Compression of iron hydride to 80 GPa and hydrogen in the Earth's inner core

Here we report a new data on the equation of state of iron hydride up to 80 GPa measured by an in situ X-ray diffraction method. We observed anomalous compression behavior at 30 to 50 GPa, and found that iron hydride might be less compressible above 50 GPa than at lower pressures. The bulk modulus above 50 GPa is larger than that of pure hcp-iron. Our data support the ab initio calculation suggesting a magnetic transition of iron hydride at around 60 GPa. The density deficit of the Earth's inner core could be explained by dissolution of smaller amount of hydrogen than previously suggested.