The elastic properties and stability of fcc-Fe and fcc-FeNi alloys at inner-core conditions

Abstract

The agreement between shear wave velocities for the Earth's inner core observed from seismology with those derived from mineral physics is considerably worse than for any other region of the Earth. Furthermore, there is still debate as to the phase of iron present in the inner core, particularly when alloying with nickel and light elements is taken into account. To investigate the extent to which the mismatch between seismology and mineral physics is a function of either crystal structure and/or the amount of nickel present, we have used ab initio molecular dynamics simulations to calculate the elastic constants and seismic velocities (V p and V s) of face centred cubic (fcc) iron at Earth's inner core pressures (360 GPa) and at temperatures up to ∼7000 K. We find that V p for fcc iron (fcc-Fe) is very similar to that for hexagonal close packed (hcp) iron at all temperatures. In contrast, V s for fcc-Fe is significantly higher than in hcp-Fe, with the difference increasing with increasing temperature; the difference between V s for the core (from seismology) and V s for fcc-Fe exceeds 40 per cent. These results are consistent with previous work at lower temperatures. We have also investigated the effect of 6.5 and 13 atm% Ni in fcc-Fe. We find that Ni only slightly reduces V p and V s (e.g. by 2 per cent in V s for 13 atm% Ni at 5500 K), and cannot account for the difference between the velocities observed in the core and those of pure fcc-Fe. We also tried to examine pre-melting behaviour in fcc-Fe, as reported in hcp-Fe by extending the study to very high temperatures (at which superheating may occur). However, we find that fcc-Fe spontaneously transforms to other hcp-like structures before melting; two hcp-like structures were found, both of hexagonal symmetry, which may most easily be regarded as being derived from an hcp crystal with stacking faults. That the structure did not transform to a true hcp phase is likely as a consequence of the limited size of the simulation box (108 atoms). At 360 GPa, in pure fcc-Fe, we find that the transition from fcc to the hcp-like structure occurs at 7000 K, whereas in the Ni bearing system, the transition occurs at higher temperature (7250 K). This reinforces previous work showing that fcc-Fe might transform to hcp-Fe just before melting, and that Ni tends to stabilize the fcc structure with respect to hcp.

Electrical Resistivity of FeS at High Pressures and Temperatures: Implications of Thermal Transport in the Core of Ganymede

“…The close resemblance of material properties among Ni, Fe and Fe‐10 wt.% Ni content as expected in terrestrial cores suggests core properties could be adequately approximated through investigations of pure Fe (e.g., Gomi & Hirose, 2015; Jephcoat et al., 1986; Lin et al., 2003, 2002; Mao et al., 1990; Mao et al., 2006; Martorell et al., 2015, 2013; Sakai et al., 2011; Wakamatsu et al., 2018; Zhang & Guo, 2000). Regarding electron transport properties, the effect due to Ni content weakens as a function of increasing P and T (Gomi & Hirose, 2015).…”

Effective‐Medium Models of Inner‐Core Anisotropy

“…Models that involve other possible allotropes of iron, such as face‐centered cubic crystals (Martorell et al, ), deserve investigation, as do mineralogies than can accommodate the possible range of light alloying elements (Hirose et al, ; Poirier, ). The second‐order J model developed here and in our previous work (Jordan, ; Song & Jordan, ) can be easily extended to mixtures of different crystals.…”

3D Transdimensional Seismic Tomography of the Inner Core

“…Each phase displays varying intrinsic anisotropy, although the magnitude of anisotropy observed seismically also depends on the degree of crystal alignment. In order to test if our seismic observations would be able to constrain the phase of iron, we calculated the anisotropy of a single crystal of each phase of iron at inner conditions using values obtained by ab initio simulations from Vočadlo (2007) and Martorell et al (2015) and calculated the corresponding seismic travel times (see Fig. S7).…”