[S-IT21] Planetary cores: Structure, formation, and evolution

[SIT21-03] Magnetism and equation of states of fcc FeH_x at high pressure

*Hitoshi Gomi¹, Kei Hirose^1,2 (1.Tokyo Institute of Technology, 2.The University of Tokyo)

Keywords:FeHx, magnetism, equation of states, KKR-CPA, inner core

Hydrogen is a strong candidate for light alloying elements in the terrestrial cores. Previous first-principles studies on non-stoichiometric hexagonal close-packed (hcp) and double hexagonal close-packed (dhcp) FeH_x predicted a discontinuous volume expansion across the magnetic phase transition from non-magnetic (NM) or antiferromagnetic (AFM) to ferromagnetic (FM) state with increasing the hydrogen content, x at 0 K. However, previous high pressure and temperature neutron diffraction experiments on face-centered cubic (fcc) FeH_x did not show such nonlinearity. The discrepancy between theory and experiment may be due to differences in the crystal structure, magnetism, or temperature. In this study, we computed the equation of states for fcc FeH_x by using the Korringa-Kohn-Rostoker method combined with the coherent potential approximation (KKR-CPA). In addition to the four types of ground-state magnetism (FM, AFM-I, AFM-II, and NM), we also calculated the local magnetic disorder (LMD) state, which approximates the paramagnetic (PM) state with local spin moment above the Curie temperature. The results show that even though FM, AFM-I, AFM-II, and NM calculations predict a discontinuity in the volume at 0 K, the volume becomes continuous above the Curie temperature, consistent with the previous high-temperature experiment. From the enthalpy comparison at 0 K, FM fcc FeH (x = 1) undergoes the NM state above ~48 GPa. The magnetic transition pressure decreases with decreasing hydrogen content. Therefore, below the magnetic transition pressure, local spin moments affect the density and elastic wave velocity of fcc FeH_x, which may be important for small terrestrial bodies such as Mercury and Ganymede. On the other hand, at the Earth’s core pressure above 135 GPa, fcc FeH_x becomes NM. Thus, we calculated the pressure and bulk sound velocity as a function of density at 0 K for NM fcc FeH_x. FeH_0.5 agrees with the preliminary reference Earth model (PREM) for pressure and bulk sound velocity at the inner core density. However, note that our first-principles calculations are conducted at 0 K, and the high-temperature effect increases pressure and decreases bulk sound velocity. Therefore, considering the thermal pressure, the hydrogen content should be smaller than 0.5. On the other hand, high-temperature effects on the bulk sound velocity require the hydrogen content greater than 0.5. Therefore, hydrogen alone cannot account for the light elements in the Earth’s inner core.