Using hard spheres, find the largest atom radius fitting an octahedral void in BCC Fe without lattice distortion, in nm to 3 decimals. Fe atomic radius is 0.124 nm.
BCC geometry gives $a=4R/\sqrt3$. The limiting separation at an octahedral site is a/2, so $r=a/2-R=(2/\sqrt3-1)R$. With R = 0.124 nm, $r=0.01918$ nm, approximately 0.019 nm.