Calorimetric Investigations of Phase Transitions Occurring in Molecule-Based Materials in Which Electrons Are Directly Involved

Abstract

Abstract New types of phase transitions occurring in molecule-based materials have recently drawn great attention, in which electrons are directly involved. In this paper are given calorimetric investigations aimed at the elucidation of the mechanisms governing such phase transitions as (i) spin crossover phenomena occurring between high- and low-spin states in transition-metal complexes, (ii) intramolecular electron transfers in mixed-valence complexes, (iii) neutral-to-ionic transitions due to charge transfer mechanism, and (iv) thermochromic phenomena due to a change in coordination geometry. Since a change in the electronic state is strongly coupled with a change in the lattice, it turns out that the transition mechanisms always include interplays with various molecular motions in the lattice. In all these systems, a large change in the electronic energy is compensated by the entropy term due to these molecular motions.