Consider the voids in an fcc unit cell of edge $a$ . Which statements are correct?

AThere are 4 octahedral voids per unit cell: one at the body centre and the others at the edge centres.
BAll 8 tetrahedral voids lie completely inside the unit cell.
CThe tetrahedral void is larger than the octahedral void.
DThe distance between the centres of two nearest tetrahedral voids is $a /2$ .

Answer: A, B, D

Octahedral:

1 + 12 \times \frac{1}{4} = 4

. Tetrahedral voids sit at

(\frac{1}{4}, \frac{1}{4}, \frac{1}{4})

-type positions, all 8 inside the cell; two nearest ones, e.g.

(\frac{1}{4}, \frac{1}{4}, \frac{1}{4})

and

(\frac{3}{4}, \frac{1}{4}, \frac{1}{4})

, are

a /2

apart. Radius ratios are

0.225

(tetrahedral)

< 0.414

(octahedral), so C is wrong.