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 (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 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, Fe 2 OBO 3 , that support the electrostatic repulsion charge-ordering mechanism: the system adopts a charge-ordered state below 317 K, in which Fe 2+ and Fe 3+ ions are equally distributed over structurally distinct Fesites. In contrast, the isostructural manganese oxoborate, Mn 2 OBO 3 , 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.