The structure of Fe‐Ni alloy in Earth's inner core

Abstract

The crystal structure of Fe‐10%Ni was investigated up to 340 GPa and 4700 K, corresponding to the Earth's inner core conditions by synchrotron X‐ray diffraction measurementsin‐situat ultrahigh pressure and temperature in a laser‐heated diamond‐anvil cell. The results show that hexagonal closed‐packed (hcp) structure is stable throughout the experimental conditions investigated with no evidence for a phase transition to body‐centered cubic (bcc) or face‐centered cubic (fcc) phases. The axialc/a ratio of the hcp crystal obtained around 330 GPa has a small temperature dependence similar to the axial ratio of pure Fe. Iron alloy with less than 10% Ni crystallizes to an hcp structure with c/a ratio of ∼1.61 at inner core conditions, although the effect of small amount of light elements remains to be examined by experiments.

The structure of Fe–Si alloy in Earth's inner core

“…axial ratio increased with increasing temperature, but the temperature effect was small. Compared with the previous data on pure Fe and Fe-10 wt.% Ni alloy obtained at ∼330 GPa (Tateno et al, 2010(Tateno et al, , 2012, Fe-9 wt.% Si alloy exhibited relatively high c/a ratio. The earlier data obtained at 80 GPa (Lin et al, 2002) show markedly higher values, but such difference can be explained by a pressure effect (Tateno et al, 2010).…”

“…Temperature variations within such 6 μm spot were less than ±10% below 5000 K and ±15% at higher temperatures as shown in supplementary Fig. S2 ( Tateno et al, 2010Tateno et al, , 2012. It is noted that the peak temperature rather than the average temperature might be important for the appearance of the B2 phase.…”

“…The compression curve of hcp Fe is also shown for comparison (dashed curve) (Dewaele et al, 2006). above 300 GPa (Tateno et al, 2010(Tateno et al, , 2012. In runs #1-3 with SiO 2 glass pressure medium, we measured unit-cell volume each time after thermal annealing at 1300-2500 K for 5-10 min depending on pressure.…”

Constraints from material properties on the dynamics and evolution of Earth’s core

“…After composition we discuss thermal properties, followed by transport properties, which must be calculated for specific (P, c, T ) conditions. however, recent experiments found that adding up to 10% of Ni does not change the hexagonal close-packed crystal structure of the solid 42 , while ab initio calculations suggest that at high T the seismic properties of Fe-Ni alloys are almost indistinguishable from those of pure iron 43 . Recent studies of core composition [44][45][46] given in section 1 of Table 1; each model is named after the corresponding molar concentration.…”

The fluid dynamics of inner-core growth

“…The preferred crystallographic orientation in all cases is thought to permit more efficient removal of solute from the boundary. Iron alloys in the inner core appear to have an hcp structure (Tateno et al, 2012), so similar processes should align the c-axes in horizontal planes. The strongest component of flow above the inner core is east-west, but there would also be a smaller poleward component in the northern and southern hemispheres.…”