as the temperature is reduced. Since annealing of these metals and alloys can affect both the ultimate and yield strengths, care must be exercised under these conditions. 

The ruthenium-copper and osmium-copper systems represent extreme cases in view of the very limited miscibility of either ruthenium or osmium with copper. It may also be noted that the crystal structure of ruthenium or osmium is different from that of copper, the former metals possessing the hep structure and the latter the fee structure. A system which is less extreme in these respects is the rhodium-copper system, since the components both possess the face centered cubic structure and also exhibit at least some miscibility at conditions of interest in catalysis. Recent EXAFS results from our group on rhodium-copper clusters (14) are similar to the earlier results on ruthenium-copper ( ) and osmium-copper (12) clusters, in that the rhodium atoms are coordinated predominantly to other rhodium atoms while the copper atoms are coordinated extensively to both copper and rhodium atoms. Also, we conclude that the copper concentrates in the surface of rhodium-copper clusters, as in the case of the ruthenium-copper and osmium-copper clusters. 

Only large clusters usually adopt the face-centered cubic structure of metallic platinum. A novel cuboctahedral cluster [Pt15Hx(CO)8(PBut3)6] has been reported by Spencer et al.512 and the first octahedral cluster [Pt6(CO)6(/i-dppm)3]2+ was only reported recently.573... 

The overlap between 5- and p-bands also occurs for the alkali metals and for the monovalent noble metals copper, silver, and gold, which have face-centered cubic structures. The noble metals differ from the alkalis because of the filled d-shell just below the 5-shell in energy the d-band and the 5-band overlap in the solid. 

There are many metal alloys that contain interstitial atoms embedded in the metal structure. Traditionally, the interstitial alloys most studied are those of the transition metals with carbon and nitrogen, as the addition of these atoms to the crystal structure increases the hardness of the metal considerably. Steel remains the most important traditional interstitial alloy from a world perspective. It consists of carbon atoms distributed at random in interstitial sites within the face-centered cubic structure of iron to form the phase austenite, which exists over the composition range from pure iron to approximately 7 at % carbon. 

Copper is a face-centered cubic (fee) metal. Band structure calculations show the valence bands to be copper d bands and hybrid bands of sd, pd, and sp character. The hybridization is essential for the conductivity of copper, as some of the bands cross the Fermi surface and are thus only partially occupied (K. Schwarz, private communication). 

Silvery-white metal when freshly cut rapidly turns yellow on exposure to air forming a thin oxide coating face-centered cubic structure malleable, ductile, and somewhat softer than calcium density 2.64 g/cm melts at 777°C vaporizes at 1,382°C vapor pressure 5 torr at 847°C and 20 torr at 953°C electrical resistivity 23 microhm-cm at 20°C thermal neutron absorption cross section 1.21 barns reacts with water soluble in ethanol. Thermochemical Properties... 

Silvery lustrous metal soft, malleahle and ductile the metal exists in two allotropic forms an alpha form, which has a face-centered cubic structure and is stable at room temperature, and a beta form, a body-centered cubic modification that forms when the alpha form is heated to 798°C. Density of the alpha modification is 6.98 g/cm and that of beta form is 6.54 g/cm. Alpha phase exhibits metallic-type conductivity at ordinary temperatures and pressures, but becomes semi-conductive above 16,000 atm. At about 40,000 atm it again becomes metallic-type conductor. (In some texts, the term beta form refers to the alpha phase). 

Although the correlation between ionic porosity and diffusivity is imperfect, there is a rough trend that oxygen diffusivity in the minerals increases with increasing IP. The trend is useful in qualitative estimation of closure temperature (among other applications). Extending the relation to metallic systems, one prediction is that diffusion in face-centered cubic structure (25.95% free space) is slower that that in body-centered structure (31.98% free space) of the same metal composition. To avoid the issue of anisotropy, it would be worthwhile to reexamine the relations between diffusivity and ionic porosity using only isometric minerals. 

