Liquid Iron Equation of State to the Terapascal Regime From Ab Initio Simulations

摘要

We present a thermodynamic model for liquid iron, based on ab initio molecular dynamics simulations, which is applicable to 2TPa and beyond 10000K, conditions that are relevant in the cores of super-Earths. We combine ab initio results for V-T-P-E with a correction scheme to match experimental properties at ambient pressure, where ab initio results show poor agreement. We explore the performance of our thermodynamic potential and various previously published models for liquid iron over a wide range of conditions: (i) at ambient pressure as a function of temperature, (ii) along the melting curve of Fe to 40GPa, relevant for the cores of smaller terrestrial bodies in our solar system, (iii) along isentropes in the Earth's outer core, and (iv) for the core of super-Earth Kepler-36b. The correction term significantly improves the agreement of computed properties with experiments and other thermodynamic models that are based on an assessment of the phase diagram at ambient and moderate pressure, showing how ab initio molecular dynamics simulations can be used at par with other thermodynamic techniques. For the Earth's core, densities from the various models are similar, but higher-order derivatives (acoustic velocities and Gruneisen parameter) show significant differences. Evaluated along a core-temperature profile in Kepler-36b, differences in density from various models are negligible, for core mass they do not exceed 2%, showing robust extrapolation of all equation of state models.