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Introduction

Condensed states of matter are basically controlled by the interplay among molecular structure, intermolecular interactions, and molecular motions. Functionalities of materials manifest themselves as the result of a concerted effect among these three factors. The former two factors are always taken into account in any theories and models for interpretation of experimental facts. However, the third factor, the molecular motions, is often neglected or forgotten in those treatments. Molecular motions are directly reflected in the entropy of substances and thus the Gibbs energy is modulated. Therefore, as far as one discusses the stability of a given phase at finite temperatures, the molecular motions play crucially important role. When a delicate balance of these three factors is broken, the condensed state faces to a catastrophe and is transformed into other phase, the so-called phase transition. Therefore, “phase transition” is a good probe for elucidation of the interplay between these three factors. Hitherto well known phase transition is an order-disorder type concerning molecular or spin orientations. Recently, however, new types of phase transitions occurring in molecule-based materials, in which electrons are directly involved, have drawn great attention.

The published paper is an account reviewing the pioneering calorimetric researches done by the present author and his collaborators over the last three decades. In this comment, we shall pick up two subjects: one is the spin crossover phenomena and the other is the intramolecular electron transfers in mixed-valence complexes.

Heat capacity calorimetry

Based on a strong belief that creative works would be achieved by own-made experimental apparatuses, we constructed many sophisticated heat-capacity calorimeters1,2 with very high accuracy and precision, covering a wide range of temperature from 0.04 K to 530 K. By use of these calorimeters, we have measured heat capacities of various kinds of materials and have revealed many essential aspects inherent in the novel phase transitions.

Heat capacity calorimetry is the most reliable experimental tool to detect the existence of phase transitions originating in the onset of a long-range ordering. Heat capacity is usually measured under constant pressure and designated as Cp. It is defined as “the energy required for raising the temperature of one mole of a given substance by 1 K”. By integration of Cp with respect to logarithmic temperature, entropy S is determined. Although entropy is a physical quantity characteristic of the macroscopic aspects of a system, the Boltzmann principle interrelates such macroscopic entropy with the number of thermally accessible microscopic states by the equation, S = kBNA ln W = R ln W, where kB and NA are respectively the Boltzmann and Avogadro constants, R is the gas constant, and W stands for the number of microscopic states. Applying this principle to the entropy of transition, one can estimate the change in the number of microscopic states at the phase transition.

 

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