Microscopic origins of the anomalous melting behaviour of high-pressure sodium

TL;DR

This paper investigates the unusual melting behavior of high-pressure sodium, revealing that electron screening causes a softening of interionic interactions, leading to a pressure-induced decrease in melting temperature.

Contribution

It introduces an ab-initio neural-network potential to accurately model sodium's phase diagram and explains the anomalous melting via electron screening effects.

Findings

Reentrant melting behavior due to electron screening.

Softening of short-range repulsion at high pressure.

Validation of the phase diagram with neural-network potential.

Abstract

Recent experiments have shown that sodium, a prototype simple metal at ambient conditions, exhibits unexpected complexity under high pressure. One of the most puzzling phenomena in the behaviour of dense sodium is the pressure-induced drop in its melting temperature, which extends from 1000 K at ~30GPa to as low as room temperature at ~120GPa. Despite significant theoretical effort to understand the anomalous melting its origins have remained unclear. In this work, we reconstruct the sodium phase diagram using an ab-initio-quality neural-network potential. We demonstrate that the reentrant behaviour results from the screening of interionic interactions by conduction electrons, which at high pressure induces a softening in the short-range repulsion. It is expected that such an effect plays an important role in governing the behaviour of a wide range of metals and alloys.