Sodium: a charge-transfer insulator at high pressures

TL;DR

This study uses first-principles calculations to explore the optical properties of high-pressure sodium, revealing a unique charge-transfer exciton and anisotropic transparency that informs about its crystal structure as an unconventional electride.

Contribution

It uncovers a novel charge-transfer exciton in dense sodium and links optical anisotropy to its crystal structure at high pressures.

Findings

Sodium exhibits anisotropic optical transparency above the metal-insulator transition.

A new type of charge-transfer exciton is predicted in dense sodium.

Optical response is strongly polarization-dependent at high pressures.

Abstract

By means of first-principles methods we analyze the optical response of transparent dense sodium as a function of applied pressure. We discover an unusual kind of charge-transfer exciton that proceeds from the interstitial distribution of valence electrons repelled away from the ionic cores by the Coulomb interaction and the Pauli repulsion. The predicted absorption spectrum shows a strong anisotropy with light polarization that just at pressures above the metal-insulator transition manifests as sodium being optically transparent in one direction but reflective in the other. This result provides a key information about the crystal structure of transparent sodium, a new unconventional inorganic electride.