Multi-phase spin crossover in Fe(ptz)₆(BF₄)₂

International audienceTransition metal complexes present solid state phase transitions associated with a change of the spin state, low-spin at low temperature to high-spin at high temperature, often with a significant change of metal-ligand bond lengths. Elastic interactions between small low-spin molecules and large high-spin ones play an important role in the phase transition. Since the discovery of spin crossover phenomenon, the complex [Fe (ptz)6](BF4)2 has successfully attracted the interest of scientists because of its complete spin conversion and of its sensitivity to light. This very rich phase multi-stability also offers a unique possibility to investigate carefully the spin crossover phenomena in diverse fundamental cases (coupled with a ferroelastic transition, after quenching, after irradiation, ...), and in an unusual case where the metal ion lies on a high-symmetry site. On the basis of results obtained by neutron scattering under high pressure, we present here the (pressure–temperature) phase diagram of the complex [Fe (ptz)6](BF4)2, which appears as seriously more complex than expected from the earlier literature, with in particular at least two new phases, one corresponding to complexes in a high-spin state but stacked in low symmetry, the other arising from an unexpected reconstructive phase transition