Phase Stability and Thermal Equation of State of Iron Carbide Fe3C to 245 GPa

• Published in: Geophysical research letters 2019-10, Vol.46 (20), p.11018-11024
• Main Authors: Hu, Xiaojun, Fei, Yingwei, Yang, Jing, Cai, Yang, Ye, Shijia, Qi, Meilan, Liu, Fusheng, Zhang, Mingjian
• Format: Article
• Language: English
• Subjects: Adiabatic; Cementite; Compression; core composition; core density; Data compression; Decomposition; Density; Density profiles; Earth; Earth core; Equations of state; Gruneisen parameter; Hugoniot curves; Iron; iron carbide; Iron carbides; Mathematical analysis; Orbital velocity; Phase stability; Pressure; Shock waves; shockwave compression; Velocity; Wave velocity
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2019GL084545

We conducted shock wave experiments on iron carbide Fe3C up to a Hugoniot pressure of 245 GPa. The correlation between the particle velocity (up) and shock wave velocity (us) can be fitted into a linear relationship, us = 4.627(±0.073) + 1.614(±0.028) up. The density‐pressure relationship is consistent with a single‐phase compression without decomposition. The inference is further supported by the comparison of the observed Hugoniot density with the calculated Hugoniot curves of possible decomposition products. The new Hugoniot data combined with the reported 300‐K isothermal compression data yielded a Grüneisen parameter of γ = 2.23(7.982/ρ)0.29. The thermal equation of state of Fe3C is further used to calculate the density profile of Fe3C along the Earth's adiabatic geotherm. The density of Fe3C was found to be too low (by ~5%) to match the observed density in the Earth's inner core, and Fe3C is unlikely a dominant component of the inner core. Key Points The stability of Fe3C is evaluated under dynamic compression, and no density discontinuity is observed up to 245 GPa Thermal equation of state of Fe3C is established by combining dynamic and static compression data The density of Fe3C is 5% lower than that of the inner core, and Fe3C is unlikely a component of the inner core