Structural chemistry of A₂MX₄ compounds (X = O, F) with isolated tetrahedral anions: search for the densest structure types

In three of the nine morphotropic A₂MX₄ series (nine different tetrahedral anions) the densest structure (spinel) is the stable structure for compounds with A cations of intermediate radius, in contrast to the classical homology rule. In some cases, denser phases have abnormally low overall binding energies.