Anisotropic Effects of Oxygen Vacancies on Electrochromic Properties and Conductivity of $\gamma$-Monoclinic WO$_3$

TL;DR

This study uses advanced first-principles calculations to analyze how oxygen vacancies affect the optical and electrical properties of monoclinic WO$_3$, revealing anisotropic effects based on vacancy orientation.

Contribution

It provides a detailed theoretical framework explaining the anisotropic effects of oxygen vacancies on WO$_3$'s properties, aligning with experimental observations.

Findings

Oxygen vacancies cause anisotropic effects depending on W-O-W chain orientation.

Different vacancy types can be classified by their electronic and excitation properties.

Theoretical results match experimental data on oxygen-deficient WO$_3$.

Abstract

Tungsten trioxide (WO$_3$) is a paradigmatic electrochromic material, whose peculiar optical properties in the presence of oxygen vacancies or intercalated alkali atoms have been observed and investigated for a long time. In this paper we propose a rationalization of experiments based on first-principles calculations of optical and electrical properties of oxygen deficient (reduced) WO$_3$. Our approach is based on a parameter-free dielectric-dependent hybrid density functional methodology, used in combination with the charge transition levels formalism, for studying excitation mechanisms in the presence of defects. Our results indicate that oxygen vacancies lead to a different physics in $\gamma$-monoclinic WO$_3$, depending on the orientation of the W-O-W chain where the vacancy is created, thus evidencing strong anisotropic effects rooted in the peculiar structural properties of the original nondefective monoclinic cell. Different types of oxygen vacancies can hence be classified on the basis of the calculated ground state properties, electronic structure, and excitation/emission energies, giving a satisfactory explanation to a range of experimental observations made on oxygen deficient WO$_3$.