Question

Which of the following unit cell represent the first nearest atom distance in fcc?

• A.
• B.
• C.
  
  
• D.

Answer 1

The correct option is C



In the fcc crytsal lattice, the atoms are present at corners of the cube and at the face-centres of the cube. Here, the corner atoms and the face-centre atoms are in contact along the face diagonal diagonal.
So, the distance between these atom is $\frac{\sqrt{2}a}{2}$. This is the nearest distance in fcc. Hence, the nearest atoms are the one which presnt at the face centres when the reference atom is at corner. There are 12 nearest atom in this unit cell.
Thus, the coordination number of fcc is 12.
Here, option (a) is the correct representation of first nearest atom.
Option (b) represent the second nearest atom in fcc which is at the distance of 'a'
Option (c) and (d) represent the bcc unit cell.