Hexagonal Close-packed Iron Hydride behind the Conventional Phase Diagram

Hexagonal close-packed iron hydride, hcp FeH x , is absent from the conventional phase diagram of the Fe–H system, although hcp metallic Fe exists stably over extensive temperature ( T ) and pressure ( P ) conditions, including those corresponding to the Earth’s inner core. In situ X-ray and neutron diffraction measurements at temperatures ranging from 298 to 1073 K and H pressures ranging from 4 to 7 GPa revealed that the hcp hydride was formed for FeH x compositions when x