Electrostatically driven charge-ordering in Fe₂OBO₃

Abstract

Charge-ordering is an important phenomenon in conducting metal oxides: it leads to metal–insulator transitions¹ in manganite perovskites (which show ‘colossal’ magnetoresistances), and the Verwey² 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 manganites³,⁴ and copper oxide superconductors⁵; in the latter case, dynamic fluctuations of the stripes have been proposed⁶ 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 crystallization⁷), 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 Fesites. In contrast, the isostructural manganese oxoborate, Mn₂OBO₃, has been previously shown⁸ to undergo charge-ordering through Jahn–Teller distortions. We therefore conclude that both mechanisms occur within the same structural arrangement.