Consider an octahedral void represented by the shaded portion `(` the shown`)`. Now suppose the length of the each arm of the square will be `2R(R` is radius of atom `)`. Let `r` be the radius of void. For right angle triangle `ABC`.
`AC=sqrt([(AB)^(2)+(BC)^(2)])`
`sqrt([(2R)^(2)+(2R)^(2)])=sqrt(8R)`
Also, `AC=R+R+2r=2r+2R=2(r+R)`
or `sqrt(8R)=2sqrt(2R)=2(r+R)`
or `r=sqrt(2R)-R`
`r=[sqrt(2)-1]R=0.414xxR`
