Electrostatically driven charge-ordering in Fe2OBO3

Charge-ordering is an important phenomenon in conducting metal oxides: it leads to metal-insulator transitions(1) in manganite perovskites (which show 'colossal' magnetoresistances), and the Verwey(2) transition in magnetite tin which the material becomes insulating at low temperatures when the conduction electrons freeze into a regular array). Charge-ordered 'stripes' are found in some manganites(3,4) and copper oxide superconductors(5); in the latter case, dynamic fluctuations of the stripes have been proposed(6) as a mechanism of high-temperature superconductivity. But an important unresolved issue is whether the charge-ordering in oxides is driven by electrostatic repulsions between the charges (Wigner crystallization(7)), or by the strains arising from electron-lattice interactions (such as Jahn-Teller distortions) involving different localized electronic states. Here we report measurements on iron oxoborate, Fe2OBO3, that support the electrostatic repulsion charge-ordering mechanism: the system adopts a charge-ordered state below 317 K, in which Fe2+ and Fe3+ ions are equally distributed over structurally distinct Fe sites. In contrast, the isostructural manganese oxoborate, Mn2OBO3, has been previously shown(8) to undergo charge-ordering through Jahn-Teller distortions. We therefore conclude that both mechanisms occur within the same structural arrangement.