Thermodynamic analysis of high-pressure phase equilibria in Fe–Si alloys, implications for the inner-core

A thermodynamic model was developed for the Fe–Si solid phases and applied to the calculation of high-pressure phase equilibria and thermophysical properties of Fe–Si alloys. The model is based on integration of available CALPHAD (CALculation of PHAse Diagrams) description for the Fe–Si system at ambient pressure with a composition-dependent equation of state, the parameters of which were fitted to available experimental data. At inner-core pressures and temperatures a two-phase structure is predicted, consisting of almost pure hcp-Fe and a stoichiometric CsCl-structured FeSi. Other outcomes of the model include predictions of pressure-enhanced B2B2 ordering in α-Fe–Si alloys and an anomalously high thermal expansion of the CsCl-structured FeSi compound at ambient pressure, connected with temperature-effect on its degree of order.