Magnetism and equation of states of fcc FeHx at high pressure

11 Hydrogen is a strong candidate for light alloying elements in the terrestrial cores. 12 Previous first-principles studies on non-stoichiometric hexagonal close-packed (hcp) 13 and double hexagonal close-packed (dhcp) FeH x predicted a discontinuous volume 14 expansion across the magnetic phase transition from non-magnetic (NM) or 15 antiferromagnetic (AFM) to ferromagnetic (FM) state with increasing the hydrogen 16 content, x at 0 K. However, previous high pressure and temperature neutron diffraction 17 experiments on face-centered cubic (fcc) FeH x did not show such nonlinearity. The 18 discrepancy between theory and experiment may be due to differences in the crystal 19 structure, magnetism, or temperature. In this study, we computed the equation of states 20 for fcc FeH x by using the Korringa-Kohn-Rostoker method combined with the 21 coherent potential approximation (KKR-CPA). In addition to the four types of 22 ground-state magnetism (FM, AFM-I, AFM-II, and NM), we also calculated the local 23 magnetic disorder (LMD) state, which approximates the paramagnetic (PM) state with 24 local spin moment above the Curie temperature. The results show that even though FM, 25 AFM-I, AFM-II, and NM calculations predict a discontinuity in the volume at 0 K, the 26 volume becomes continuous above the Curie temperature, consistent with the previous 27 high-temperature experiment. From the enthalpy comparison at 0 K, FM fcc FeH ( x = 28 1) undergoes the NM state above ~48 GPa. The magnetic transition pressure decreases 29 with decreasing hydrogen content. Therefore, below the magnetic transition pressure, 30 local spin moments affect the density and elastic wave velocity of fcc FeH x , which may 31 be important for small terrestrial bodies such as Mercury and Ganymede. On the other 32 hand, at the Earth’s core pressure above 135 GPa, fcc FeH x becomes NM. Thus, we 33 calculated the density and bulk sound velocity as a function of pressure at 0 K for NM