What are two types of crystal structures shown by ice at different pressures?

In a crystal of substance $A_xB_y$,$A$ atoms are present at each corner of the face-centered cubic $(FCC)$ unit cell,and $B$ atoms are present at each face center. What is the correct formula of the substance $A_xB_y$?

Coordination numbers of $Zn^{2+}$ and $S^{2-}$ in the crystal structure of wurtzite are

$A$ and $B$ atoms in a crystal are positioned at the corners and face centers respectively. What is the formula of the crystal?

The ratio of cationic radius to anionic radius in an ionic crystal is greater than $0.732$. Its coordination number is

Which statement is correct for the antifluorite structure of $Na_2O$?