Simple cubic diatomic structure.

The centres of the right anions from a simple cubic structure with the cation located example of this configuration is $CsCl$ structure in which the $Cl^-$ ions are situated at the eight corners of the cube and the $Cs^+$ ion is found at the body centre.

In critical condition the eight corner anions touch the central cation along the body diagonals. The eight anions touch each other along the cube edges.

The body diagonals pass through two opposite corner anions and the central cation. Here $a = 2r_A $and $GD = √3a = 2(r_C+r_A).$

Therefore ,$ r_C/r_A =0.732$