Coupled spin cross-over and ferroelasticity: revisiting the prototype [Fe(ptz)6](BF4)2 material

Spin-crossover (SCO) materials exhibit thermal conversion from low to high-spin states. We review different models developed to describe this entropy-driven process and the occurrence of cooperative conversions resulting from elastic interactions. There is a growing number of SCO materials exhibiting unusual thermal conversions when symmetry breaking occurs. To illustrate the importance of considering both phenomena, we review studies of the prototype [Fe(ptz)(6)](BF4)(2) system, exhibiting at atmospheric pressure a single step thermal transition with hysteresis, where a ferroelastic distortion occurs from the high-spin high-symmetry (HShs) phase, towards the low-spin low-symmetry (LSls) phase. Under pressure, sequential conversions occur on cooling from the HShs phase towards a high-spin low-symmetry (HSls) phase, followed by a spin crossover towards the LSls phase. In addition, a metastable low-spin high-symmetry (LShs) state forms upon fast cooling. We revisit this coupling and decoupling of spin crossover and ferroelastic phase transition through the Landau theory model adapted by Collet, which provides qualitative agreement with the experimental data, such as the phase diagram and the evolution of spin transition curves or lattice deformations under pressure. This Ferroelastic Instability coupled to Spin Crossover (FISCO) approach should be generalized to many materials undergoing coupled spin transition and symmetry breaking.