1. Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK
2. IRC in Superconductivity, University of Cambridge, Madingley Road, Cambridge CB3 0HE, UK
3. Laboratoire de Physique de l'Etat Condensé, UPRESA CNRS 6087, Université du Maine, 72085 Le Mans, France
4. Synchrotron Radiation Source, CLRC Daresbury Laboratory, Warrington WA4 4AD, UK
5. Chemical Crystallography Laboratory, University of Oxford, Parks Road, Oxford, UK

Correspondence to: J. P. Attfield^1,2 Correspondence and requests for materials should be addressed to J.P.A. (e-mail: Email: jpa14@cam.ac.uk).

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.