Thermal equation of state and thermodynamic properties of iron carbide Fe3C to 31 GPa and 1473 K

Konstantin D. Litasov, Igor S. Sharygin, Peter I. Dorogokupets, Anton Shatskiy, Pavel N. Gavryushkin, Tatiana S. Sokolova, Eiji Ohtani, Jie Li, Kenichi Funakoshi

Research output: Contribution to journalArticlepeer-review

36 Citations (Scopus)

Abstract

Resent experimental and theoretical studies suggested preferential stability of Fe3C over Fe7C3 at the condition of the Earth's inner core. Previous studies showed that Fe3C remains in an orthorhombic structure with the space group Pnma to 250 GPa, but it undergoes ferromagnetic (FM) to paramagnetic (PM) and PM to nonmagnetic (NM) phase transitions at 6-8 and 55-60 GPa, respectively. These transitions cause uncertainties in the calculation of the thermoelastic and thermodynamic parameters of Fe3C at core conditions. In this work we determined P-V-T equation of state of Fe3C using the multianvil technique and synchrotron radiation at pressures up to 31 GPa and temperatures up to 1473 K. A fit of our P-V-T data to a Mie-Gruneisen-Debye equation of state produce the following thermoelastic parameters for the PM-phase of Fe3C: V 0 = 154.6 (1) Å3, KT0 = 192 (3) GPa, KT′ = 4.5 (1), γ0 = 2.09 (4), θ0 = 490 (120) Cyrillic capital letter ka, and q = -0.1 (3). Optimization of the P-V-T data for the PM phase along with existing reference data for thermal expansion and heat capacity using a Kunc-Einstein equation of state yielded the following parameters: V0 = 2.327 cm3/mol (154.56 Å3), KT0 = 190.8 GPa, KT′ = 4.68, ΘE10 = 305 K (which corresponds to θ0 = 407 K), γ0 = 2.10, e0 = 9.2 × 10 -5 K-1, m = 4.3, and g = 0.66 with fixed parameters m E1 = 3n = 12, γ = 0, β = 0.3, and a 0 = 0. This formulation allows for calculations of any thermodynamic functions of Fe3C versus T and V or versus T and P. Assuming carbon as the sole light element in the inner core, extrapolation of our equation of state of the NM phase of Fe3C suggests that 3.3 ± 0.9 wt % Cyrillic capital letter es at 5000 Cyrillic capital letter ka and 2.3 ± 0.8 wt % Cyrillic capital letter es at 7000 Cyrillic capital letter ka matches the density at the inner core boundary. Key Points We present a P-V-T EOS for PM-Fe3C with support from thermodynamic analyses We discuss uncertainties in magnetic transitions We applied EOS data for modeling carbon content in the core

Original languageEnglish
Pages (from-to)5274-5284
Number of pages11
JournalJournal of Geophysical Research: Solid Earth
Volume118
Issue number10
DOIs
Publication statusPublished - 2013 Oct

Keywords

  • X-ray diffraction
  • equation of state
  • iron carbide
  • synchrotron

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

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