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

Transition metal oxoborates are of interest for magnetic and optical properties. Crystals of many (M2MO2BO3)-M-II-O-III and (MMOBO3)-M-II-O-III materials can be grown from borate fluxes. In the Fe-II-Fe-III-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.