A model for the Verwey transition based on an effective interionic potential

A model for the Verwey transition has been developed and tested against exp erimental data. The model describes the electron-ordering process in terms of an average interionic Coulomb potential and a simplified but realistic e nergy level scheme. The strength of the interionic interaction is captured in a set of parameters related to the Madelung energy of a single ion. Thes e constants can be evaluated easily through a computational procedure. A me an field analysis, in which the experimentally determined value of the ener gy gap between the octahedral 2(+) and 3(+)-levels serves as an input param eter, reproduces the observed value of the Verwey transition of 125 K quite well, thereby indicating dominant long range interactions. Madelung calcul ations confirm that these long range interactions are most likely Coulomb-t ype interactions. Finally, the strong effect of cation substitutions on the Verwey transition is discussed and explained quantitatively. Interionic Co ulomb interactions and balancing of the local charge distribution, as impli ed by the Anderson-condition, are the basis of this discussion. (C) 1999 El sevier Science B.V. All rights reserved.