Final answer:
The tetrahedral interstitial site is a void between four densely packed atoms in an FCC crystal lattice. It allows for the insertion of foreign atoms without disrupting the lattice structure.
Step-by-step explanation:
In a face-centered cubic (FCC) crystal lattice, the tetrahedral interstitial site is a void or empty space situated between four densely packed atoms on the corners of a tetrahedron. It can be visualized as a tetrahedron formed by connecting the midpoints of the edges of an FCC unit cell.
The significance of the tetrahedral interstitial site in crystallography is that it provides a location for foreign atoms or ions to be inserted into the crystal lattice without disrupting its overall structure. This allows for the formation of solid solutions, where different types of atoms or ions can coexist within the same crystal lattice.