First-principles study of iron and (iron,nickel) alloys up to core pressures.

摘要

The elasticity of Fe and (Fe,Ni) alloys are investigated up to core pressures using Density Functional Theory. At pressures below 52 GPa an uncharacteristically large discrepancy exists between theoretically predicted and experimentally observed properties of elemental iron. The differences in compressibility and of most elastic contants can be reduced significantly by introducing an anti-ferromagnetic spin arrangement. This spin arrangement induces significant lattice distortions that change the crystal symmetry from hexagonal to orthorhombic. The lattice distortions are small and decrease rapidly with increasing pressure and thus may be difficult to observe experimentally. Available experiments that explore the elasticity of iron at high pressure rely on scaling between vibrational frequencies and an elastic constant (C44). This relationship was tested and it was found that it holds to within 4%. Therefore the remaining differences between theory and experiment for elemental iron cannot be explained by flawed scaling.;Iron-nickel alloys with up to 12.5 at%, encompassing likely nickel concentrations in the Earth's core, were investigated up to core pressures. The effect of Nickel on compressibility, density, and cell parameters are small at least up to 12.5 at% Ni. The only elastic constant that is significantly affected by nickel content is the shear elastic constant C44, it is lowered by 16% at an inner core pressures of 340 GPa. This effect leads to a lowering of the isotropic wavespeeds as nickel content increases. The knowledge of the complete elastic constant tensor affords to address the effect of nickel on seismic observables such as the compressional and shear-wave velocity anisotropy. Possible implications for the chemical composition of the inner core are discussed.