The role of volume effects in the Verwey transition in magnetite

Fe2+/Fe3+ ordering in magnetite is described in terms of a simple mean-fiel d approach based on an effective interionic Coulomb potential. It is shown that first-order electronic order-disorder transformations, as reported in the literature, can only be reproduced when the dependence of the interioni c potential on the unit-cell dimensions is taken into account. In this case the first-order transitions can then be viewed as the result of an interpl ay between the lattice-deformation energy and the free-energy contribution related to the electronic ordering of the octahedral Fe lattice. Furthermor e, the effect of the lattice deformation by hydrostatic pressure on the Ver wey transition can be successfully explained to some extent within the cont ext of the same framework. In comparison to experimental data available fro m the literature, the mean-field approach developed in this paper yields ve ry acceptable results with respect to both qualitative and quantitative asp ects, thus opening an interesting new viewpoint on the mechanism of the Ver wey transition.