Cubic closed-packed crystal structure

Austenitic stainless steels are a class of materials that are extremely relevant for conventional and advanced reactor technologies. They are Fe-Cr-Ni alloys with a fully or quasifuUy face-centered-cubic close-packed crystal structure which imparts most of their physical and mechanical properties [7]. Various chemical additions enhance their properties over a wide range of temperatures. Three main alloy classes are to be considered here 304, 316, and alloy 800 series. 

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. 

Cubed compound, in PVC siding manufacture, 25 685 Cube lattice, 8 114t Cubic boron nitride, 1 8 4 654 grinding wheels, 1 21 hardness in various scales, l 3t physical properties of, 4 653t Cubic close-packed (CCP) structure, of spinel ferrites, 11 60 Cubic ferrites, 11 55-57 Cubic geometry, for metal coordination numbers, 7 574, 575t. See also Cubic structure Cubic symmetry Cubic silsesquioxanes (CSS), 13 539 Cubic structure, of ferroelectric crystals, 11 94-95, 96 Cubic symmetry, 8 114t Cubitron sol-gel abrasives, 1 7 Cucurbituril inclusion compounds,... 

STRONTIUM. [CAS 7440-24-6], Chemical element, symbol Sr, at. no. 38, at. wt. 87.62. periodic table group 2, mp 769°C, bp 1384°C, density 2.54 g/cm- (20°C). Below 215°C, elemental strontium has a face-centered cubic crystal structure between 215-605 C. a hexagonal close-packed crystal structure and above 605°C, a body-cen tered cubic crystal structure. 

Woodcock, L., Entropy difference between the face-centred cubic and hexagonal close-packed crystal structures. Nature, 385, 141, 1997. 

The majority of new materials for SOFCs are perovskite stmctured oxides of general form ABO3.5 [23]. The ideal perovskite structure is a cubic close-packed ABO3 structure where the B-site cation sits within the octahedral interstices. Fig. 3.5. This stmcture is very flexible toward cation composition and tolerates large substitution fractions on either cation site. The Goldschmidt factor, a ratio of A, B, and O ionic radii, is often utilized to predict if a metal oxide will crystallize into the perovskite structure [24]. The A site of the commonly utilized perovskites is typically occupied by La, Ca, Sr, or Ba. The B site is typically a transition metal. Other stmctures investigated include double perovskites, apatites, and fluorites. 

Silicon carbide exists in a large number of structural forms called polytypes (more than 140), which represent modifications of hexagonal (wurtzite) and cubic (sphalerite) close-packed crystal structures. 

Describe how face-centered cubic and hexagonal close-packed crystal structures may be generated by the stacking of close-packed planes of atoms. 

Unalloyed (i.e., commercially pure) titanimn has a hexagonal close-packed crystal structure, sometimes denoted as the a phase at room temperature. At 883°C (1621 °F), the HCP material transforms into a body-centered cubic (or 0) phase. This transformation temperature is strongly influenced by the presence of alloying elements. For example, vanadium, niobium, and molybdemun decrease the a-to-jS transformation temperature and promote the formation of the p phase (i.e., are )3-phase stabOizers), which may exist at room temperatme. In addition, for some compositions, both a and p phases coexist. On the basis of which phase(s) is (are) present after processing, titanimn alloys fall into four classifications a, p, a + p, and near a. 

Figure 29.1 Crystal structures of ZnS. (a) Zinc blende, consisting of two, interpenetrating, cep lattices of Zn and S atoms displaced with respect to each other so that the atoms of each achieve 4-coordination (Zn-S = 235 pm) by occupying tetrahedral sites of the other lattice. The face-centred cube, characteristic of the cep lattice, can be seen — in this case composed of S atoms, but an extended diagram would reveal the same arrangement of Zn atoms. Note that if all the atoms of this structure were C, the structure would be that of diamond (p. 275). (b) Wurtzite. As with zinc blende, tetrahedral coordination of both Zn and S is achieved (Zn-S = 236 pm) but this time the interpenetrating lattices are hexagonal, rather than cubic, close-packed.

The surface of a single crystal of nickel, showing the regularity of its cubic close-packed structure. 

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. 

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