First-principles theory of coloration of WO$_3$ upon charge insertion

TL;DR

This study uses first-principles calculations to explain how charge insertion in WO$_3$ causes systematic color changes, highlighting the importance of interband and intraband optical contributions.

Contribution

It provides a detailed first-principles explanation of the coloration mechanism in Na$_x$WO$_3$, emphasizing the roles of interband and intraband effects.

Findings

Color changes from dark blue to golden-yellow with increasing sodium content.

Proper modeling of optical response requires both interband and intraband contributions.

Theoretical results align well with experimental color observations.

Abstract

We report first-principles investigations of the coloration of WO$_3$ upon charge insertion, using sodium tungsten bronze (Na$_x$WO$_3$) as a model system. Our results explain well the systematic color change of Na$_x$WO$_3$ from dark blue to violet, red-orange, and finally to golden-yellow as sodium concentration $x$ increases from 0.3 to unity. Proper accounts for both the interband and the intraband contributions to the optical response are found to be very important for a detailed understanding of the coloration mechanism in this system.