Localisation vs. delocalisation in the dimeric mixed-valence clusters in the generalised vibronic model. Magnetic manifestations

The problem of localisation-delocalisation in the dimeric mixed-valence clusters is considered in the framework of the generalised vibronic model. The model takes into account both the local vibrations on the metal sites (Piepho-Krausz-Schatz model) and the multicenter (molecular) vibrations changing the intermetallic distances (as suggested by Piepho). In the framework of the semiclassical adiabatic approach the potential surfaces are analysed and different kinds of localised and delocalised states are found. On the basis of the calculated degrees of the localisation the conventional Robin and Day classification of mixed-valence compounds is reconsidered in view of the generalised vibronic model. The magnetic properties of the many-electron mixed-valence dimers are considered as well. The multicenter vibrations are shown to produce a ferromagnetic effect.