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A geometric heuristic favouring the largest coordination number compatible with neighbouring opposite-charge ions touching the central ion without overlap. A limiting radius ratio rule comes from additionally making the surrounding ions just touch. It is a hard-sphere model, not an exact stability criterion.

The actual ionic radii need not equal the limiting ratio, and electronic charge distributions are not rigid sphere boundaries. In the same unlike-contact hard-sphere model, put $q=r_a/r_c$ and use $a=4(r_a+r_c)/\sqrt{3}$ to getA cation somewhat larger relative to the anion than the limiting size opens gaps between the anions. Starting from $q=1.366$ and reducing $q$ modestly reduces this packing fraction; thus a lower value is expected for that departure from the ideal contact construction. The formula is not monotonic for all imaginable ratios, so no universal inequality should be inferred without specifying radii. Coordination-dependent ionic radii, polarizability, nonionic contributions to bonding, thermal expansion and point defects also limit the model. No numerical real-crystal packing fraction is determined by the supplied data alone.

A hard-sphere model relating the radius ratio of opposite-charge ions to a possible coordination number. For a cube of eight mutually touching anions around a touching cation, $r_c/r_a=\sqrt{3}-1$; real bonding and coordination-dependent radii limit its predictive power.