Question

Each ion or atom that forms an $fcc$ unit cell is surrounded by

• A. eight octahedral voids and four tetrahedral voids
• B. six octahedral voids and six tetrahedral voids
• C. eight octahedral voids and six tetrahedral voids
• D. six octahedral voids and eight tetrahedral voids

Answer 1

Answer: (D)

In $fcc$ unit cell, a corner atom can support $8$ unit cells. In each cube, at a distance of $\sqrt{3}a/4$ from corner atom, there is tetrahedral void; it implies each atom is surrounded by eight tetrahedral voids.
At the centre of edge, there is an octahedral void. Each corner can support six edges and hence it is surrounded by six octahedral voids.