Thermal and magnetic field switching in a two-step hysteretic Mn^III spin crossover compound coupled to symmetry breakings

A Mn^III spin crossover complex with atypical two-step hysteretic thermal switching at 74 K and 84 K shows rich structural-magnetic interplay and magnetic field-induced spin state switching below 14 T with an onset below 5 T. The spin states, structures, and the nature of the phase transitions are elucidated via X-ray and magnetization measurements. An unusual intermediate phase containing four individual sites, where ¼ are in a pure low spin state, is observed. The splitting of equivalent sites in the high temperature phase into four inequivalent sites is due to a structural reorganization involving a primary and a secondary symmetry-breaking order parameter that induces a crystal system change from orthorhombic → monoclinic and a cell doubling. Further cooling leads to a reconstructive phase transition and a monoclinic low temperature phase with two inequivalent low spin sites. The coupling between the order parameters is identified in the framework of Landau theory.