Intraatomic exchange

U - A is reduced significantly by the intraatomic-exchange stabilization Ae. Where w = U, this energy is reduced still further by screening, and a charge-transfer disproportionation reaction becomes possible. This most favorable situation applies in the case of CaFeOa-... 

The localized-electron model or the ligand-field approach is essentially the same as the Heitler-London theory for the hydrogen molecule. The model assumes that a crystal is composed of an assembly of independent ions fixed at their lattice sites and that overlap of atomic orbitals is small. When interatomic interactions are weak, intraatomic exchange (Hund s rule splitting) and electron-phonon interactions favour the localized behaviour of electrons. This increases the relaxation time of a charge carrier from about 10 s in an ordinary metal to 10 s, which is the order of time required for a lattice vibration in a polar crystal. 

Localized electrons in a partially filled shell carry a net spin (provided Hund s rule is obeyed), and interactions between these localized electrons and the collective electrons gives rise to a large effective field Hex acting on the collective electrons. Since the intraatomic exchange correlations minimize the electrostatic interactions between electrons of parallel spin, Hcx is directed parallel to the atomic moment due to localized electrons. Below a magnetic-ordering temperature, this internal field induces a contribution to the atomic moment from the collective electrons whether the localized electrons are ordered parallel or antiparallel. From equation 58, this contribution is... 

Although Fe304 is cited as the classical example of this effect, it should be noted that the transition temperature is about four times smaller than is predicted from electrostatic considerations. Also the room-temperature Hall mobilities of the charge carriers are 0.5 cm2/V-sec (719), which might be thought to represent narrow-band conductivity. (In Fe304 there arc 3.5 t2g electrons per B-site cation, so that if R < Rc, and upper H-site t2g band, which is split from a lower t2g band by intraatomic exchange via the localized eg electrons, would be one-sixth filled.) However, intermediate mobil-... 

---