Structural analysis of metastable pseudobrookite ferrous titanium oxides with neutron diffraction and Mossbauer spectroscopy
Four synthetic iron titanium oxides with the pseudobrookite ( AB2O 5, Cmcm, Z = 4) structure have been prepared and characterized by neutron diffraction and zero-field, natural abundance 57Fe Mossbauer effect spectroscopy (MES). The combination of the element specificity of MES with the different neutron scattering lengths of Ti and Fe (-0.33 and 0.95 × 10 -12 cm, respectively) offers a unique opportunity to distinguish between cation distributions on the two (" A" and " B") sites. Two of the samples have been prepared in low temperature experiments (quenched from 1200°C) and have the stoichiometry FeTi 2O 5, and Fe .6Mg .6Ti 1.8O 5. The third and fourth samples are commercial iron titanium oxides prepared by the reduction of ilmenite ore with carbon above 1700°C. The stoichiometries of these samples are Mn 0.05Fe 0.33Ti 2.52O 5 and Fe .33Mg .31Ti 2.36O 5. Results from these experiments indicate that for each of these samples the B site is predominantly (>65%) occupied by Ti, while the A site contains a mixture of Ti, Fe, and/or Mg. However, only at higher temperatures (>1700°C) is the B site devoid of ferrous cations. These results suggest that an "ordered" model for ferrous titanium-rich oxides of the pseudobrookite structure (100% Ti occupancy of the B site) is descriptive only at elevated temperatures, and that at lower temperatures a "disordered" model (partial iron occupation of the B site) is a more accurate representation of the structure. Because of this difference, it may be possible to predict the thermal history of naturally occurring samples based on cation distributions.