Question

Position of octahedral voids in fcc structure is/are:

• A. Edge centres of unit cell
  
• B. Corners of unit cell
  
• C. Body centres of unit cell
• D. Face-centres of unit cell

Answer 1

Answer: (A), (C)

Body centres of unit cell

In a unit cell of fcc lattice, the body centre of the lattice is not occupied but it is surrounded by the six face centres. Thus, the unit cell has one octahedral void at the body center.
Each of the 12 edge centres in a unit cell is surrounded by 6 atoms, and so are octahedral voids. Of the six atoms, four belongs to the same unit cell and two belongs to two other adjacent unit cells. Each edge of the cube (octahedral void) is shared by 4 adjacent unti cells. So, only ${\frac{1}{4}}^{th}$ of each void is belong to a particular unit cell.
Hence,
Octahedral voids are present at:
1. Body centre (1) of unit cell is shown in figure (a)
2. Edge centres (12) of unit cell is shown in figure (b)