Tutorial on elastic interaction models for multistep spin-crossover transitions

Abstract

Spin-crossover (SC) compounds are fascinating materials that exhibit colorful phase transitions induced by temperature, pressure, photoirradiation, etc. From the microscopic point of view, the electronic (spin) state of a molecule changes between the low-spin (LS) and high-spin (HS) states by such stimuli, and a variation of the molecular size follows through the vibronic coupling between the electronic state and structure in the molecule. This causes an elastic distortion and then an elastic interaction. The elastic interaction is essential in cooperative properties of SC phenomena. In this paper, we present a tutorial study on elastic interaction models for SC phenomena, which are the recent trend of modeling of SC compounds. We focus on multistep transitions, which are a topic of SC phenomena. We analyze the phase diagrams including the metastable phases for several SC systems, in which antiferromagnetic-like and ferrimagnetic-like phases are associated in addition to the LS and HS phases. Making use of the phase diagrams, we show various patterns of thermally induced SC transition with steps. We also investigate SC transitions with steps in a core-shell SC nanocomposite composed of two different SC compounds. We focus on two cases: the core has a lower transition temperature than the shell, and the core has a higher transition temperature. We show characteristic features of difference in the two systems.