Phase transitions give rise to different and unique modifications of physical properties in materials. Understanding these changes of state and their origin is of great interest for the further tuning of such properties, through the application of external stimuli, just as temperature, pressure, electric or magnetic fields, and light irradiation.1,2In this sense, the classical Landau theory is a useful tool for the description of different ordering phenomena, taking place during phase transitions, originally through the evolution of symmetry-breaking order parameters, which measure the deviation from the high-symmetry phase.2 Due to entropy gain, it is the disordered high symmetry phase that is stable at high temperature, in contrast to the ordered low symmetry phase, which is stable at low temperature. In addition, there are many materials, like charge-transfer, spin-crossover, and Mott insulator systems, in which the phase transition is related to an electronic instability without symmetry-breaking. In these cases, the non-symmetry-breaking phase transition should be depicted with the evolution of another order parameter, accounting for the aforementioned electronic instability.1For some materials, like in the case of spin-crossover systems, both, electronic and structural order instabilities may couple. This can result in the widening of hysteresis loops between high- and low- spin phases or in a step-wise evolution of the transition.3 In order to describe the aforementioned phenomena, the addition of the symmetry-allowed coupling terms to the Landau theory main equation is needed.In this work, we are interested in the [Fe(PM-PEA)2(NCS)2] system exhibiting a spin transition coupled to a ferroelastic symmetry-breaking. In particular, this system is unusual because it presents a high symmetry configuration (orthorhombic – Pccn) for the low-temperature low-spin state, while the high-temperature high-spin state is in a low-symmetry phase (monoclinic – P21/c).3,4 In this poster, we will show a way to rationalize this unusual behavior within the Landau theory framework, towards the reproduction and understanding of the experimentally observed asymmetric spin-transition hysteresis loop, and coupled changes of spin state and symmetry of the crystal.< Réduire