Synthesis, structure and properties of a semivalent iron oxoborate, Fe2OBO3

Transition metal oxoborates are of interest for magnetic and optical properties. Crystals of many MII2MIIIO2BO3 and MIIMIIIOBO3 materials can be grown from borate fluxes. In the FeII–FeIII–B–O system, flux growth results in Fe3O2BO3 crystals, but solid state reaction at higher temperatures has yielded Fe2OBO3 as a polycrystalline powder. This has been characterised by synchrotron and neutron diffraction, electron microscopy, Mossbauer spectroscopy, and conductivity and magnetic measurements. Two notable transitions occur, a broad semiconductor– semiconductor change accompanied by a structural transition at 317 K, and L-type ferrimagnetic order below a Curie temperature of 155 K. An average (Fe2+)0.5(Fe3+)0.5 valence is observed at the two crystallographically distinct sites in Fe2OBO3, indicating that charge ordering occurs.