${\mathrm{SmFeO}}_3$ has attracted considerable attention very recently due to its reported multiferroic properties above room temperature. We have performed powder and single crystal neutron diffraction as well as complementary polarization dependent soft X-ray absorption spectroscopy measurements on floating-zone grown ${\mathrm{SmFeO}}_3$ single crystals in order to determine its magnetic structure. We found a $k=0$ $G$-type collinear antiferromagnetic structure that is not compatible with inverse Dzyaloshinskii-Moriya interaction driven ferroelectricity. While the structural data reveal a clear sign for magneto-elastic coupling at the Néel-temperature of $\sim 675\mathrm{K}$, the dielectric measurements remain silent as far as ferroelectricity is concerned.

The Fe(III)-reducing bacterium Geobacter sulfurreducens produces nanometer-sized magnetite by transferring electrons from organic matter or hydrogen to ferrihydrite, resulting in the reductive transformation of the ferrihydrite to magnetite, and the generation of ATP for growth. Electron transfer can occur by direct contact between the cell surface and the mineral or via a soluble “electron shuttle,” for example a quinone-containing humic species. The minerals produced at different stages of ferrihydrite reduction during two experiments, one with and one without the humic analog anthraquinone-2, 6-disulphonate (AQDS), were measured using high-resolution synchrotron powder X-ray diffraction. Amorphous 2-line ferrihydrite converts to goethite, then to a mixture of goethite and magnetite, and finally to magnetite. Samples with and without AQDS showed the same general mineralogical trends, and the rate of reaction was faster in the presence of AQDS. In addition, two transient minerals structurally similar to goethite and magnetite were observed to form as intermediates between ferrihydrite and goethite and goethite and magnetite, but only in samples produced in the absence of the electron shuttle. These transient minerals were named proto-goethite and proto-magnetite. Proto-goethite has a shorter c-axis [4.467(20) Å] than crystalline goethite, a function of size (<2 nm) where quantum properties prevail. Proto-magnetite is identified by long tetrahedral (2.113 Å) and short octahedral (1.943 Å) Fe-O bonds compared to stoichiometric magnetite, possibly indicative of a coordination crossover caused by charge density [Fe(II)] migration to tetrahedral sites. Fe(II) in solution or sorbed to the mineral surface is considered to be the catalyst causing the mineral transformations. The Fe(II) is thought to form predominantly from the reductive dissolution of 2-line ferrihydrite by G. sulfurreducens.

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.