A lot of space in crystals is empty and not occupied by lattice atoms(recall the definition of atomic packing factor). However, understanding interstitial sites in all the common lattice structures is an important step to understanding more complex crystal structures. Furthermore, interstitial impurity/alloying elements and self-interstitial atoms generally locate themselves in such sites. Figure 2.15a illustrates the octahedral position at the center of an FCC unit cell. Similar octahedral locations can be imagined at the center of each cube edge indicated by an “open” circle. However, to depict fully these octahedral positions just as shown in the center of the cube, one needs to construct addi­tional neighboring unit cells. Tetrahedral interstitial positions (indicated by the “open” circles) of the FCC unit cell are shown in Figure 2.15b. Note that all the tetrahedral positions are entirely within the unit cell unlike the octahedral positions. Figures 2.16 and 2.17 show similar octahedral and tetrahedral sites in both BCC and HCP unit cells.

Figure 2.16 BCC unit cell. (a) Octahedral positions. (b) Tetrahedral positions (the “filled” circles represent the lattice atoms and the “open” circles interstitial positions).

2.1.7