Heisenberg‐type and double exchange in mixed‐valence clusters

A theory of the vibronic reduction of the energy levels of transition metal mixed valence clusters is developed. The ²T₂(t₂) ‐ ³T₁(t 22)‐type system is considered in detail. The vibronic model takes into account the interaction with the totally symmetric (A₁) mode as well as the interaction with the Jahn‐Teller tetragonal (E) displacement of each d‐ion ligand. The dispersion of crystal vibrations is considered. The correlation interionic interactions via the phonon field are found and discussed in detail. It is indicated that the Heisenberg‐type exchange interaction is partially reduced by the Jahn‐Teller interaction, and the expressions for the reduction factors are calculated. The double exchange parameter is reduced completely by Jahn‐Teller and totally symmetric vibrations. It is shown that the reduction of the double exchange leads to the antiferromagnetic ground state in mixed valence systems with ferromagnetic sround state arising from the double exchange.