The correct relationships between unit cell edge length '$$a$$' and radius of sphere '$$r$$' for face-centred and body-centred cubic structures respectively are :

A compound is formed by two elements $$\mathrm{X}$$ and $$\mathrm{Y}$$. The element $$\mathrm{Y}$$ forms cubic close packed arrangement and those of element $$\mathrm{X}$$ occupy one third of the tetrahedral voids. What is the formula of the compound?

Which of the following represents the lattice structure of $$\mathrm{A_{0.95}O}$$ containing $$\mathrm{A^{2+},A^{3+}}$$ and $$\mathrm{O^{2-}}$$ ions?