Correlation, superexchange

If a pair of cations is separated by two anions, delocalization superexchange is still possible, though very much weakened, since the cation d orbitals spread out over the entire anion with which they form a partial covalent bond. Correlation superexchange is also weakened by the anion-anion correlation factor. Polarization super-exchange may be competitive in this case, but it is cooperative. It follows that the rules for the sign of 180° cation-anion-anion-cation interactions are the same as those for 180° cation-anion-cation interactions, but the magnitude is reduced, probably by an order of magnitude. 

An antiferromagnetic 90° correlation superexchange may predominate at larger cation-cation separations. 

Gruschus J, Kuki A, multiconfigurational computation of superexchange including correlation and non-local exchange (in progress)... 

According to the rules for 180° superexchange the dynamic orbital correlations lead to ferromagnetic interaction between the Mn3+ ions [4], Hence, one can expect that ferromagnetism in manganites can arise even in the absence of Mn4+ ions, if only the JT effect is dynamic. For instance, Mn substitution with non-Jahn Teller diamagnetic Nb5+, Al3+, Sc3+, etc, ions should result in the appearance of ferromagnetic order. Below we show that this assumption is correct. 

CASE OUTER-ELECTRON CONFIGURATION CORRELATION SUPER EXCHAN6E DELOCALIZATION SUPEREXCHANGE SUM STRENGTH K... 

There are three principal contributions to the superexchange a correlation effect, a delocalization effect, and a polarization effect. If orthogonal orbitals arc used, the last of these appears to be definitely the smallest (478). 

Thus the data for FeS are compatible both with an Re 3.00 A for Fe2+ in a sulfur lattice and with the rules for narrow-band (ft zzs Rc), collective-electron correlations that are extrapolated from the theory of localized-electron (ft > Re), cation--cation superexchange. This fact gives the interpietation of FeS even more significance than a recognition of an additional class of electron ordering at a TV < Tmp. 

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