What is the minimum limiting radius ratio for an octahedral arrangement?

The interionic distance for $Cesium$ $chloride$ $(CsCl)$ crystal will be (where $a$ is the edge length of the unit cell):

Select the correct expression for determining the Packing Fraction $(P.F.)$ of an $NaCl$ unit cell (assume ideal),if ions along an edge diagonal are absent.

In the face-centred cubic $(FCC)$ lattice structure of gold,the closest distance between gold atoms is ('a' being the edge length of the cubic unit cell).

The number of nearest neighbours in a $BCC$ unit cell is

In a crystal of $LiI$, $I^{-}$ ions are arranged in a $ccp$ structure and $Li^{+}$ ions occupy octahedral voids. If $a = 600 \ pm$, what is the radius of the $Li^{+}$ ion in $pm$?