The influence of hydrogen on the seismic properties of solid iron

We estimate the implications of having hydrogen dissolved in the solid core from a mineral physics perspective. Based on first-principles calculations, we observe that hydrogen has a strong effect on decreasing the density of the hexagonal close-packed structure of Fe. The structural effect is weakly dependent on the ordering pattern of the hydrogen defects. However, hydrogen also increases the elastic moduli of Fe at core pressures. Both the compressional and the shear wave velocities increase with addition even of small amounts of hydrogen, the increase of shear going against the geophysical requirements for light elements. This aspect makes hydrogen an undesirable light element in the Earth's core if we try to match the seismological observations.