THERMODYNAMIC PROPERTIES OF COUPLED MIXED-VALENCE MOLECULES USING A COOPERATIVE PKS THEORY - A 2ND-ORDER LOCALIZED DELOCALIZED TRANSITION

To model the thermodynamic properties of mixed-valence molecular solid s, we have considered a set of coupled bistable oscillators; the oscil lators are described by a vibronic Piepho-Krausz-Schatz Hamiltonian, a nd are coupled to each other by asymmetric potential terms. The mean-f ield treatment of the problem leads to a continuous transition, from a localized to a delocalized state as the temperature increases, with a n associated discontinuity of the specific heat; the thermal evolution of the latter significantly depends on the value of the intramolecula r transfer integral. Numerical values have been considered in relation to the situation of biferrocenium salts.