For C.N. = 6 and a radius ratio greater than the minimum, the crystal is more stable since the cation is still in contact with six anions, but the anions are further from each other so that their mutual repulsion is reduced. An octahedron may then form with a radius ratio greater than or equal to .414, but as the ratio rises above .732, a cubic geometry becomes more stable. This explains why Na+ in NaCl with a radius ratio of 0.55 has octahedral coordination, whereas Cs+ in CsCl with a radius ratio of 0.93 has cubic coordination.