Re-Appearance of Cooperativity in Ultra-Small Spin-Crossover [Fe(pz){Ni(CN)4}] Nanoparticles

A reverse nanoemulsion technique was used for the elaboration of [Fe(pz){Ni(CN)4}] nanoparticles. Low‐temperature micellar exchange made it possible to elaborate ultra‐small nanoparticles with sizes down to 2 nm. When decreasing the size of the particles from 110 to 12 nm the spin transition shifts to lower temperatures, becomes gradual, and the hysteresis shrinks. On the other hand, a re‐opening of the hysteresis was observed for smaller (2 nm) particles. A detailed 57Fe Mössbauer spectroscopy analysis was used to correlate this unusual phenomenon to the modification of the stiffness of the nanoparticles thanks to the determination of their Debye temperature. Spin‐crossover nanoparticles: Mössbauer spectroscopy was used to determine the variation of the Debye temperature (θD) for [Fe(pz){Ni(CN)4}] (py=pyrazine) spin‐crossover nanoparticles ranging from 2 to 110 nm (see picture). The investigations explain the re‐appearance of cooperativity in the smallest nanoparticles.