Bcc structures

The BCC structure is illustrated in figure Al.3,3. Elements such as sodium, tungsten and iron fonn in the BCC structure. The conventional unit cell of the BCC structure is cubic, like FCC, with the length of the edge given by the lattice parameter, a. There are two atoms in the conventional cell. In the primitive unit cell, there is only one atom and the lattice vectors are given by... 

FIG. 5 A crystalline structure. The particles are located near preferred lattice sites. A body-centered cubic (bcc) structure is shown. 

Several polymorphs of Bi have been described but there is as yet no general agreement on their structures except for a-Bi (above) and -Bi which forms at 90 kbar and has a bcc structure with 8 nearest neighbours at 329.1 pm. 

The elements all have typically metallic bcc structures and in the massive state are lustrous, silvery, and (when pure) fairly soft. However, the most obvious characteristic at least of... 

In the solid state all three elements have typically metallic structures. Technetium and Re are isostructural with hep lattices, but there are 4 allotropes of Mn of which the o-fomi is the one stable at room temperature. This has a bcc structure in which, for reasons which are not clear, there are 4 distinct types of Mn atom. It is hard and brittle, and noticeably less refractory than its predecessors in the first transition series. 

In three dimensions, Ohta and Kurokawa [32] reported that a BCC arrangement was only slightly more favored than the FCC arrangement. In fact, many BCC structures have been reported for AB type block copolymers and the blends of homopolymer-block copolymer systems [27,33-35]. However, the lattice structure of the core-shell type polymer microspheres was FCC. This FCC formation resulted in the lower viscosity of... 

In contrast to this the austenite - martensite transition temperature depends on the concentration of vacancies. The configurations A and E show a transition temperature of 10 K, while in C and D the bcc structure already occurs at 100 K. 

We found that without any exception in all of our simulations Bain s lattice correspondence actually applies, i.e. one set of (110) planes of the bcc structure corresponds to a set of fee (111) planes, while the bcc [001] direction lying in these planes is transformed into the [110] direction of the fee phase. Moreover, these directions are exactly parallel to each other. This would correspond to a Nishiyama-Wassermann orientational relationship if the (110) and (111) planes would also be parallel to each other. But this is not the case. These planes are rotated around [001] by an angle between 0 and 9 during the transformation. This angle differs between the simulations in a non-systematic way. 

Recent papers [4-6] of the NRL group have concentrated on a tight-binding methodology that simultaneously fits the energy bands and the total ener — of the fee and bcc structures as a function of volume, and correctly predicts the ground state for those metals that crystallize in the hep or even the Of-Mn structure. 

Hydrogen has a very low solubility in the iron lattice, which makes direct observation of the location of the hydrogen atom in the lattice very difficult. The hydrogen definitely occupies an interstitial site in the bcc iron lattice. Two such sites are normally associated with interstitial solutes in bcc structures, the tetrahedral and the octahedral sites (see Fig. 8.39). Indirect evidence suggests that hydrogen occupies the tetrahedral site. 

FIGURE 5.32 The body-centered cubic (bcc) structure. This structure is not packed as closely as the others that we have illustrated. It is less common among metals than the close-packed structures. Some ionic structures are based on this model. 

Iron crystallizes in a bcc structure. The atomic radius of iron is 124 pm. Determine (a) the number of atoms per unit cell (b) the coordination number of the lattice (c) the length of the side of the unit cell. 

Calculate the density of each of the following metals from the data given (a) nickel, fee structure, atomic radius 125 pm (b) rubidium, bcc structure, atomic radius 250 pm. 

Metals with bcc structures, such as tungsten, are not close packed. Therefore, their densities would be greater if they were to change to a ccp structure (under pressure, for instance). 

Iron corrodes in the presence of oxygen to form rust, which for simplicity can be taken to be iron(lll) oxide. If a cubic block of iron of side 1.5 cm reacts with 15.5 L of oxygen at 1.00 atm and 25°C, what is the maximum mass of iron(III) oxide that can be produced Iron metal has a bcc structure, and the atomic radius of iron is 124 pm. The reaction takes place at 298 K and 1.00 atm. 

Calcium is miscible with Sr in the liquid and in all the solid bcc, hep and fee allotropic forms (Fig. 1). Barium exhibits no hep or fee forms, however, so that solid solubility between the close-packed structures of Ca and Sr, and the bcc structure of Ba is restricted in the Ca-Ba and also in the Sr-Ba systems. A continuous series of solid solutions is only achieved in Ca-Ba and Sr-Ba for the high-T bcc modifications. In Ca-Ba, the solid solutions are separated by a narrow heterogeneous field between 32 and 36 mol% Ba in Sr-Ba this occurs between 24 and 30 mol% Ba (Fig. 1). 

The tetragonal distortion of the ccp structure also leads to the bcc structure. 

In the tetragonal transformation, the bcc structure is not the local maximum of the D parameter. Although the bcc stracture has a higher density if it is compressed along the tetragonal axis, many elements take this structure. On the other hand, the bet structure is the local maximum of the D parameter in the tetragonal distortion but no metal takes this structure. These results indicate that density is not an important factor for the choice of crystal stracture. 

Figure 57.15. The structural correlation between the trigonal Ni2Al3 (the thick outline) and the bcc structure of the disordered Ni-Al phase. The large circles are Ni, the medium ones are A1 and the small ones are voids (11).

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