radius ratio considerations are only one factor in determining the crystal structures of ionic and covalent compounds and they are not usually referred to as interstitial compounds even though the pattern or distribution of atoms in the unit cells may be exactly the same. For example, the TiN structure Fig. 1.14(a) is isomorphous with the NaCl structure. Similarly, the TiH2 structure(Fig. 1.14 b) is identical to the Li2O structure and the TiH structure: (Fig. 1.14(c) is isomorphous with ZnS(zinc blende or sphalerite) structure.