TY - JOUR

T1 - Equations of state of iron nitrides ε-Fe3Nx and γ-Fe4Ny to 30 GPa and 1200 K and implication for nitrogen in the Earth's core

AU - Litasov, K. D.

AU - Shatskiy, A.

AU - Ponomarev, D. S.

AU - Gavryushkin, P. N.

PY - 2017/5/1

Y1 - 2017/5/1

N2 - Nitrogen abundance is one of the most uncertain among all elements in the Earth's interior. Recent data indicate an affinity between Fe-nitrides and Fe-carbides in the Earth's mantle and inner core. In this work P-V-T equations of state of ε-Fe3N0.8 and ε-Fe3N1.26 (which is close to Fe7N3) have been determined using a combination of multianvil and synchrotron radiation techniques at pressures up to 30 GPa and temperatures up to 1473 K. A fit of the P-V-T data to the Vinet-Rydberg and Mie-Grüneisen-Debye equations of state yields the following thermoelastic parameters for the ε-Fe3N0.8: V0 = 81.44(2) Å3, KT0 = 157(3) GPa, KT′ = 5.3 (fixed), θ0 = 555 K (fixed), γ0 = 1.83(1), and q = 1.34(18). For ε-Fe3N1.26 we obtained V0 = 86.18(2) Å3, KT0 = 163(2) GPa, KT′ = 5.3(2), θ0 = 562(90) K, γ0 = 1.85(2), and q = 0.55(24). It is likely that all presumably paramagnetic ε-Fe3Nx with x = 0.75–1.5 have similar thermoelastic properties with a minor increase of the bulk modulus with increasing N content. The melting temperature of ε-Fe3Nx increases from approximately 1473 to 1573 K in the pressure range from 5 to 30 GPa. We also determined a preliminary equation of state for γ-Fe4Ny and calculated y = 0.35(2) from the data at 20–30 GPa. Combining the results with a recent experimental study on the stability of β-Fe7N3, isostructural with Fe7C3, and a theoretical study of the magnetic transitions in ε-Fe3Nx, we estimate the density of Fe-nitrides at the Earth's inner core conditions. Our results indicate that at 5000–6000 K, 2.0–3.2 wt % N can explain the density deficit in Earth's inner core.

AB - Nitrogen abundance is one of the most uncertain among all elements in the Earth's interior. Recent data indicate an affinity between Fe-nitrides and Fe-carbides in the Earth's mantle and inner core. In this work P-V-T equations of state of ε-Fe3N0.8 and ε-Fe3N1.26 (which is close to Fe7N3) have been determined using a combination of multianvil and synchrotron radiation techniques at pressures up to 30 GPa and temperatures up to 1473 K. A fit of the P-V-T data to the Vinet-Rydberg and Mie-Grüneisen-Debye equations of state yields the following thermoelastic parameters for the ε-Fe3N0.8: V0 = 81.44(2) Å3, KT0 = 157(3) GPa, KT′ = 5.3 (fixed), θ0 = 555 K (fixed), γ0 = 1.83(1), and q = 1.34(18). For ε-Fe3N1.26 we obtained V0 = 86.18(2) Å3, KT0 = 163(2) GPa, KT′ = 5.3(2), θ0 = 562(90) K, γ0 = 1.85(2), and q = 0.55(24). It is likely that all presumably paramagnetic ε-Fe3Nx with x = 0.75–1.5 have similar thermoelastic properties with a minor increase of the bulk modulus with increasing N content. The melting temperature of ε-Fe3Nx increases from approximately 1473 to 1573 K in the pressure range from 5 to 30 GPa. We also determined a preliminary equation of state for γ-Fe4Ny and calculated y = 0.35(2) from the data at 20–30 GPa. Combining the results with a recent experimental study on the stability of β-Fe7N3, isostructural with Fe7C3, and a theoretical study of the magnetic transitions in ε-Fe3Nx, we estimate the density of Fe-nitrides at the Earth's inner core conditions. Our results indicate that at 5000–6000 K, 2.0–3.2 wt % N can explain the density deficit in Earth's inner core.

KW - Earth's core

KW - equation of state

KW - iron

KW - nitride

KW - nitrogen

KW - HIGH-PRESSURE

KW - THERMODYNAMIC PROPERTIES

KW - CARBON

KW - INCLUSIONS

KW - DIAMOND

KW - OF-STATE

KW - FE3C

KW - 1ST-PRINCIPLES CALCULATIONS

KW - DIFFRACTION

KW - GPA

UR - http://www.scopus.com/inward/record.url?scp=85019921988&partnerID=8YFLogxK

U2 - 10.1002/2017JB014059

DO - 10.1002/2017JB014059

M3 - Article

AN - SCOPUS:85019921988

VL - 122

SP - 3574

EP - 3584

JO - Journal of Geophysical Research: Solid Earth

JF - Journal of Geophysical Research: Solid Earth

SN - 2169-9313

IS - 5

ER -