A compound formed by elements A and B crystallises in the cubic structure where A atoms are at the corners of a cube and B atoms are at the face centre. The formula of the compound is :

Which compound has antifluorite structure?

An element occurs in two crystalline forms $$\alpha$$ and $$\beta$$. The $$\alpha$$-form has an fcc with $$a = 3.68\,\mathop A\limits^o $$ and $$\beta$$ form has a bcc with $$a = 2.92\,\mathop A\limits^o $$. Calculate the ratio of their densities.