Disorder-order transformation and significant dislocation motion cooperating with a surprisingly large hysteretic magnetic transition in a nickel-bisdithiolene spin system

The compound [4′-CF₃bzPy][Ni(mnt)₂] (1) (where 4′-CF₃bzPy = 1-(4′-(trifluoromethyl)benzyl)pyridinium and mnt^2- = maleonitriledithiolate) was synthesized and displays a magnetic bistability with a surprisingly large thermal hysteresis loop (∼49 K). X-ray crystallographic studies reveal that in the high-temperature (HT) phase the anions and cations form mixed stacks, with alternating anion dimers (AA) and cation dimers (CC) in an...AACCAACC... fashion along the crystallographic a + b direction, and disordered CF₃ groups in the cations are aligned into a molecular layer parallel to the crystallographic (001) plane. However, in the low-temperature (LT) phase, the c-axis length of the unit cell is roughly doubled, and the asymmetric unit switches from one [4′-CF₃bzPy][Ni(mnt)₂] pair in the HT phase to two [4′-CF₃bzPy][Ni(mnt)₂] pairs. Most interestingly, the CF₃ group in the cations becomes ordered, and the conformation of one of two crystallographically different cations changes significantly. A dislocation motion between the neighboring molecular layers emerges as well. The analyses of the magnetic susceptibilities and the density functional theory calculations suggest that the antiferromagnetic exchange interaction within one of two types of [Ni(mnt)₂]₂^2- dimers in the LT phase is much stronger than that within the [Ni(mnt)₂] ₂^2- dimer in the HT phase. The lattice reorganization during this phase transition is proposed to be responsible for the wide thermal hysteresis loop.