Microscopic spin-distortion model for switchable molecular solids: Spatiotemporal study of the deformation field and local stress at the thermal spin transition

We design a microscopic model for switchable molecular solids (e.g., spin crossover), based on the elastic properties of a discrete lattice made of switchable sites, denoted high spin (HS) or low spin (LS). The elastic interactions and equilibrium distances between sites are written as explicit functions of their HS or LS states. The model was solved by Monte Carlo technique, alternatively running on the electronic and position variables. In the present work we investigate the thermal transition in the case of a square two-dimensionsal lattice, including short-range interactions up to the second neighbors in order to maintain the stability of the lattice. The input values of the elastic parameters are selected so as to lead to realistic values of the bulk modulus and Debye temperature. We show that the elastic interactions act as effective Ising interactions, leading to the expected transition and phase diagram, in terms of transition temperature vs elastic interaction parameter. We study the domain growth of the LS or HS species at different temperatures along the thermal loop and obtain features consistent with the experimental data. We also follow the mechanical properties of the system by calculating the displacement field and the internal stresses produced by the domain growth process. The resulting maps evidence the leading role of the HS/LS interface and the crucial effect of the edges of the lattice, thus paving the way to a real understanding of the shape effects in spin transition nanocrystals.