We analyzed the size effect on a first-order spin transition governed by elastic interactions. This study was performed in the framework of a nonextensive thermodynamic core-shell model. When decreasing the particle size, differences in surface energies between the two phases lead to the shrinking of the thermal hysteresis width, the lowering of the transition temperature, and the increase of residual fractions at low temperature, in good agreement with recent experimental observations on spin transition nanomaterials. On the other hand, a modification of the particle-matrix interface may allow for the existence of the hysteresis loop even at very low sizes. In addition, an unexpected reopening of the hysteresis, when the size decreases, is also possible due to the hardening of the nanoparticles at very small sizes, which we deduced from the size dependence of the Debye temperature of a series of coordination nanoparticles.

In recent years, study of nano-objects with first-order phase transitions has raised new interesting questions. Indeed, the phase stability and the transformation kinetics are highly dependent on the object size. In the case of the spin crossover phenomenon size effects may lead to a loss of the hysteresis (bistability) properties, a shift of the transition temperature or the occurrence of an incomplete transition and also reappearance of the hysteresis upon extreme size reduction (∼1–4 nm). Research in this area is growing, but the mechanisms of the spin transition at the nanoscale and in particular the size dependence of the collective behaviour are still not completely understood. The aim of this Focus paper is to point out new issues brought about by the size-reduction effects. We also review size-induced effects for a few selected phase transition phenomena, such as structural or electronic transitions, with the aim of highlighting the possible analogies with the spin crossover phenomenon.

This review reports on the recent progress in the synthesis, modelling and application of hybrid spin-crossover materials, including core–shell nanoparticles and multilayer thin films or nanopatterns. These systems combine, often in synergy, different physical properties (optical, magnetic, mechanical and electrical) of their constituents with the switching properties of spin-crossover complexes, providing access to materials with unprecedented capabilities.

We used nuclear inelastic scattering (NIS) to investigate the lattice dynamics in [Fe(pyrazine)(Ni(CN)${}_4\left)\right]$ spin crossover nanoparticles. The vibrational density of states of iron was extracted from the NIS data, which allowed to determine characteristic thermodynamical and lattice dynamical parameters as well as their spin-state dependence. The optical part of the NIS spectra compares well with the Raman scattering data reflecting the expansion/contraction of the coordination octahedron during the spin transition. From the acoustic part, we extracted the sound velocity in the low-spin $(v_{\mathrm{LS}}=2073\pm 31$ m ${\mathrm{s}}^{-1})$ and high-spin $(v_{\mathrm{HS}}=1942\pm 23$ m ${\mathrm{s}}^{-1})$ states of the particles. The spin-state dependence of this parameter is of primary interest to rationalize the spin-transition behavior in solids as well as its dynamics and finite size effects.