Cluster molecular orbital description of the electronic structures of mixed-valence iron oxides and silicates

A molecular orbital description, based on spin-unrestricted Xα-scattered wave calculations, is given for the electronic structures of mixed valence iron oxides and silicates. The cluster calculations show that electron hopping and optical intervalence charge-transger result from weak FeFe bonding across shared edges of FeO₆ coordination polyhedra. In agreement with Zener's double exchange model, FeFe bonding is found to stabilize ferromagnetic coupling between Fe²⁺ and Fe³⁺ cations.