Materials with mixed valency that show a verwey transition

A discussion is given of various models for the Verwey transition in Fe3O4and Ti4O7, and its apparent absence in Sm3S4. In allsystems there is evidence for polaron formation, and for bipolarons in the first two. It is suggested that, in spite of polaron formation, in low-temperature Fe3O4 there must be considerable overlap of the wavefunction from the sites occupied by Fe2+onto those occupied by Fe3+, and that the disappearance of long-range order willtherefore affect short-range order.