Electrostatically driven charge-ordering in magnetite below the Verwey temperature

This work describes the stability of different charge models for magnetite, Fe3O4, below the Verwey temperature (Tv). Thirty-seven configurations allo wed by the Anderson condition are analysed by means of Madelung calculation s, including models proposed by Verwey, Mizoguchi and Zuo. A new approach b ased on summation over cubic cells is applied that ensured rapid convergenc e of lattice sums. The lowest Madelung energy has been found for the model in which coupled chains of Fe2+ and Fe3+ ions form structures parallel to t he c-axis. These structures are similar to double helices having a period o f twice the cubic lattice constant. Thus it has been shown, contrary to pre vious studies, that the Coulomb energy alone is sufficient to stabilize the charge order involving doubling of the c-axis. (C) 2001 Published by Elsev ier Science Ltd.