Vibronic theory of the stability and hysteresis in bistable mixed-valence molecular salts

We previously proposed a two-orbital cooperative adaptation of the vibronic Piepho-Krausz-Schatz model, for modeling the solid-state properties of biferrocenium mixed-valence salts. The resulting molecular ferroelectric model, treated in mean-field approach, leads to remarkable double-reetrance at low temperature, following the sequence ferro-para-ferro-para-electric with increasing temperatures. Here we investigate some aspects of the phase diagram of the system: we unambiguously assess the metastable character of the second-reentrant state and we study the hysteresis loops associated with the first-order transitions which result from orbital crossovers in the model. © 1996 The American Physical Society.