Estimating melting curves for Cu and Al from simulations at a single state point

TL;DR

This paper demonstrates a method to accurately estimate melting curves of metals like Cu and Al from simulations at a single state point by leveraging the concept of isomorphs and hidden scale invariance.

Contribution

It applies the isomorph theory to metals, validating the approach with Cu and Al, and proposes that metals may exhibit re-entrant melting due to density-scaling behavior.

Findings

The method accurately predicts Cu melting curves validated by two-phase simulations.

For Al, the method matches experimental and DFT melting data.

Metals may undergo re-entrant melting with a maximum melting temperature at certain pressures.

Abstract

Determining the melting curves of materials up to high pressures has long been a challenge experimentally and theoretically. A large class of materials, including most metals, has been shown to exhibit hidden scale invariance, an approximate scale invariance of the potential-energy landscape that is not obvious from the Hamiltonian. For these materials the isomorph theory allows the identification of curves in the phase diagram along which structural and dynamical properties are invariant to a good approximation when expressed in appropriately scaled form. These curves, the isomorphs, can also be used as the basis for constructing accurate melting curves from simulations at a single state point [U. R. Pedersen et al., Nat. Comm. 7, 12386 (2016)]. In this work we apply this method to the metals Cu simulated using the effective medium theory and Al simulated using density functional theory (DFT). For Cu the method works very well and is validated using two-phase melting point simulations. For Al there are likewise good isomorphs, and the method generates the melting curve accurately as compared to previous experimental and DFT results. In support of a recent suggestion of Hong and van de Walle [Phys. Rev. B 100, 140102 (2019)], we finally suggest that the tendency for the density-scaling exponent $\gamma$ to decrease with increasing density in metals implies that metals in general will undergo re-entrant melting, i.e., have a maximum of melting temperature as a function of pressure.