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.

Toward High-Temperature Light-Induced Spin-State Trapping in Spin-Crossover Materials: The Interplay of Collective and Molecular Effects

“…An alternative approach to understanding collective effects is to study phenomenological models that describe the spin state via a binary variable and model the interactions between SCO centers either via a phenomenological “Ising-like” interaction , or via elastic interactions . Indeed these two forms of interactions between SCO centers have been shown to be equivalent. − In the mean-field approximation these models give rise to Landau theories of spin-state transitions .…”

“…Indeed these two forms of interactions between SCO centers have been shown to be equivalent. − In the mean-field approximation these models give rise to Landau theories of spin-state transitions . These models have been shown to reproduce many of the important conclusions drawn from macroscopic rate constant models, particularly in the thermally activated decay regime. − Binary models have also been shown to describe the multistep transitions that are observed in some SCO materials and have been used to predict exotic new phases of these materials. ,,,− A serious limitation of binary models is that they contain many phenomenological parameters, and the connection of many of these phenomenological parameters to the underlying physics and chemistry of materials remains obscure.…”

Towards High Temperature Light-Induced Spin State Trapping in Spin Crossover materials: the Interplay of Collective and Molecular Effects

“…Finally, we include the elastic coupling between SCO centers, which is known to be vital for understanding collective effects in the solid state. 18,69,70,[70][71][72][73][74][75][76][77] Table 1: Realistic values of the parameters in our model. We give both the range found in typical SCO materials and a "default" value in the center of this range that will be used in many of the calculations below.…”

“…An alternative approach to understanding collective effects is to study phenomenological 'Ising-like' models. 69,70 These models have been shown to reproduce most of the important conclusions drawn from macroscopic rate constant models, particularly in the thermally activated decay regime. [71][72][73] Ising-like models have also been shown to describe multistep transitions that are observed in some SCO materials and have been used to predict exotic new phases of these materials.…”

“…[71][72][73] Ising-like models have also been shown to describe multistep transitions that are observed in some SCO materials and have been used to predict exotic new phases of these materials. 18,70,[74][75][76][77] In principle, this approach also allows one to study correlation effects beyond mean field; however, to date, most studies are in the mean field approximation. [71][72][73] A serious limitation of Ising-like models is that they contain many phenomenological parameters; and the connection of many of these phenomenological parameters to the underlying physics and chemistry of materials remains obscure.…”