Abc arrangement

The aba arrangement has the hexagonal unit cell shown in Fig. 16.14, and the resulting structure is called the hexagonal closest packed (hep) structure. The abc arrangement has a face-centered cubic unit cell, as shown in Fig. 16.15 on page 779, and the resulting structure is called the cubic closest packed (ccp) structure. Note that in the hep structure the spheres in every other layer occupy the same vertical position (ababab. . . ), while in the ccp structure the spheres in every fourth layer occupy the same vertical position (abcabca. . . ). A characteristic of both structures is that each sphere has 12 equivalent nearest... 

When spheres are packed in the abc arrangement, the unit cell is face-centered cubic To make the cubic arrangement easier lo see, the vertical axis has been tilted as shown. 

A metallic crystal can be pictured as containing spherical atoms packed together and bonded to each other equally in all directions. We can model such a structure by packing uniform, hard spheres in a manner that most efficiently uses the available space. Such an arrangement is called closest packing. The spheres are packed in layers, as shown in Fig. 10.13, in which each sphere is surrounded by six others. In the second layer the spheres do not lie directly over those in the first layer. Instead, each one occupies an indentation (or dimple) formed by three spheres in the first layer. In the third layer the spheres can occupy the dimples of the second layer in two possible ways They can occupy positions so that each sphere in the third layer lies directly over a sphere in the first layer [the aba arrangement Fig. 10.13(a)], or they can occupy positions so that no sphere in the third layer lies over one in the first layer [the abc arrangement Fig. 10.13(b)]. 

The cubic symmetry of the ABC arrangement of spheres (cubic closest packing) is more difficult to visualize. A portion of four layers of the ABC arrangement is shown in Figure 33. 

The aba arrangement has the hexagonal unit cell shown in Fig. 10.14, and the resulting structure is called the hexagonal closest packed (hep) structure. The abc arrangement has a face-centered cubic unit cell, as shown in Fig. 10.15, and the resulting structure is... 

The atoms are arranged in close-packed layers stacked in the ABC sequence (see the triangular net stacking symbol). 

In P", (a), the oxide packing arrangement comprises close packed layers in a cubic close packed,. .. ABC. .., stacking sequence, which continues throughout the structure. The spinel blocks are four oxide layers thick and contain AP" ions (and stabilising Li, Mg ions) distributed over tetrahedral and octahedral sites, as in spinel, MgAl204. These sites... 

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