Unexpected 3+valence of iron in FeO2, a geologically important material lying in between oxides and peroxides

Recent discovery of the pyrite FeO2, 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 - FeS2. Doping, which is most likely to occur in the Earth's mantle, makes FeO2 much more magnetic. In addition we show that unique electronic structure places FeO2 in between the usual dioxides and peroxides making this system interesting both for physics and solid state chemistry.