Polaron formation, native defects, and electronic conduction in metal tungstates

TL;DR

This study uses first-principles calculations to explore defect physics and polaron formation in FeWO4 and MnWO4, revealing their native defects and charge transport mechanisms relevant for energy storage applications.

Contribution

It provides a detailed first-principles analysis of defect physics and polaron formation in FeWO4 and MnWO4, advancing understanding of their electronic conduction properties.

Findings

Hole polarons form at transition-metal sites.

Negatively charged vacancies are dominant defects.

Materials exhibit good p-type conductivity and pseudocapacitive energy storage.

Abstract

Iron tungstate (FeWO$_4$) and manganese tungstate (MnWO$_4$) belong to a family of wolframite-type materials that has applications in various areas, including supercapacitors, batteries, and multiferroics. A detailed understanding of bulk properties and defect physics in these transition-metal tungstates has been lacking, however, impeding possible improvement of their functional properties. Here, we report a first-principles study of FeWO$_4$ and MnWO$_4$ using screened hybrid density-functional calculations. We find that in both compounds the electronic structure near the band edges are predominantly the highly localized transition-metal $d$ states, which allows for the formation of both hole polarons at the Fe (Mn) sites and electron polarons at the W sites. The dominant native point defects in FeWO$_4$ (MnWO$_4$) under realistic synthesis conditions are, however, the hole polarons at the Fe (Mn) sites and negatively charged Fe (Mn) vacancies. The presence of low-energy and highly mobile polarons provides explanation for the good p-type conductivity observed in experiments and the ability of the materials to store energy via a pseudocapacitive mechanism.