Hydrogen in solid iron under core conditions

Abstract:

Based on first-principles calculations, we observe that hydrogen has a strong effect on decreasing the density of the hexagonal close-packed (hcp) structure of iron. The structural effect is weakly dependent on the ordering pattern of the H defects. However, hydrogen also hardens iron at core pressures. Our theoretical results show that about 0.3 wt.% are needed to fit the density and the compressional velocities of the inner core at equal density and static conditions. Considering the electronic temperature implies larger H amounts. But hydrogen also increases the shear wave velocities of iron. The latter aspect goes against seismic observations that require a decrease of Vs by light elements, even at high-temperature conditions. Any addition of hydrogen to iron must then be compensated by further addition of other light elements, which in turn will make impossible the fitting of the core density. In conclusion, hydrogen is not a "good" light element for the core, in a geophysical sense. We equally discuss the behavior of H at high-temperature in hcp iron with a special emphasis on its mobility.