Configurational thermodynamics of Fe–Ni alloys at Earth's core conditions

Ekholm, Marcus; Mikhaylushkin, A. S.; Simak, Sergey; Johansson, Börje; Abrikosov, Igor A.

doi:10.1016/j.epsl.2011.05.035

Cited by 14 publications

(10 citation statements)

References 39 publications

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“…Alloying iron with nickel therefore has a measurable effect on the c / a axial ratio of the alloy, as does alloying iron‐nickel with silicon. This is in agreement with theoretical calculations from Asker et al () and Ekholm et al () and existing experimental results from Lin et al (), Tateno et al (), Sakai et al (), and Tateno et al ().…”

Section: Unit Cell Axial Ratio and Anisotropysupporting

confidence: 93%

Equations of State and Anisotropy of Fe‐Ni‐Si Alloys

et al. 2018

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We present powder X‐ray diffraction data on body centered cubic (bcc)‐ and hexagonal close packed (hcp)‐structured Fe0.91Ni0.09 and Fe0.8Ni0.1Si0.1 at 300 K up to 167 and 175 GPa, respectively. The alloys were loaded with tungsten powder as a pressure calibrant and helium as a pressure transmitting medium into diamond anvil cells, and their equations of state and axial ratios were measured with high statistical quality. These equations of state are combined with thermal parameters from previous reports to improve the extrapolation of the density, adiabatic bulk modulus, and bulk sound speed to the pressures and temperatures of Earth's inner core. We propagate uncertainties and place constraints on the composition of Earth's inner core by combining these results with available data on light‐element alloys of iron and seismic observations. For example, the addition of 4.3 to 5.3 wt% silicon to Fe0.95Ni0.05 alone can explain geophysical observations of the inner core boundary, as can up to 7.5 wt% sulfur with negligible amounts of silicon and oxygen. Our findings favor an inner core with less than ∼2 wt% oxygen and less than 1 wt% carbon, although uncertainties in electronic and anharmonic contributions to the equations of state may shift these values. The compositional space widens toward the center of the Earth, considering inner core seismic gradients. We demonstrate that hcp‐Fe0.91Ni0.09 and hcp‐Fe0.8Ni0.1Si0.1 have measurably greater c/a axial ratios than those of hcp‐Fe over the measured pressure range. We further investigate the relationship between the axial ratios, their pressure derivatives, and elastic anisotropy of hcp‐structured materials.

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Section: Unit Cell Axial Ratio and Anisotropysupporting

confidence: 93%

Equations of State and Anisotropy of Fe‐Ni‐Si Alloys

et al. 2018

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“…are extremely small. For example, recent calculations using the quasiharmonic approximation show that the fcc phase of pure iron may be more stable than the hcp phase beyond 6000 K (Côté et al 2012) at inner-core pressure, in agreement with results from classical molecular dynamics simulations at 6600 K (Ekholm et al 2011). The fcc structure cannot, therefore, yet be fully ruled out as a core-forming phase and so in this paper we address an important missing aspect of the story by calculating the high-temperature elasticity of fcc-Fe and the fcc-FeNi system.…”

supporting

confidence: 79%

“…Static Diamond Anvil Cell (DAC) experiments with 10 wt% Ni in Fe have shown only the hcp structure up to 340 GPa and 4700 K (Tateno et al 2012;Sakai et al 2013), but other studies with different concentrations of Ni (15, 25 and 32 wt%) suggest that Ni favours the fcc structure, although milder conditions (25 GPa, room temperature) were used in these experiments (Shabashov et al 2009). A trend towards increasing fcc stability is also observed in quasi-harmonic calculations on Fe-Ni alloys (Côté et al 2012), but classical molecular dynamics simulations at 6600 K (Ekholm et al 2011) suggest that Ni will destabilize the fcc structure with respect to hcp at 360 GPa.…”

mentioning

confidence: 84%

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

Martorell

Brodholt

Wood

et al. 2015

Geophysical Journal International

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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.

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“…Subsequent laser‐heated DAC experiments demonstrated that hcp Fe‐18.4 wt.% Ni is stable to at least 278 GPa at 2000 K [ Kuwayama et al , 2008]. More recent DAC studies by Sakai et al [2011]also showed hcp Fe‐10%Ni up to 250 GPa at 2730 K. The stability of hcp Fe 0.9 Ni 0.1 under inner core conditions is supported by ab‐initio calculations as well [ Ekholm et al , 2011]. In contrast, experiments performed by Dubrovinsky et al [2007] reported a phase transition in Fe 0.9 Ni 0.1 from hcp to bcc above 225 GPa and 3400 K. Since nickel is a fcc stabilizer, it expands the stability of fcc relative to hcp [ Lin et al , 2002; Mao et al , 2006; Kuwayama et al , 2008; Komabayashi et al , 2012].…”

Section: Introductionmentioning

confidence: 85%

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

Tateno¹,

Hirose²,

Komabayashi³

et al. 2012

Geophysical Research Letters

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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.

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Configurational thermodynamics of Fe–Ni alloys at Earth's core conditions

Cited by 14 publications

References 39 publications

Equations of State and Anisotropy of Fe‐Ni‐Si Alloys

Equations of State and Anisotropy of Fe‐Ni‐Si Alloys

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

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

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