Publication Date:
2013-10-11
Description:
[1] Resent experimental and theoretical studies suggested preferential stability of Fe 3 C over Fe 7 C 3 at the condition of the Earth's inner core. Previous studies showed that Fe 3 C remains in an orthorhombic structure with the space group Pnma to 250 GPa, but it undergoes ferromagnetic (FM) to paramagnetic (PM) and PM to non-magnetic (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 Fe 3 C at core conditions. In this work we determined P–V–T equation of state of Fe 3 C using the multi-anvil 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 Fe 3 C: V 0 = 154.6 (1) Å 3 , K T 0 =192 (3) GPa, K T ’ = 4.5 (1), γ 0 =2.09 (4), θ 0 = 490 (120) К, 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: V 0 = 2.327 cm 3 /mol (154.56 Å 3 ), K T 0 = 190.8 GPa, K T ’ = 4.68, Θ E10 = 305 K (which corresponds to θ 0 = 407 K), γ 0 = 2.10, e 0 = 9.2 × 10 -5 K -1 , m = 4.3, and g = 0.66 with fixed parameters m E 1 = 3 n = 12, γ ∞ = 0, β = 0.3, and a 0 = 0. This formulation allows for calculations of any thermodynamic functions of Fe 3 C 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 Fe 3 C suggests that 3.3 ± 0.9 wt.% С at 5000 К and 2.3 ± 0.8 wt.% С at 7000 К matches the density at the inner core boundary.
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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