Electrostatically driven charge-ordering in Fe2OBO3

Charge-ordering is an important phenomenon in conducting metal oxides: it l eads 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 cond uction 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 unresolv ed issue is whether the charge-ordering in oxides is driven by electrostati c repulsions between the charges (Wigner crystallization(7)), or by the str ains arising from electron-lattice interactions (such as Jahn-Teller distor tions) involving different localized electronic states. Here we report meas urements on iron oxoborate, Fe2OBO3, that support the electrostatic repulsi on charge-ordering mechanism: the system adopts a charge-ordered state belo w 317 K, in which Fe2+ and Fe3+ ions are equally distributed over structura lly distinct Fe sites. In contrast, the isostructural manganese oxoborate, Mn2OBO3, has been previously shown(8) to undergo charge-ordering through Ja hn-Teller distortions. We therefore conclude that both mechanisms occur wit hin the same structural arrangement.