Model for the thermodynamics of iron at high pressures near melting

Iron phase diagram and specifically, its melting line at high pressures, have a wide range of applications, including the study of Earth’s inner core boundary (ICB). We propose an equation of state (EOS) model based on the interstitial theory of simple condensed matter (Granato, 1992) (ITCM) focused on melting at high pressures. Melting is described as resulting from a strong structural perturbation due to interstitial-like defects. The nonlinearity of their interaction leads to the stabilization of interstitial-rich phase. The model is expanded to describe melting over a wide range of pressures. The model is fitted to measured melting data and extrapolated to cover ICB conditions. The ITCM naturally yields two distinct liquid phases: HCP and FCC-based liquids. These phases and the transition between them are consistent with previous measurements. The additional liquid phase offers a new interpretation of the previously suggested near-melting high-pressure phase (Boehler, 1986) and the inner core nucleation paradox (Huguet et al., 2018). • A new consistent equation of state for iron at high pressures. • The model is based on viewing liquids as highly perturbed solids. • A new prediction of a liquid-liquid phase transition in Earth’s inner core. • The model reconciles discrepancies between various experiments. • We demonstrate that liquid iron near melting may be richer than previously suggested.