Two magnetic phase transitions, driven by symmetry breaking and isostructural phase transitions, in a nickel-bis-dithiolene spin system

A one-dimensional (1-D) magnetic chain compound, [(CD₃) ₂CN-Py][Ni(mnt)₂], where (CD₃) ₂CN-Py⁺ = 1-(propan-d₆-2-ylideneamino) pyridinium and mnt^2- = maleonitriledithiolate, was synthesized and characterized. This compound undergoes two structural phase transitions, at around 267 K and 325 K. The three phases are sequentially labeled as α (below 267 K), β (between 267 and 325 K) and γ (above 325 K). The [Ni(mnt)₂]^- anions form irregular stacks, with two values of neighboring Ni⋯Ni distances. The cations align in a straight and regular arrangement along the crystallographic a-axis direction in the β phase. The asymmetric unit switches from one anion-cation pair to two anion-cation pairs in the transition from the β to the α phase. This leads to a doubling of the crystallographic axis length, parallel to the anion stack. It also results in the irregular anion stack showing three non-equivalent neighboring Ni⋯Ni distances, as well as making the straight cation arrangement irregular. The β and γ phases are isostructural and exhibit quite similar packing structures. The anion stack shrinks a little with increasing temperature, with all crystallographic axes showing small reduction from the β to γ phase. The two observed magnetic phase transitions are driven by these various structural phase transitions. A thermal hysteresis loop, with a separation of ∼8 K, appears in the transition between the β and α phases. However, this is absent in the transition between the β and γ phases. A dielectric anomaly appears across the phase transition between the β and γ phases but not between the β and α phases.