Double exchange process

Due to the presence of the S+1/2 term in (2), it is straightforward to see that the double exchange process will favor the stabilization of the spin state with the highest multiplicity, S=Sm.v.. More details on the SH formalism can be found on the web site... 

As in other transition metal oxides such as NiO, the spins on the transition metal ions are coupled via the oxide ions. The Mn " ions couple antiferromagnetically with each other via superexchange as in NiO. The coupling of the Mn " ions varies and can be ferro- or antiferro-magnetic. Mn " ions couple to Mn " ions in a process known as double exchange. In this process, a simultaneous hop of an electron from Mn " to an 0 Ip orbital and from the 0 2p orbital to an Mn ion takes place. 

In many cases molecular displacements depend on other transition processes occurring previously in the neighborhood. Let us consider Fig. 27. The double kink may move from 1 to l Then segment 2 has the possibility to go from 2 to 2. Flowing units 4,5 and 6 may undergo similar processes. At last the molecule may reach the dashed structure. In every case the final position will be reached after some successive place exchange processes each determining the next step. We therefore get a system of simultaneous differential equations. 

Having introduced the concept of exchange in a pictorial way, we treat now the more general case of exchange between two sites of different population and different intrinsic relaxation times. Under such conditions relaxation is not a single exponential but rather a double exponential process, where the individual... 

For this double electron exchange process to operate, there should be a molecular orbital overlap between the excited donor and the acceptor molecular orbital. For a bimolecular process, intermolecular collisions are required as well. This mechanism involves short-range interactions ( 6—20 A and shorter). Because it relies on tunneling, it is attenuated exponentially with the intermolecular distance between the donor and the acceptor.17 The rate constant can be expressed by the following equation. 

Figure 1 shows the double-well potential which is often used to represent the electron transfer act in oxidation-reduction. The special case of a self-exchange process was chosen for simplicity. (For a full discussion of the issues being discussed in relation to the potential energy diagram, see Ref. 11. Ref. 13 gives a more compact treatment.) The implications of the diagram may not be immediately obvious, and the... 

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