Experimental evidence for the elastic long-range character of the spin crossover transition in cooperative single crystals
Résumé
We evidenced by means of optical microscopy the long-range character of the interactions acting in spin crossover (SCO) single crystals through a study of the optical density (OD) variation during the spin transition between high-spin (HS) and low-spin (LS) states. The time evolution of the local OD, measured in four single crystals, revealed the existence of precursor phenomena, which are manifest through the appearance of significant changes in the OD of the probed area well before the arrival of the front interface in the case of HS to LS transition and after the passage of the interface in the case of LS to HS transition. These two effects are attributed to the manifestation of long-range interactions that develop during the propagation of the HS/LS interface, which is accompanied by a volume change, generating elastic stresses in the material that deploy far from the sources. Interestingly, the experimental investigations indicate that the precursor and the after effect phenomena occur mostly in the HS phase, which is softer than the LS phase. These original experimental results are well reproduced using an electroelastic modeling in which we considered, in addition to the lattice misfit between HS and LS states, the existence of different elastic constants in the HS and LS phases.