A Complex Spin-Transition Thermal Hysteresis Unlocked by Two Competing Symmetry-Breakings.

The spin-crossover material [Fe(NH2trz)3](NO3)2 belongs to the class of 1D triazole-based polymeric SCO complexes. It exhibits a thermal phase transition, with a 28 K wide thermal hysteresis, characterized by phase transition temperatures T↑ = 346 K upon warming and T↓ = 318 K upon cooling. This hysteretic thermal phase transition is unusual because it is associated with two competing symmetry-breaking phenomena: a ferroelastic distortion upon warming from the low-spin (LS) trigonal phase to the high-spin (HS) triclinic phase, and a unit cell tripling upon cooling from the HS phase to the LS phase. Magnetic, spectroscopic and X-ray diffraction measurements reveal a clear coupling between the spin transition and the two competing symmetry changes. Since there is no group-subgroup relationship between the LS and HS phases, the phase transition can only be discontinuous. However, the detailed analysis shows that the situation is more complex, since both LS and HS phases are two daughter phases of a parent high-symmetry phase. The phase transitions are then rationalized within the frame of our theoretical model based on the Landau theory of phase transitions, which considers the coupling of the spin transition to both types of competing symmetry-breakings between the LS and HS phases. The theoretical approach reproduces the experimental findings and explains the origin of this hysteretic spin transition.