(Hexagonal close packed layers)
Size of octahedral and tetrahedral voids
If the R is the radius of the sphere in the close packed structure
Radius (r) of tetrahedral voids = 0.225 R
Radius (r) of octahedral voids = 0.414 R
In ionic solids, the bigger ions (usually anions) form the close packed structure and the smaller ions (usually cations) occupy the voids.
Radius of cation r+ occupy the tetrahedral voids = 0.225 r-
Radius of cation r+ occupy the octahedral voids = 0.414 r-
If the latter ion is small enough then tetrahedral voids are occupied, if bigger, then octahedral voids.
Not all octahedral or tetrahedral voids are occupied.