A Electronic band structure of iron superconductor BaFe2As2

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how can I determinate the electronic structure of a multiband system? for example in the case of the superconductor BaFe2As2 , the electronic properties are as known dominated by 5 Fe d states at the fermi energy, so 5 bands cross the Fermi level and form 5 Fermi surface sheets (if we consider 1Fe/unit cell) and they are responsable for conduction, and then, we can determinate the electron and hole pockets of the structure....

I don't get first how to identify the bands that participate in the conduction, the iron is in the configuration of Fe2+, isn't it? Doesn't that mean that the electronic configuration is like [Ar] 4s2 3d4 and then we have only 4 valence electrons in d orbitals? so the last d orbital isn't empty? if so, from where it comes the 5 d bands that crosses the Fermi level?

my mind is in a mess, maybe I'm totally wrong and important considerations should be taken into account , I need some clarifications if possible please
 
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I don't get first how to identify the bands that participate in the conduction, the iron is in the configuration of Fe2+, isn't it? Doesn't that mean that the electronic configuration is like [Ar] 4s2 3d4 and then we have only 4 valence electrons in d orbitals? so the last d orbital isn't empty? if so, from where it comes the 5 d bands that crosses the Fermi level?
As general rule, it is not very reasonable to expect that atoms, when condensed into a material, will follow a simple Hund's filling. Here is an article on the 122 pnictides that includes both theory and experiment that demonstrate the important role of correlations in establishing the electronic properties, including filling.
 

ZapperZ

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how can I determinate the electronic structure of a multiband system? for example in the case of the superconductor BaFe2As2 , the electronic properties are as known dominated by 5 Fe d states at the fermi energy, so 5 bands cross the Fermi level and form 5 Fermi surface sheets (if we consider 1Fe/unit cell) and they are responsable for conduction, and then, we can determinate the electron and hole pockets of the structure....

I don't get first how to identify the bands that participate in the conduction, the iron is in the configuration of Fe2+, isn't it? Doesn't that mean that the electronic configuration is like [Ar] 4s2 3d4 and then we have only 4 valence electrons in d orbitals? so the last d orbital isn't empty? if so, from where it comes the 5 d bands that crosses the Fermi level?

my mind is in a mess, maybe I'm totally wrong and important considerations should be taken into account , I need some clarifications if possible please
You can't simply look at the configuration of each individual atoms and expect that you can get an accurate idea of what bands do what. This is a many-body effect, and you have overlaps and hybridizations, etc... all complicate the picture.

For example, just look at the tight-binding band structure. Look at how the band dispersion changes just by going from including nearest neighbor to next-nearest neighbor in the hopping integral.

This is why people have to use techniques such as DFT to calculate band structures. And even then, the band-structure picture can have limitations.

Zz.
 

TeethWhitener

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Roald Hoffman wrote a phenomenal book on solid state physics starting from the point of view of atomic and molecular chemistry that helped me wrap my head around some of the concepts mentioned by other posters:
 

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