Revised α/ε′–γ phase boundaries for the Fe–H system

At high temperatures and high pressures, iron reacts with hydrogen to form hydrides. Three hydride phases are present at moderate temperature and pressure ranges below 2000 K and 20 GPa: α, γ, and e′ phases, respectively, with bcc, fcc, and double-hcp (dhcp) structures. Recent reports have described studies of phase transitions at temperatures and pressures that differ greatly from those predicted from the conventional phase diagram for the Fe–H system established in the early 2000s. We conducted in-situ X-ray diffraction experiments to investigate α/e′–γ phase boundaries at temperatures up to 900 K and pressures up to 9.1 GPa. Phase transition temperatures were determined by averaging those observed upon heating and cooling under constant pressure. The α/e′–γ phase boundaries were found at temperatures 150–300 K higher than the conventional phase boundaries. The α–γ–e′ triple point was expected to relocate accordingly.