Effects of high-spin-low-spin lattice misfit on the nucleation and propagation velocities of elastic interfaces in cooperative spin-crossover solids

Switchable Spin crossover (SCO) materials have been studied for many years for their promising applications as sensors of pressure, memories, and molecular switches. Recent optical microscopy studies demonstrated that, in cooperative SCO single crystals, the first-order spin transition proceeds through nucleation of a macroscopic single domain with a clear elastic high-spin=low-spin interface that propagates over the whole crystal, with very small velocities, measured in the range: 2–10 µm·s−1. The present theoretical work is devoted to investigate the nucleation and propagation of high-spin (HS) low-spin (LS) domains in spin crossover materials during the thermally-induced relaxation of a metastable HS state towards the stable LS state. The analysis is performed on a 2D rectangular lattice using an electro elastic model, which takes into account for the change of spin states and the volume along the transition process. We found that the increase of lattice parameters misfits affects the nucleation dynamics of the spin-crossover transformations, successively from homogenous domain nucleation to two sites nucleation leading to two coexisting domains and ending up with a macroscopic single domain features beyond a threshold value of lattice misfit. The evolution of the HS=LS interface has been monitored during the relaxation and a universal law of the HS–LS interface velocity has been derived and discussed.