Iron under conditions close to the α − γ − ϵ triple point

The stability domains and equations of state of α-Fe, ϵ-Fe, and γ-Fe have been measured using X-ray diffraction under conditions close to their triple point: 7 ≤ P ≤ 20 GPa and 480 ≤ T ≤ 820 K. Special attention was paid to ensure the hydrostatic compression of the sample, which was a single crystal at start. Narrow α − γ and α − ϵ coexistence domains were observed, while the γ − ϵ transformation appeared sluggish. The triple point is measured at 8.7 ± 1.0 GPa and 750 ± 30 K. Anharmonic effects are evidenced in the equation of state of ϵ-Fe and partly reproduced using ab initio molecular dynamics simulations.