Physics of iron and earth's inner core

The Earth's core is primarily composed of iron and I studied the physics of this element. This dissertation addresses two important questions concerning the physical properties of hexagonally close packed (hcp) iron, the high pressure polymorph. In the first part (chapter II and III) the focus is on the thermoelastic properties of hcp iron at the thermodynamic conditions of the deep interior of the Earth, advancing our understanding of inner core processes and structure. Combining accurate band structure total energy methods for electronic contributions to the Helmholtz free energy with a mean field approach to the lattice vibrations, I have predicted the structure and elastic constants of dense hcp iron at high temperature from first principles. The axial ratio c/ a in the hcp cell increases substantially with temperature, reaching a value of nearly 1.7 at 5700 K, where aggregate elastic properties match those of the inner core. As a consequence of increasing c/ a the single crystal longitudinal anisotropy at high temperature has the opposite sense from that at low temperature. Combining the single crystal elasticity with a simple model of inner core texture, in which basal planes are preferentially aligned with Earth's rotation axis, one can account for seismological observations of inner core anisotropy. The second part (chapters IV and V) investigates the effect of a theoretically predicted anti-ferromagnetic ground state at pressure on spectroscopic and elastic properties, the results adding to a growing body of theoretical evidence for magnetism in hcp iron at pressure. The orthorhombic anti-ferromagnetic ordering leads to two Raman active transverse optical modes, as found experimentally, while only one such mode is predicted in the hcp structure. Moreover, we find good agreement with experimental observations of the frequencies of the two modes. The influence of anti-ferromagnetic ordering on the elasticity of hcp iron reduces the discrepancy between theory and experiment in equation of state and aggregate elastic properties.