On the origin of the phase transitions in a class of mixed valence compounds

Crystals of several mixed-valence compounds featuring (FeIII)2(FeII)O units undergo a variety of phase transitions with the three main phases differing in the extent of the intramolecular delocalization of the extra electron. We point out that a published theory for this phenomenon is basically equivalent to a classical spin model. Although some intriguing quantal aspects are ignored, solution of the spin model by mean-field theory reveals a much richer phenomenology than expected. In particular, the model predicts that the middle phase (of the three observed) should be one in which only a sublattice contains a large fraction of delocalized molecules.