Crystalline solid face-centered cubic

Krypton is present in the air to the extent of about 1 ppm. The atmosphere of Mars has been found to contain 0.3 ppm of krypton. Solid krypton is a white crystalline substance with a face-centered cubic structure which is common to all the "rare gases."... 

Niobium metal absorbs nitrogen, similar to hydrogen, forming interstitial solid solution. The absorption occurs at 300°C and the solubility of nitrogen in the metal is directly proportional to the square root of the partial pressure of nitrogen. The reaction is exothermic and the composition of such interstitial solid solution varies with the temperature. When the metal is heated with nitrogen at temperatures between 700 to 1,100°C, the product is niobium nitride, Nb2N or (NbNo.s) [12033-43-1]. When heated with ammonia at these temperatures, niobium forms this nitride. Another niobium nitride exists, NbN [24621-21-4], with a face-centered cubic crystalline structure. 

Similarly, charged solid particles (such as latex spheres) —kinetically stable lyophobic colloids —may exist in colloidal crystalline phases (with body-centered or face-centered cubic structures) as a consequence of thermodynamically favored reduction in free energies (see Chapter 13). Even neutrally charged spherical particles ( hard spheres ) undergo a phase transition from a liquidlike isotropic structure to face-centered cubic crystalline structures due to entropic reasons. In this sense, the stability or instability is of thermodynamic origin. 

The SMA effect can be traced to properties of two crystalline phases, called martensite and austenite, that undergo facile solid-solid phase transition at temperature Tm (dependent on P and x). The low-temperature martensite form is of body-centered cubic crystalline symmetry, soft and easily deformable, whereas the high-temperature austenite form is of face-centered cubic symmetry, hard and immalleable. Despite their dissimilar mechanical properties, the two crystalline forms are of nearly equal density, so that passage from austenite to a twinned form of martensite occurs without perceptible change of shape or size in the macroscopic object. 

Multiple Melting Points A compound may have different crystal structures (i.e., solid phases). For example, carbon tetrachloride has three known solid phases at atmospheric pressure la (face-centered cubic), lb (rhombohedral), and II (monoclinic). Ia and lb melt at temperatures some 5K apart [3]. Multiple melting points have been reported for a large set of compounds, such as many of those listed in the Merck Index [4], Dearden and Rahman improved a structure-melting point correlation for substituted anilines by excluding two outliers on the ground that their Tm values were inadequate, due to different crystalline forms [5]. 

In 1991, scientists at AT T Bell Laboratories discovered a new class of high-temperature superconductors based on fullerene, the allotrope of carbon that contains Cgo molecules (Sections 10.10 and 19.6). Called "buckyballs," after the architect R. Buckminster Fuller, these soccer ball-shaped Cgo molecules react with potassium to give K3C6o- This stable crystalline solid contains a face-centered cubic array of buckyballs, with K+ ions in the cavities between the Cgo molecules (Figure 21.16). At room temperature, K3Q,o is a metallic conductor, but it becomes a superconductor at 18 K. The rubidium fulleride, Rb C o, and a rubidium— thallium-Cfio compound of unknown stoichiometry have higher Tc values of 30 K and 45M8 K, respectively. 

Copper (Cu), silver (Ag), and gold (Au) are also atomic crystalline solids. Their atoms pack in a face-centered cubic arrangement. Use 14 2-in.-diameter balls and 12 toothpicks to form three layers of a face-centered cubic crystal, as illustrated in Figure 4.7 (the top layer is the same as the bottom layer). Put the layers together like a sandwich. Draw a picture of this... 

A ceramic work of art is constructed by hand and formed into a predetermined shape. A crystalline solid grows to a assume a predetermined shape. Aluminum atoms always pack into a face-centered cubic unit. 

All silver crystals have the same geometric shape. Therefore, the crystalline shape of a metallic solid is a function of the size of the metal solid atoms and their electron configuration. Each metal has its own geometric crystalline shape. Aluminum atoms pack into a face-centered cubic cell. Iron s solid structure is body-centered cubic. 

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. 

Sodium chloride occurs as a white crystalline powder or colorless crystals it has a saline taste. The crystal lattice is a face-centered cubic structure. Solid sodium chloride contains no water of crystallization although, below 0°C, salt may crystallize as a dihydrate. 

The crystalline form of interest in Zr-based ceramic compounds is the cubic fluorite structure based on the mineral CaF2. In this structure, consisting of interpenetrating face-centered-cubic and simple cublic arrays of cations (Zr ) and anions (O ), respectively, oxygen ion conductivity is enhanced by replacing zirconium (Zr ) ions on the cation lattice with soluble dopant cations having a valence less than 4, typically divalent (Mg, Ca ) and trivalent (Y, Yb , Sc ) cations. These dopants, which are in solid solution, are incorporated into the zirconia structure by the following types of defect reaction ... 

For crystalline solids, the equilibrium interatomic distance, r0, can be estimated from knowledge of lattice site separation distances and is typically expressed as some fraction of the lattice parameter ac. Aluminum forms a face-centered-cubic (fee) lattice, with lattice parameter ac = 0.405 nm. Since the densest packing direction is along the face diagonal, i.e., along the (110) direction, the equilibrium interatomic distance in Al is 4la /2 = 0 29nm. We can also calculate the distance... 

Under the influence of such a potential, the spheres easily settle into a crystalline form when the temperature is lowered or the pressure is raised. The solid phase can be either a face-centered cubic (fee) lattice or a body-centered cubic (bee) lattice, depending on the detailed nature of the potential. In the case of argon, the crystal freezes into an fee lattice, while liquid sodium freezes into a bee lattice. The transformation of simple spherical molecules (such as argon and sodium) into the crystalline solid phase is now rather well understood, largely because of the extensive use of computer simulation studies, accompanied by theoretical analysis using methods of statistical mechanics. 

Properties of solids differ from those of fluids because in solids the motions of molecules are highly restricted. The molecules may be confined to periodic arrays, producing crystalline structures such as the face-centered cubic (fee) and body-centered cubic (bcc), or they may be periodic only in certain directions, producing layered or amorphous structures such as graphite. Besides equilibrium structures, many solids can exist for prolonged periods in metastable structures examples include glasses. 

Figure 15.1 shows the three ways the atoms of a crystalline solid can be arranged. As a molecule goes from a simple cubic structure to a face-centered cubic structure, the density increases. The less space between the atoms, the more tightly packed the entire molecule, and the harder and less flexible. Unlike amorphous solids, a lattice structure provides for predictable breaks along set lines. This is the reason why diamonds and gemstones can be cut into facets. The round, oval, pear, emerald cut, and diamond-shaped cuts used in jewelry can be cut by dilferent gem cutters all over the world due to their characteristic lattice structures. 

The reader would be familiar with the packing of atoms in crystalline solids to produce regular, repeating, three-dimensional patterns such as the simple cubic, body-centered cubic, face-centered cubic, and hexagonal close-packed structures. The packing density and coordination number of these crystal structures for a pure metal are listed in Table 6.2. 

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