Packing Structures and Unit Cells
Let's take a closer look at the ababab... stacking pattern, and focus on a cluster of atoms in this pattern (Figure 11.6). The cluster forms a hexagonal (six-sided) prism of closely packed spheres. In fact, they are as tightly packed as they can be, so the structure is called Hexagonal closest-packed (hcp) The atoms in 16 metallic elements (see Figure 11.7) have hcp structures. Within these structures the 15-atom cluster in figure 11.6(c) serves as an atomic-scale building block--a pattern of atoms repeated over and over again in all three dimensions inside the solid.