- High-pressure and high-temperature structural and electronic properties of (Mg,Fe)O and FeO (2007)
- Magnesium-rich MgO-FeO solid solution, known as the mineral ferropericlase, constitutes a significant part of the Earth as the second most abundant mineral in the lower mantle after (Mg,Fe)SiO3 perovskite. A combined experimental and theoretical study was carried out in order to determine structural and electronic properties of ferropericlase over a broad pressure and temperature range. The phase diagram of FeO (wüstite), the end member of the (Mg,Fe)O solid solution, was found to be more complex than previously thought. It was discovered that the magnetic ordering (Néel) transition does not coincide with the structural cubic-to-trigonal symmetry breaking transition in non-stoichiometric FeO. The magnetic ordering transition was determined for the first time by a combined Mössbauer spectroscopy and neutron diffraction study. A full agreement between these two methods was observed, indicating that in the case of FeO the Mössbauer spectra reflect long-range magnetic ordering. A quasi-single crystal X-ray diffraction study of FeO compared with previous results shows that the transition pressure depends not only on stress conditions, but also on wüstite composition, and probably the order of the transition (second- or weak first-order) is also stress dependent. Above ~70 GPa after laser annealing the X-ray diffraction pattern of FeO could not be explained as a trigonal structure, but as a monoclinic structure with space group P21/m. (Mg,Fe)O solid solution was studied over a wide pressure and temperature range and over a compositional range from 5 to 20 mole % of FeO component. The detailed analysis of (Mg,Fe)O Mössbauer spectra shows clear evidence for the distribution of the hyperfine parameter quadrupole splitting (D), which provides a key to determining its local structure. It is shown that by analyzing the D distribution, a short-range order parameter could be estimated for the low-Fe (Mg,Fe)O solid solution. Samples quenched from high temperature at ambient pressure during synthesis show local cation distribution close to randomness, as was reported previously. Upon compression, however, a rapid increase of short-range order with the tendency for Fe clusterization was observed. This non-random atomic distribution was shown to be stable at high pressures and also at high temperatures. Such a tendency for Fe ions to separate could lead to the miscibility gap in the (Mg,Fe)O solid solution series at high pressures and temperatures, as was observed. At pressures higher than 50 GPa a spin-pairing transition of Fe2+ was observed. Clear and pronounced changes in the Mössbauer spectra are fully consistent with a high- to low-spin transition: the centre shift decreases, indicating an increase of electron density at the nuclei. Quadrupole splitting also vanishes to zero, indicating significant spherical symmetrisation of the valence electrons and electrical field gradient disappearance. The absolute magnitude of these changes is in full agreement with ab initio calculations made in this study. The onset of the spin transition is similar for all the samples studied, but the width is strongly composition dependent. The higher the iron content, the broader the transition width, which reaches about 50 GPa for the (Mg0.8Fe0.2)O sample. Such a broad transition range is not typical for phase transitions with significant volume collapse. Analysis of literature data together with the results of this study lead to an interpretation of spin crossover as a thermal equilibrium process without phase transition. The compositional and temperature dependence of spin crossover in ferropericlase can be described fairly well within such a model, taking into account the local structure of the solid solution. The results of this model were also confirmed by ab initio simulations. The model proposed in this work predicts that spin crossover in ferropericlase will occur over a large depth range of the lower mantle. No discontinuities in density or elastic properties are expected to be produced in the lower mantle due to spin crossover in ferropericlase, contrary to previous suggestions.