Richness of the nature of intermolecular interactions in spin-crossover materials and its implications

Thermally induced spin transition materials are known for their potential applications in a wide range of fields, from pressure and strain sensing to high-density information storage and control. The latter are described by Ising-like models and elaborate elastic theories including the coupling between the change of spin state and the accompanying local deformation of the lattice during the transition. In the present work, the Morse potential is used to model interactions between spin-transition molecules. The resultant Hamiltonian is solved by searching first for the mechanical equilibrium, revealing an isomorphism with an Ising-like model, including competing short- and long-range interactions with the presence of anharmonic effects. Studies within the molecular field approximation, Bethe-Peierls, and Monte Carlo methods allow reproducing several experimental situations among which gradual, first-order, and two-step transitions are observed, demonstrating the ability of the model to capture the diversity of spin-crossover materials while comparing the different methods and their limitations.