Role of surface vibrational properties on cooperative phenomena in spin-crossover nanomaterials

The influence of surface/interface on the lattice dynamics of spin crossover nanoparticles has been investigated by a spring-ball model solved by Monte Carlo methods. The bond cohesion energy of the model has been extracted from M\"ossbauer spectroscopy measurements performed on the model compound Ni₃[Fe(CN)₆]. We show that the coupling between bulk and surface vibrational properties, which drastically affects the mechanical properties of the whole particle below a characteristic size, has a major impact on the phase stability of the particles. In the case of free surfaces, the Debye temperature decreases with the size and the first-order nature of the spin transition disappears. On the other hand, a hardening of the surface bonds leads to increasing particle stiffness with the size reduction. In this case, a persistence of the hysteretic behavior in the spin transition curve is also predicted in good agreement with previous theoretical and experimental results.