- iron oxide (1) (remove)
- Elasticity measurements at extreme conditions: application to FeO and FeNi-alloy (2008)
- The picture of Earth’s deep interior is rapidly improving from the seismic tomography data and indicates more complexity than previously thought. The presence of Earth’s seismic anisotropy requires the knowledge of fully anisotropic elasticity data for mineral phases. The single-crystal elastic constants of minerals, Cij, are elements of the fourth-rank elasticity tensor, which relates stress to strain. The fact that elastic strain also defines seismic wave propagation, the elastic tensor of minerals can be applied to interpret the bulk mineralogy of the interior from seismological observation. Knowledge of the elasticity of crystalline materials as a function of pressures and temperature is also of primary interest for solid state physics because elastic tensors reveal the nature of interatomic interactions. In order to determine the full elastic tensor of minerals under high pressure and temperature, several techniques are available, including ultrasonic interferometry and inelastic x-ray scattering methods. One of the most accurate techniques is high-frequency acoustic interferometry, which is capable for measuring sound wave velocities in very small samples under high pressures. The ultrasonic interferometry system operating at 0.5-2.0 gigahertz (GHz) frequencies was developed in the Bavarian Geoinstitut of the University of Bayreuth for in situ high pressure and temperature experiments. Here, GHz-ultrasonic interferometry has been used to study the elastic properties of monoxide minerals such as FeO, liquids and nanocrystalline samples, each with particular importance to Earth or material sciences. FexO, wüstite, is the end-member phase of the (Mg,Fe)O solid solution, thought to be the most abundant non-silicate oxide in the mantle. The full elastic tensor of wüstite is determined by three elastic constants (C11, C12, C44), which have been probed at high-pressures. At about 17-20 GPa, FeO is known to undergo a displacive cubic-to-rhomobhedral phase transformation. Prior to this transformation, we observe a pressure-induced mode softening of the C44 elastic constant. In addition, previously undetected discontinuities in the pressure derivatives of C11 and C12 at 4.7  0.2 GPa were observed. This pressure is consistent with that of the magnetic ordering commencement, as was observed by high-pressure Mössbauer spectroscopy in a 57Fe-enriched sample of FeO. The results indicate that an intermediate, partially magnetic but still cubic phase of FeO probably exists at room temperature and in pressure range from ~5 GPa to ~17 GPa. In order to provide deeper knowledge of the magneto-elastic coupling in the material, neutron diffraction experiments were performed under ambient pressure and low temperatures. The results indicate that the magnetically ordered cubic phase of FexO that was observed at high pressures also exists at ambient pressure at temperatures between 160 and 201 K. Combined inelastic x-ray scattering and x-ray diffraction studies on a single crystal of Fe0.95O were performed up to 20 GPa at room temperature. The results show strong anelastic behaviour of wüstite, which should be accounted for at high pressure. Transition-metal oxides, non-stoichiometric compounds, and materials with complex mesostructure have some internal degree of freedom, and could therefore experience internal relaxation and show deviations from normal elastic behaviour. A methodology to measure inelastic x-ray scattering in externally heated diamond anvil cells have also been developed. This technique was used to study polycrystalline fcc-Fe0.78Ni0.22 alloy at high pressures (up to 72 GPa) and temperature (up to 715 K). The bulk elasticity and its P and T derivatives were obtained for the material. No significant deviation of the elastic properties from those of pure iron was observed and furthermore no deviation from Birch’s law. Although the bulk elasticity of fcc Fe-Ni alloy and Fe seem to be very similar, the elastic anisotropy of hexagonal and cubic phases should be quite different. If the metal phase in the inner core is not hexagonal, but cubic (or a mixture of the two phases exists), seismic anisotropy may provide a better way to discriminate between them two.