Electrostatically driven charge-ordering in Fe2OBO3

Charge-ordering is an important phenomenon in conducting metal oxides: it leads to metal-insulator transitions in manganite perovskites (which show `colossal' magnetoresistances), and the Verwey transition in magnetite (in 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 and copper oxide superconductors; in the latter case, dynamic fluctuations of the stripes have been proposed 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), 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 to undergo charge-ordering through Jahn-Teller distortions. We therefore conclude that both mechanisms occur within the same structural arrangement.