Simple Two-Dimensional Model for the Elastic Origin of Cooperativity among Spin States of Spin-Crossover Complexes

TL;DR

This paper presents a simple two-dimensional elastic model to explain the cooperative spin crossover behavior, showing how intermolecular elastic interactions influence the transition nature from gradual to first-order.

Contribution

It introduces a novel elastic interaction model incorporating inhomogeneity effects and demonstrates its impact on spin crossover transition characteristics.

Findings

Increasing intermolecular interaction strength changes transition from gradual to first-order.

Elastic interactions among molecules are key to understanding cooperativity in spin crossover.

The model reproduces temperature-dependent behavior of spin states.

Abstract

We study the origin of the cooperative nature of spin crossover (SC) between low spin (LS) and high spin (HS) states from the view point of elastic interactions among molecules. As the size of each molecule changes depending on its spin state, the elastic interaction among the lattice distortions provides the cooperative interaction of the spin states. We develop a simple model of SC with intra and intermolecular potentials which accounts for the elastic interaction including the effect of the inhomogeneity of the spin states, and apply constant temperature molecular dynamics based on the Nos\'e-Hoover formalism. We demonstrate that, with increase of the strength of the intermolecular interactions, the temperature dependence of the HS component changes from a gradual crossover to a first-order transition.