Metallic Icosahedron Phase of Sodium at Terapascal Pressures

TL;DR

This paper predicts that sodium, under extremely high pressures reaching 15.5 TPa, transitions from an insulator back to a metal due to the formation of a unique Na12 icosahedral electride structure, revealing complex high-pressure behavior.

Contribution

It introduces the first-principles prediction of sodium's reentrant metallicity and the discovery of a metallic icosahedral phase at terapascal pressures.

Findings

Sodium remains insulating from 0.2 to 15.5 TPa.

Na adopts an oP8 structure at 117-125 GPa and 1.75-15.5 TPa.

Metallization occurs via a cI24 electride structure with Na12 icosahedra.

Abstract

Alkali metals exhibit unexpected structures and electronic behavior at high pressures. Compression of metallic sodium (Na) to 200 GPa leads to the stability of a wide-band-gap insulator with the double hexagonal hP4 structure. Post-hP4 structures remain unexplored, but they are important for addressing the question of the pressure at which Na reverts to a metal. Here we report the reentrant metallicity of Na at the very high pressure of 15.5 terapascal (TPa), predicted using first-principles structure searching simulations. Na is therefore insulating over the large pressure range of 0.2-15.5 TPa. Unusually, Na adopts an oP8 structure at pressures of 117-125 GPa, and the same oP8 structure at 1.75-15.5 TPa. Metallization of Na occurs on formation of a stable and striking body-centered cubic cI24 electride structure consisting of Na12 icosahedra, each housing at its center about one electron which is not associated with any Na ions.