Combined magnetic and structural investigation of nano crystalline iron oxides: The interplay between crystal size and phase composition during FeS2 oxidation

The oxides that form during thermal oxidation of natural FeS₂ (pyrite and marcasite) consist of nanometer-sized crystals of α-Fe₂O₃ and γ-Fe₂O₃. This is shown with heating experiments that were made up to 650 °C, which resembles temperatures used in metallurgical processes. It is shown that magnetic measurements can play a key role in the investigation of this reaction, due to the unwanted blurring effects associated with finite crystal sizes if other methods are used. According to Mössbauer spectra combined with pXRD, many α-Fe₂O₃ crystals are in a stable magnetic state only due to the formation of bridging superexchange interactions in between them, but the γ-Fe₂O₃ experiences super-paramagnetic relaxation ceasing first at 20 K. Magnetisation measurements were used for two main purposes (1) determination of the amounts of γ-Fe₂O₃ in the products, and (2) for characterization of γ-Fe₂O₃ with respect to crystal size and possible magnetic surface effects such as spin-glass. It is proven that fine FeS₂ grains produce more γ-Fe₂O₃ than coarse. At 500 °C the fine FeS₂ grains oxidised into c. 30% γ-Fe₂O₃ and ca. 70% α-Fe₂O₃. At 525 °C, the γ-Fe₂O₃ amounts were also estimated in coarse oxidised FeS₂, and results were ca. 20% and 10% γ-Fe₂O₃ for the fine and coarse FeS₂ respectively. The γ-Fe₂O₃ crystal sizes were a function of both temperature and grain size, and it decreased with decreasing grain size, and upon rising the temperature from 450 to 550 °C. It is argued that the estimated errors during γ-Fe₂O₃ amount determination are due mainly to disordered magnetic sublattices at the crystal faces of γ-Fe₂O₃, giving an error of ca. 15% for those samples that have the smallest crystals.     

Fe-Hydroxysulphates from Bacterial Fe2+ Oxidation

The perturbed angular correlation method (PAC) was applied to investigate the lattice location of implanted ¹¹¹In probe ions in Hf₂Ni and Zr₂Ni intermetallic compounds. It is concluded that the ¹¹¹In/¹¹¹Cd probe nuclei experiencing the highly asymmetric electric field gradient (EFG) occupy the unique hafnium or zirconium 8(h) sites in the investigated phases. Above room temperature, the EFGs decrease linearly with temperature. The results are compared with that of previous PAC measurements with ¹⁸¹Ta probes.