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Iron in oxides, silicates and alloys under extreme pressure-temperature conditions
(2011)
- (1) There is a general agreement, that magnesium silicate perovskite (Pv) comprises around 80 vol% of the Earth's lower-mantle, making it by volume the most abundant mineral in our planet, and and there is no doubt that Pv in the mantle contains Fe and Al. However, the exact concentrations are unknown, as well as the effect of pressure on physical properties of Pv at conditions of Earth lower mantle. In our study we investigate Pv with one of the less explored substitution Mg2+A+Si4+B→Fe3+A+Al3+B. Here we explore as a function of pressure and temperature the crystal structure of the material, the distribution of chemical elements between different crystallographic sites and the evolution the spin state of ferric iron, as one of crucial parameters determining electrical and radiative conductivity of the Earth's lower mantle. We perform single-crystal x-ray diffraction on magnesium silicate perovskite with the composition Mg0.63Fe0.37Si0.63Al0.37O3 (MgFeAlPv) using a combination of in-situ diamond anvil cell technique and laser heating in order to simulate the extreme conditions of the Earth's lower mantle. We provide a complete description of the behavior of MgFeAlPv in terms of crystal structure and ferric iron occupying its dodecahedral (A-)site. We observe no spin transition of ferric iron at A-site, confirming theoretical predictions and recent experimental observations. However, even upon heating MgFeAlPv samples to 1800 K at ~78 GPa we see no indication of a spin crossover or a pressure/temperature induced redistribution of ferric iron and aluminum between the different crystallographic sites as suggested previously. We combine these data with high pressure-high temperature measurements to obtain a thermal equation of state. (2) As a model Fe-O system, magnetite is a mixed valence iron oxide incorporating both ferric and ferrous iron. Being essential part of some sedimentary (banded iron formations) and igneous rocks, magnetite can be subjected to high pressure in natural systems, for instance, during subduction of oceanic crust, or during serpentinization (metamorphic reaction). In order to shed light on the complex physical properties of magnetite under compression we conducted a combined single crystal x-ray diffraction and Mössbauer spectroscopy at pressures below 25 GPa. We find no evidence for the transition from inverse to normal spinel in magnetite. Analyzing the collected Mössbauer data, we show that a high spin – intermediate spin transition cannot occur in magnetite in the pressure range of 10-20 GPa, and finally, based on a careful analysis of the data and results reported in the literature, we provide a model consistently describing the behavior of electronic and magnetic properties of magnetite in terms of a gradual charge delocalization induced by pressure. (3) Our study of wüstite (FexO) is focused on the high pressure – low temperature phase diagram of the Fe-end member in the (Mg,Fe)O system. We perform high resolution neutron diffraction experiments in order to investigate the low temperature phase diagram of Fe0.925O and Fe0.94O. We determine the critical temperatures of antiferromagnetic ordering (the Neél temperature TN) and structural transitions (TS) of the two compounds. We report divergence of TN and TS as a function of pressure. We argue that a modification of the defect structure in wüstite can be invoked explaining the drastically different response of Fe0.925O and Fe0.94O to compression. With that we show that although ferric iron is a minor structural component of wüstite, it is an essential component of defect structures and induces profound effects on the low temperature phase diagram of wüstite. (3) We investigate effect of pressure (P) on the elastic and electronic properties of Fe, Fe0.9Ni0.1 hcp phases below 70 GPa. After processing our experimental data, we report a gradual decrease in the ratio of the hcp lattice parameters c/a for Fe in the pressure range below 45-50 GPa, and a non-linear behavior of Mössbauer isomer shift for hcp phases of pure Fe and Fe0.9Ni0.1, suggesting an isostructural transition in these phases. We investigate paramagnetic hcp Fe under compression by employing state-of-art calculations (LDA+DMFT) and including many-body correlation effects. Based on the results of the calculations, we predict an electronic topological transition (ETT). After comparing data on materials with already known ETT with our observations and theoretical predictions, we conclude that results obtained from the independent experimental measurements can be explained in the framework of an ETT. (4) The development of a portable laser heating system was a necessary requirement for our work done on minerals at conditions of Earth’s lower mantle in general, and for the study of magnesium silicate perovskite containing iron and aluminum in particular. The main advantages of the system developed are compactness, versatility for different in-house and synchrotron based techniques, including high pressure measurements of resistivity, Raman spectroscopy, energy and time-resolved Mössbauer spectroscopy, powder and single crystal x-ray diffraction, nuclear inelastic x-ray scattering, and x-ray absorption. These advantages, the low times of assembly, stable and homogeneous conditions for heating, in-situ measurement of sample temperature, as well as the direct visual control over the heating area distinguish our system from similar, but bulkier devices.
