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

Transition metal oxoborates are of interest for magnetic and optical properties. Crystals of many M II 2 M III O 2 BO 3 and M II M III OBO 3 materials can be grown from borate fluxes. In the Fe II -Fe III -B-O system, flux growth results in Fe 3 O 2 BO 3 crystals, but solid state reaction at higher temperatures has yielded Fe 2 OBO 3 as a polycrystalline powder. This has been characterised by synchrotron and neutron diffraction, electron microscopy, Mössbauer 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 (Fe 2+ ) 0.5 (Fe 3+ ) 0.5 valence is observed at the two crystallographically distinct sites in Fe 2 OBO 3 , indicating that charge ordering occurs.