High Temperature Simulations of the Seismic Wave Velocities on FeNi Alloys at the Conditions of Earth's Inner Core

Understanding the physical properties of the Earth's core is a key step in the study of the evolution and dynamics of our planet. The Earth's inner core is a solid Fe-Ni alloy at high temperature (~6000 K) and high pressure (330<P<360 GPa). Furthermore, to account for the lower than expected density in the Earth's core, it has been suggested that light elements must also be present. While the effect of light elements on the properties of iron have been the subject of an extensive literature, the effect of nickel on the properties of iron has often been overlooked, possibly due to the expectation that the properties of Ni and Fe will be similar. This expectation was not true in the athermal limit (results presented in AGU 2011), where 12.5% Ni decreased the compressional and shear velocities (Vp and Vs) more than 2 and 7% respectively respect to pure iron. Encouraged by this result, we performed abinitio molecular dynamics simulations coupled with periodic plane-wave density functional within the generalized gradient approximation on the fcc and hcp structures of Fe1-XNiX alloys (X= 0, 0.0625, 0.125) in order to obtain the high temperature elastic properties and wave propagation velocities at 5500 K and 360 GPa [1]. At 5500 K, the previously mentioned reduction in the wave velocities disappears and the velocities of hcp and fcc Fe-Ni alloys remain very similar to those of pure iron throughout the range of compositions studied. Moreover, at 5500K the values for Vp and Vs are too high in comparison with PREM. To solve this we have performed simulations at higher temperature, closer to the melting point of Fe (6300 K at 330 GPa from GGA calculations [2]). Finally, the evaluation of maximum anisotropy in Vp at 5500 K in hcp and fcc structure shows that neither the temperature nor the Ni has an important effect, although it is dependent on the structure studied. We conclude, therefore, that Ni does indeed behave similarly to Fe and can safely be ignored when considering its effect on the seismic properties of Fe under core pressures and temperatures. [1] Martorell, B., Bordholt, J., Wood, I., Vočadlo, L., The effect of nickel on the properties of iron at the conditions of Earth's inner core: ab initio calculations of seismic wave velocities of Fe-Ni alloys. Earth Planet. Scie. Let. (2012) On review. [2] Alfè, D., Temperature of the inner-core boundary of the Earth: Melting of iron at high pressure from first-principles coexistence simulations. Phys. Rev. B 79 (2009) 060101.