[xenonp]

Ok, so I'm shown the fluorite lattice and told the anion is the oxide ion and an unknown cation.
I'm told the unit cell edges are 509pm and the oxide ion radius is 140pm.

I'm asked to calculate the radius of the cation. Is this just 140 / 4 since the unit cell edge is 4 x Rc (oxide filling the tet holes) ?

Then I am asked if either the anions or cations could be considered to be in contact with like charges. I'm not really sure how to go about this. I know rc/ra = 0.225 for tetrahedral holes (is this the maximum limit or minimum limit?). Rc/Ra from my calculations gives me 0.91 so what does this mean?

Thanks