Beside dislocation density, dislocation orientation is the primary factor in determining the critical shear stress required for plastic deformation. Dislocations do not move with the same degree of ease in all crystallographic directions or in all crystallographic planes. There is usually a preferred direction for slip dislocation movement. The combination of slip direction and slip plane is called the slip system, and it depends on the crystal structure of the metal. The slip plane is usually that plane having the most dense atomic packing (cf. Section 1.1.1.2). In face-centered cubic structures, this plane is the (111) plane, and the slip direction is the [110] direction. Each slip plane may contain more than one possible slip direction, so several slip systems may exist for a particular crystal structure. Eor FCC, there are a total of 12 possible slip systems four different (111) planes and three independent [110] directions for each plane. The... 

A FIGURE 14.3 One plane of the structure of an interstitial metallic hydride. The metal atoms (larger spheres) have a face-centered cubic structure, and the hydrogen atoms (smaller spheres) occupy interstices (holes) between the metal atoms. 

Magnesia (MgO), used as an insulator for electrical heating devices, has a face-centered cubic structure like that of NaCl. Draw one unit cell of the structure of MgO, and explain why MgO is more brittle than magnesium metal. 

Strontium. Sr at no 38 at wt 87.62 valence 2 two important radioactive isotopes (out of 12), Sr-89 and Sr-90 four stable isotopes, 88 (82.56% abundance), 86 (9.86% abundance), 87 (7.02% abundance), 84 (0.56% abundance) silvery-white metal, face-centered cubic structure brief exposure to air results in the yel oxide mp 752°, 757°, 769° (separate values) bp 1366°, 1384°, 1390° (separate values) d 2.6g/cc. Sol in acids, ethanol and liq ammonia. CA Registry No [7440-24-6]. Occurs in nature as the sulfate celestine or the carbonate strontianite also found in small quantities associated with Ba, Ca, Pb or K minerals. Prepn is by a) electrolysis of molten Sr chloride in a graphite crucible with cooling of the upper cathodic space to isolate the Sr vapors, or b) thermal redd of the oxide... 

When we determined the crystalline structure of solids in Chapter 4, we noted that most transitional metals form crystals with atoms in a close-packed hexagonal structure, face-centered cubic structure, or body-centered cubic arrangement. In the body-centered cubic structure, the spheres take up almost as much space as in the close-packed hexagonal structure. Many of the metals used to make alloys used for jewelry, such as nickel, copper, zinc, silver, gold, platinum, and lead, have face-centered cubic crystalline structures. Perhaps their similar crystalline structures promote an ease in forming alloys. In sterling silver, an atom of copper can fit nicely beside an atom of silver in the crystalline structure. 

Metals with body-centered and face-centered cubic structures are more ductile than other metals. 

Calculate the Fermi wave number for C, Si, Ge, and Sn by minimizing the energy in Eq. (15-16) for both a face-centered cubic structure and a diamond structure. The values of /cp given for C, Si, and Ge in the Solid State Table are for an electron density corresponding to the diamond structure. That for Sn is for the metallic structure (not face-centered cubic but a structure of similar packing density). Compare the predicted ratio of kp values for the two structures in tin by noticing that the observed specific gravity is 5.75 in the diamond structure and 7.31 in the metallic white-tin structure. 

In all properties studied with pseudopotenlial theory, the first step is the evaluation of the structure factors. For simplicity, let us consider a metallic crystal with a single ion per primitive cell -either a body-centered or face-centered cubic structure. We must specify the ion positions in the presence of a lattice vibration, as we did in Section 9-D for covalent solids. There, however, we were able to work with the linear force equations and could give displacements in complex form. Here the energy must be computed, and that requires terms quadratic in the displacements. It is easier to keep everything straight if we specify displacements as real. Fora lattice vibration of wave number k, we write the displacement of the ion with equilibrium position r, as... 

Froyen and Harrison (1979) also used this procedure to obtain for a nearest-neighbor fit to the bands for a face-centered cubic structure. The four values (in comparison to the tetrahedral values) were —0.62 (— 1.39), 2.33 (1.88), 2.47 (3.24), and 0 (—0.93). These are major differences, but the nearest-neighbor LCAO theory of close-packed structures is of little use in any case. The tetrahedral parameters seem to be adequate for covalent and ionic systems but not to be relevant to the simple metals. 

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