A COOPERATIVE 2-ORBITAL VIBRONIC MODEL FOR THE SOLID-STATE PROPERTIESOF MOLECULAR MIXED-VALENCE SALTS AND SIMILAR FERROELECTRIC SOLIDS

We present a two-orbital extension of a previous cooperative vibronic Piepho-Krausz-Schatz model formerly designed for modeling the localiza tion properties of molecular mixed-valence solids. The two-orbital ext ension involves a thermodynamic competition between electronic states characterized by different localizing properties. In addition to descr ibing the ''usual'' second-order transitions between the low-temperatu re ordered phase (if present), and the high-temperature disordered pha se, the present model also predicts first-order transitions, re-entran t phases and two-step transitions; discontinuous variations of the ord er parameter are associated with cross-overs of the electronic states.