Phase Diagram of Iron by in Situ X-ray Diffraction: Implications for Earth's Core

The phase diagram of iron has been studied to 130 gigapascals (1 gigapascal = 10 4 atmospheres) and 3500 kelvin by a combined laser-heated diamond-anvil cell and x-ray diffraction technique that provides direct identification of the solid phases. Iron in the hexagonal close-packed (hcp) phase (ε-Fe) is stable from 50 to at least 110 gigapascals at high temperatures. The wide stability field of ε-Fe indicates that this polymorph should currently be considered the most relevant solid phase for Earth's core. The triple point between the γ, ε, and liquid phases is located at 2500 ± 200 kelvin and 50 ± 10 gigapascals. There is evidence for a phase with a double hcp structure below 40 gigapascals and for another transition above 110 gigapascals and 3000 kelvin.