What is the limiting radius ratio for a tetrahedral shape?

What is the crystal structure of $Na_2O$?

The sum of the number of first and second nearest neighbors in an $FCC$ closed packing is:

In a face-centered cubic $(FCC)$ lattice,$A$ atoms are present at the corners and $B$ atoms are present at the face centers. If one $B$ atom is removed from one of the face centers,what will be the molecular formula of the compound?

Body centered cubic lattice has a coordination number of

The crystal structure of $AB$ is of $NaCl$ type. If the radius of the $A^+$ ion is $x \ pm$ and the radius of the $B^-$ ion is $y \ pm$, then the edge length of the unit cell will be ..........