An octahedral void is surrounded by how many spheres?

The relationship between the radius $(r)$ of the atom and the edge length $(a)$ of the cubic close packing $(CCP)$ structure is:

Ferric oxide crystallises in a hexagonal close-packed array of oxide ions with two out of every three octahedral holes occupied by ferric ions. Derive the formula of the ferric oxide.

Calculate the packing efficiency in a body-centred cubic $(BCC)$ structure.

What does a tetrahedral void in a crystal represent?

The edge of unit cell of $FCC$ $Xe$ crystal is $620 \ pm$. The radius of $Xe$ atom is $..........$ $pm$.