Absolute free energies from ab-initio calculation: application to phase stability of liquid metal alloys

Absolute liquid state entropies calculated using the "S2" method can be combined with ab-initio enthalpies to predict absolute Gibbs free energies. We apply this method to model the phase behavior of the liquid alkali metal alloy Li-Na, which is known to phase separate, compared with Na-K, which exhibits a eutectic. Our ab-initio simulations employ hybrid Monte Carlo/molecular dynamics to accelerate sampling of the ensemble and provide accurate pair correlation functions for the entropy calculation. Both alloys exhibit effective unlike-atom repulsion, but the weaker repulsion of Na-K allows an entropically stabilized eutectic, while the stronger repulsion of Li-Na overwhelms the entropy below a high temperature critical point which we approximately locate.