Thermoelastic Anomaly of Iron Carbonitride Across the Spin Transition and Implications for Planetary Cores

Carbon and nitrogen are considered as candidate light elements present in planetary cores. However, there is limited understanding regarding the structure and physical properties of Fe-C-N alloys under extreme conditions. Here diamond anvil cell experiments were conducted, revealing the stability of hexagonal-structured Fe7(N0.75C0.25)3 up to 120 GPa and 2100 K, without undergoing any structural transformation or dissociation. Notably, the thermal expansion coefficient and Gr & uuml;neisen parameter of the alloy exhibit a collapse at 55-70 GPa. First-principles calculations suggest that such anomaly is associated with the spin transition of iron within Fe7(N0.75C0.25)3. Our modeling indicates that the presence of similar to 1.0 wt% carbon and nitrogen in liquid iron contributes to 9-12% of the density deficit of the Earth's outer core. The thermoelastic anomaly of the Fe-C-N alloy across the spin transition is likely to affect the density and seismic velocity profiles of (C,N)-rich planetary cores, thereby influencing the dynamics of such cores. A significant amount of light elements are believed to be present in the cores of Earth and other planets to explain the density difference between iron-nickel alloys and geophysical observations. This study used experiments at high-pressure and high-temperature conditions and theoretical simulations to investigate a specific candidate phase of the core called Fe7(N0.75C0.25)3, which has a hexagonal structure, at high pressures similar to those in planetary cores. This phase did not undergo any major structural changes under the conditions investigated. However, its properties related to the thermoelastic behaviors showed significant changes between 55 and 70 GPa. Theoretical calculations indicate that this anomalous behavior is linked to the magnetic transition within h-type Fe7(N0.75C0.25)3. These findings suggest that the characteristics of a planetary core would be altered in the transition region, leading to a more complex thermal evolution than previously believed. This study provides insights into the behavior of light elements in planetary cores and its implications for planetary dynamics. The h-type Fe7(N0.75C0.25)3 is stable to 120 GPa and 2100 K, but undergoes a pressure-induced spin transition Spin transition causes a significant reduction in gamma 0 and alpha 0, and the thermal equation of state of h-type non-magnetic Fe7(C,N)3 is determined The density profile and seismic features of a planetary core could be altered across the spin transition, which may affect core dynamics