Phase transition boundary between fcc and hcp structures in Fe-Si alloy and its implications for terrestrial planetary cores

Komabayashi, Tetsuya; Pesce, G.; Morard, Guillaume; Antonangeli, Daniele; Sinmyo, Ryosuke; Mézouar, M.

doi:10.2138/am-2019-6636

Cited by 21 publications

(44 citation statements)

References 45 publications

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“…The substitutional solid solution commonly occurs in Fe-Si alloys. Both theoretical and experimental results show that Fe-Si alloys with hcp structure always have a larger unit-cell volume comparing to that of the pure hcp-Fe [39,40]. On the other hand, theoretical calculation shows that the hcp phase containing ∼6 at.…”

Section: Carbon Substitution Mechanismmentioning

confidence: 93%

Effect of Carbon on the Volume of Solid Iron at High Pressure: Implications for Carbon Substitution in Iron Structures and Carbon Content in the Earth’s Inner Core

Yang

Fei

et al. 2019

Minerals

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Understanding the effect of carbon on the density of hcp (hexagonal-close-packed) Fe-C alloys is essential for modeling the carbon content in the Earth’s inner core. Previous studies have focused on the equations of state of iron carbides that may not be applicable to the solid inner core that may incorporate carbon as dissolved carbon in metallic iron. Carbon substitution in hcp-Fe and its effect on the density have never been experimentally studied. We investigated the compression behavior of Fe-C alloys with 0.31 and 1.37 wt % carbon, along with pure iron as a reference, by in-situ X-ray diffraction measurements up to 135 GPa for pure Fe, and 87 GPa for Fe-0.31C and 109 GPa for Fe-1.37C. The results show that the incorporation of carbon in hcp-Fe leads to the expansion of the lattice, contrary to the known effect in body-centered cubic (bcc)-Fe, suggesting a change in the substitution mechanism or local environment. The data on axial compressibility suggest that increasing carbon content could enhance seismic anisotropy in the Earth’s inner core. The new thermoelastic parameters allow us to develop a thermoelastic model to estimate the carbon content in the inner core when carbon is incorporated as dissolved carbon hcp-Fe. The required carbon contents to explain the density deficit of Earth’s inner core are 1.30 and 0.43 wt % at inner core boundary temperatures of 5000 K and 7000 K, respectively.

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Section: Carbon Substitution Mechanismmentioning

confidence: 93%

Effect of Carbon on the Volume of Solid Iron at High Pressure: Implications for Carbon Substitution in Iron Structures and Carbon Content in the Earth’s Inner Core

Yang

Fei

et al. 2019

Minerals

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“…While thermodynamic models including mixing property of liquids were established for the system Fe-FeO under core P-T conditions (Frost et al 2010;Komabayashi 2014), a self-consistent thermodynamic model for the system Fe-(Fe)Si which can be applicable to Earth's core has yet to be constructed although the system has extensively been studied by both experiment and first-principles calculation (Alfe et al 2002;Dobson et al 2002;Lin et al 2002;Kuwayama and Hirose 2004;Tateno et al 2015;Ozawa et al 2016;Komabayashi et al 2019). A thermodynamic model for the system Fe-Si needs to address mixing property of Fe and Si in iron phases, since these phases form solutions and their nonideality affects the P-T locations of chemical reactions involving them.…”

Section: Introductionmentioning

confidence: 99%

“…In this study, the mixing properties of Si-bearing iron phases are first examined by thermodynamic calculation of a phase transition loop. Komabayashi et al (2019) constrained the width of the transition between fcc and hcp structures in Fe-4wt%Si to 71 GPa and 2000 K in an internally resistiveheated diamond anvil cell (DAC) which enabled very precise measurements ( Fig. 1).…”

Section: Introductionmentioning

confidence: 99%

Thermodynamics of the system Fe–Si–O under high pressure and temperature and its implications for Earth’s core

Komabayashi

2020

Phys Chem Minerals

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The thermodynamics of the system Fe-Si-O under high pressure (P) and temperature (T) was examined, starting with modelling the phase transition between a face-centred cubic (fcc) and hexagonal close-packed (hcp) structure in Fe-Si alloy which was previously examined by experiment under high P-T conditions. The mixing properties of Fe and Si for the iron phases were found to be approximated by ideal mixing under high P and T conditions. The entropy changes upon melting of the end-members of the system are fairly large, and therefore the melting temperature of the Si-bearing fcc or hcp phases needs to be insensitive to the Si content, to account for the reported close compositions of coexisting liquid and solid (< 1 wt%Si at P > 50 GPa). The solidus and liquidus temperatures of Fe-Si iron alloy would therefore, not significantly be changed by the presence of Si at the inner core-outer core boundary, which enables us to evaluate the melting curve of Fe-Si fcc and hcp phases. From thus-constrained melting curve, I assessed a thermal equation of state of Si-bearing iron liquid. I then estimated a seismologically consistent outer core composition as a function of Si and O contents using the EoS for liquids constructed in this study and the literature. The best-fit composition is Fe-5.8(0.6) wt%Si-0.8(0.6) wt%O, which however does not precipitate a solid iron phase that is consistent with the inner core density. Therefore, Earth's core cannot be fully represented by the system Fe-Si-O and it should include another light element.

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“…Additionally, the comparative effect of nickel and light elements on the elasticity and equation of state of a ternary alloy needs further investigation, with only few experimental studies in literature that have been limited to ambient temperature [7,8,10,12]. In particular, amongst the light elements that have been proposed to enter into the inner core composition, silicon has recently received much attention [7,8,10,[19][20][21][22][23], as both core differentiation models [24,25] and isotopic arguments [26] have supported its presence at a small level of wt%.…”

Section: Introductionmentioning

confidence: 99%

Axial Compressibility and Thermal Equation of State of Hcp Fe–5wt% Ni–5wt% Si

et al. 2020

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Knowledge of the elastic properties and equations of state of iron and iron alloys are of fundamental interest in Earth and planetary sciences as they are the main constituents of telluric planetary cores. Here, we present results of X-ray diffraction measurements on a ternary Fe–Ni–Si alloy with 5 wt% Ni and 5 wt% Si, quasi-hydrostatically compressed at ambient temperature up to 56 GPa, and under simultaneous high pressure and high temperature conditions, up to 74 GPa and 1750 K. The established pressure dependence of the c/a axial ratio at ambient temperature and the pressure–volume–temperature (P–V–T) equation of state are compared with previous work and literature studies. Our results show that Ni addition does not affect the compressibility and axial compressibility of Fe–Si alloys at ambient temperature, but we suggest that ternary Fe–Ni–Si alloys might have a reduced thermal expansion in respect to pure Fe and binary Fe–Si alloys. In particular, once the thermal equations of state are considered together with velocity measurements, we conclude that elements other than Si and Ni have to be present in the Earth’s inner core to account for both density and seismic velocities.

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Phase transition boundary between fcc and hcp structures in Fe-Si alloy and its implications for terrestrial planetary cores

Cited by 21 publications

References 45 publications

Effect of Carbon on the Volume of Solid Iron at High Pressure: Implications for Carbon Substitution in Iron Structures and Carbon Content in the Earth’s Inner Core

Effect of Carbon on the Volume of Solid Iron at High Pressure: Implications for Carbon Substitution in Iron Structures and Carbon Content in the Earth’s Inner Core

Thermodynamics of the system Fe–Si–O under high pressure and temperature and its implications for Earth’s core

Axial Compressibility and Thermal Equation of State of Hcp Fe–5wt% Ni–5wt% Si

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