Materials with mixed valency that show a verwey transition

Abstract

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.