for BCC crystal structure, tetrahedral interstitial
sites are found at 0,1/2,1/4 position that is, lying on {100}
faces and situated midway between two unit
cell edges on this face and one-quarter of the
distance between the other two unit cell
edges compute the radius r of an impurity atom that will just fit into one of these sites in terms of
the atomic radius R of the host atom.

Question

for BCC crystal structure, tetrahedral interstitial
sites are found at 0,1/2,1/4 position that is, lying on {100}
faces and situated midway between two unit
cell edges on this face and one-quarter of the
distance between the other two unit cell
edges compute the radius r of an impurity atom that will just fit into one of these sites in terms of
the atomic radius R of the host atom.