Structure and properties of deep Earth materials containing light elements under high pressure

This review summarizes recent advances in the study of hydrous and hydrogen-bearing materials under high-pressure conditions relevant to the Earth's interior. Combining neutron and synchrotron X-ray diffraction with vibrational spectroscopy, we have investigated hydrogen bonding, hydrogen site occupancies, and structure–property relationships in materials associated with the Earth's mantle and core. These studies were enabled by the development of the high-pressure neutron beamline PLANET at J-PARC. Neutron diffraction experiments revealed that hydrogen-induced volume expansion in Fe–Ni and Fe–Si alloys is significantly greater than in pure iron, implying that hydrogen contents estimated for the Earth's core may require downward revision. Investigations of dense hydrous magnesium silicates and hydroxides clarified pressure responses of hydrogen bonds and their implications for deep-Earth water transport. High-pressure studies of organic compounds further demonstrated polymerization, dehydration–condensation, peptide formation, and enantiomeric enrichment, suggesting possible pathways for molecular evolution in icy planetary interiors.