Control of the phase stability in spin crossover core-shell nanoparticles through elastic interface energy

Size reduction leads to drastic changes in thermodynamical properties in spin crossover (SCO) nano-objects in comparison with bulk materials. In particular, an important modification of the phase stability has been observed, reflected by a shift of the transition temperature. These changes are mostly attributed to the increasing role of surface properties at the nanoscale, especially the elastic properties, which play a key role in SCO phenomena. In this work, we propose a continuum mechanics approach to explore the possibility to tune the phase stability by controlling the interfacial elastic energy in core-shell nanoparticles with (semi-)coherent interface. To this aim, the pressure at the particle surface/interface is analytically derived in the case of hollow and core-shell nano-objects. Then, the extracted interfacial energy is injected in a thermodynamical model to mimic the spin transition curves in the framework of the effective medium approximation. A structural misfit between a SCO core and an inactive shell produces a negative or positive pressure on the SCO material, favoring either the high spin or the low spin states and thus can downshift or upshift the transition temperature. These theoretical results provide real perspectives for the control of the spin transition at the nanoscale via interfacial energies since the design of core-shell coordination nanoparticles and other heterostructures has been considerably developed in recent years.