In-situ high pressure and temperature neutron diffraction study of the Fe-H system and hydrogen in the planetary core

Hydrogen is one of the candidate for the light elements in the Earth's core. In order to quantify the hydrogen content in the core, many researches have been made to determine phase relations, equations of state, and sound velocity measurements of iron hydride, FeH_x. In spite of intensive researches on iron hydride, precise determination of volume expansion due to hydrogen dissolution in the iron lattice using the in-situ neutron diffraction study has not been conducted yet at high pressure and temperature.

We conducted in situ neutron diffraction studies of iron hydride up to 12 GPa and 1200 K and clarified its phase relations at high pressure and temperature by using the cubic press, Atsuhime, installed at the PLANET beamlime of J-PARC. The phase boundaries among bcc, fcc, and dhcp-FeH_x determined by the present in situ neutron diffraction experiments are significantly higher than those reported previously.

At pressures below 5 GPa, the hydrogen content, x, in the fcc-iron lattice is relatively small less than 0.3, whereas it increases to above 0.8 with increasing pressure. We made a first observation of site occupation of hydrogen atoms in both octahedral (O) and tetrahedral (T) sites in iron lattice, and their typical site occupancies are 0.86(1) in O-sites and 0.06(1) in T-sites at 12 GPa and 1200 K.

The fcc-iron lattice expands approximately linearly with a rate of 2.22(36) ų per hydrogen atom, which is comparable with deuterium in the fcc-iron lattice. The pressure dependency of the fcc-iron lattice expansion by dissolution of hydrogen atoms is negligible in the pressure range from 3 to 12 GPa. The present direct determination of the hydrogen interstitial volume expansion in fcc-iron lattice can modify the estimation of the hydrogen content in the core. The hydrogen contents in the core may be overestimated, since they were based on a smaller volume expansion by interstitial hydrogen atoms (1.9 ų per hydrogen atom). The revised analyses indicate that whole core may contain hydrogen of 80(±31) times of the ocean mass with 79(±30) and 0.8(±0.3) ocean mass for the outer and inner cores, respectively.

This work was supported by JSPS Kakenhi Grant 15H05748 to EO. LY was supported by International Joint Graduate Program in Earth and Environmental Science (GP-EES) and the JSPS Japanese-German Graduate Externship.