Length of body diagonal in FCC unit cell is $a$ $\stackrel{°}{\mathrm{A}}$. Distance between two octahedral voids in it is :

The length of body diagonal in FCC unit cell = $a$ $\stackrel{°}{\mathrm{A}}$ 

Length of the body diagonal, $=\sqrt{3}a$.

Distance between two octahedral voids in it is :

$\begin{array}{r}\frac{a}{\sqrt{3}\sqrt{2}}=\frac{a}{\sqrt{6}}\end{array}$.