# Unexpected 3+ valence of iron in FeO$_2$, a geologically important material lying "in between" oxides and peroxides

## TL;DR

This study reveals that FeO₂ contains iron with nearly 3+ valence, significantly influencing its magnetic properties and electronic structure, positioning it uniquely between oxides and peroxides, with implications for Earth's geochemistry.

## Contribution

The paper uncovers the unexpected Fe valence state in FeO₂ and its effects on magnetic and electronic properties, highlighting its intermediate position between oxides and peroxides.

## Key findings

- Fe is nearly 3+ in FeO₂, affecting magnetic properties.
- Doping enhances magnetism in FeO₂.
- FeO₂'s electronic structure is intermediate between oxides and peroxides.

## Abstract

Recent discovery of pyrite FeO$_2$, which can be an important ingredient of the Earth's lower mantle and which in particular may serve as an extra source of water in the Earth's interior, opens new perspectives for geophysics and geochemistry, but this is also an extremely interesting material from physical point of view. We found that in contrast to naive expectations Fe is nearly 3+ in this material, which strongly affects its magnetic properties and makes it qualitatively different from well known sulfide analogue - FeS$_2$. Doping, which is most likely to occur in the Earth's mantle, makes FeO$_2$ much more magnetic. In addition we show that unique electronic structure places FeO$_2$ "in between" the usual dioxides and peroxides making this system interesting both for physics and solid state chemistry.