The refractive index and electronic gap of water and ice increase with increasing pressure

TL;DR

This study uses ab initio calculations to show that both the refractive index and electronic gap of water and ice increase with pressure up to 30 GPa, challenging previous assumptions of inverse correlation.

Contribution

It provides new insights into the pressure dependence of water and ice electronic properties through detailed electronic structure calculations.

Findings

Refractive index of water and ice increases with pressure.

Electronic gap of water and ice increases with pressure.

Electronic effects under pressure explain the anomalous behavior.

Abstract

Determining the electronic and dielectric properties of water at high pressure and temperature is an essential prerequisite to understand the physical and chemical properties of aqueous environments under supercritical conditions, e.g. in the Earth interior. However optical measurements of compressed ice and water remain challenging and it has been common practice to assume that their band gap is inversely correlated to the measured refractive index, consistent with observations reported for hundreds of materials. Here we report ab initio molecular dynamics and electronic structure calculations showing that both the refractive index and the electronic gap of water and ice increase with pressure, at least up to 30 GPa. Subtle electronic effects, related to the nature of interband transitions and band edge localization under pressure, are responsible for this apparently anomalous behavior.