An ionic solid of $\mathrm{XY}$ the type having anions in $\mathrm{CCP}$ the lattice and cations in the octahedral voids. Let $\mathrm{a}$ be the edge length of an $\mathrm{FCC}$ cube. The radius ratio of cation $\left(R_{+}\right)$ to that of anion $\left(R_{-}\right)$ is greater than $0.415$. Then which of the following is true?

The radius ratio should be equal to $0.414$ for an ideal fit in the octahedral vacuum, but in this case, the cation is larger than required. Anions can no longer touch one another. Because they are separated by octahedral spaces, cations do not contact. However, side length, an is still equal to the sum of the radius of the anion and the cube's centre cation's diameter.

$a=2\left(R_{+}+R_{-}\right)$

But to have ${\mathrm{R}}_{-}=\frac{\mathrm{a}}{2\sqrt{2}},$

${\mathrm{R}}_{-}=\frac{2\left({\mathrm{R}}_{+}+{\mathrm{R}}_{-}\right)}{2\sqrt{2}}$

Or, ${\mathrm{R}}_{+}=0.414{\mathrm{R}}_{-}$ but it's not always the case.