Dedicated to the memory of Prof. John J. McGarveyThe lattice dynamical aspects of the spin crossover phenomenon in molecular solids-displaying intricate couplings between the electronic spin state of the molecules and the lattice properties-are reviewed. Emphasis is on experimental and theoretical approaches giving access to the vibrational spectra and to key properties, such as the heat capacity, vibrational entropy and enthalpy, lattice rigidity, elastic constants, and elastic interactions. Recent results in relation to surface and finite size effects as well as with ultrafast out-of-equilibrium phenomena are also covered.

Size reduction effects on the lattice dynamics of spin crossover (SCO) thin films have been investigated through molecular dynamics (MD) simulations of the density of vibrational states. The proposed simple model structure and reduced force field allows us to obtain good orders of magnitude of the sound velocity in both spin states and takes into account the contribution of free surfaces in the vibrational properties of very thin films (below a thickness of 12 nm). The slab method issue from the field of surface physico-chemistry has been employed to extract surface thermodynamic quantities. In combination with the related slab-adapted method, the slab approach provides a powerful numerical tool to separate surface contributions from finite-size effects. Due to the relatively low stiffness of SCO materials, the lattice dynamics seems to be governed by surface instead of confinement effects. The size evolution of thermodynamic quantities is successfully reproduced, especially the increase of the vibrational entropy with the size reduction, in good agreement with experimental observations.