Title: Unexpected 3+ valence of iron in FeO$_2$, a geologically important material lying "in 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.