The Structure of Solids

· The octahedral and tetrahedral interstitial sites of an fcc or hcp structure are shown. There is a part of an atom associated with each vertex and an interstitial site at the center of each polyhedron.
· The "interstitial" spaces have a radius, r_B , that is a fraction of the anion radius, r_A For the tetrahedral interstices: (r_B/r_A) = 0.225. For the octahedral interstices: (r_B/r_A) = 0.414

From: Guy and Hren, "Elements of Physical Metallurgy," Addison Wesley (1974)