Crystalline solid body-centered cubic

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. 

The alkali metals barium (Ba) and uranium (U) are atomic crystalline solids. Their atoms pack in a body-centered cubic arrangement. 

Use nine 2-in.-diameter balls and eight toothpicks to form three layers of a body-centered cubic crystal, as illustrated in Figure 4.6 (the top layer is the same as the bottom layer). Put the layers together like a sandwich. Draw a picture of this atomic crystalline solid. 

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. 

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. 

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. 

Section 11.7 In a crystalline soUd, particles are arranged in a regularly repeating pattern. An amorphous solid is one whose particles show no such order. The essential structural features of a crystalline solid can be represented by its unit cell, toe smallest part of toe crystal that can, by simple displacement, reproduce the three-dimensional structure. The three-dimensional structures of a crystal can also be represented by its crystal lattice. The points in a crystal lattice represent positions in toe structure where there are identical environments. The simplest unit cells are cubic. There are three kinds of cubic unit cells primitive cubic, body-centered cubic, and face-centered cubic. 

Solids can be crystalline or amorphous. A crystalline solid has an ordered arrangement of structural units placed at crystal lattice points. We may think of a crystal as constructed from unit celb. Cubic unit cells are of three kinds simple cubic, body-centered cubic, and face-centered cubic. One of the most important ways of determining the structure of a crystalline solid is by x-ray diffraction. 

Refer to Figure 22.16, which illustrates a surface of a body-centered cubic crystalline solid. What are the Miller indices of the surface indicated ... 

Chromium crystallizes with a body-centered cubic unit cell. The radius of a chromium atom is 125 pm. Calculate the density of solid crystalline chromium in g/cm. ... 

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. 

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