So far, we have noted FCC and HCP metals to be close-packed structures with the highest atomic packing factor (0.74), but we have not discussed how a close-packed structure results. Both FCC and HCP crystals can be constructed by stacking close — packed layers of atoms on top of one another. There are two ways the stacking can be achieved leading to the creation of two different stacking sequences and thus two different crystal structures. Figure 2.8a shows a close-packed plane A where six equal-sized spheres surround a central atom. To create a close-packed three-dimen­sional structure, more spheres need to be placed on top of the A layer to fill up the triangular cavities created by the first layer. There are two sets of cavities on the A layer, B and C positions, as shown in the Figure 2.8a. A second plane of atoms can be placed on top of either B or C position. If one assumes that the second atom layer is over B, there are two ways the third close-packed layer can be stacked either vertically above C positions or directly above A positions. This scheme leads to the possibility of two types of close-packed packing. An FCC crystal shows a stacking sequence of ABCABCABC … (Figure 2.8d). On the other hand, a HCP crystal shows a stacking sequence of ABABAB… as shown in Figure 2.9. The lower and upper basal plane layers constitute the A layer and middle layer forms the B layer.

ABAB… stacking sequence

3D projection

Top layer

Middle layer

Bottom layer

Figure 2.9 Stacking of close-packed planes resulting in a HCP structure.

Due to this kind of stacking, a set of planes known as close-packed planes in FCC and HCP crystals are the densest (i. e., the planar atom density is the greatest). We will discuss this further in a later section.

2.1.4