Wide Magnetic Thermal Memory Effect (∼55 K) above Room Temperature Coupled to a Structure Phase Transition of Lattice Symmetry Reduction in High-Temperature Phase in an S = 1/2 Spin Chain Molecule Crystal

One-dimensional (1D) S = 1/2 Heisenberg antiferromagnetic (AFM) chain system shows frequently a spin-Peierls-type transition owing to strong spin-lattice coupling. From high-temperature phase (HTP) to low-temperature phase (LTP), the spin chain distortion leads to the reduction in lattice symmetry in LTP, called the symmetry breaking (SB) phase transition. Herein, we report the first example of 1D S = 1/2 AFM molecular crystal, [Et ₃ (n-Pr)N][Ni(dmit) ₂ ] (Et ₃ (n-Pr)N ⁺ = triethylpropylammonium, dmit ^2- = 2-thioxo-1,3-dithiole-4,5-dithiolate), which shows a structural phase transition with lattice symmetry increase in LTP, which is contrary to the SB phase transition. Particularly, the structure phase transition leads to magnetically bistable state with T _C ↑ ≈ 375 K, T _C ↓ ≈ 320 K, and surprisingly large thermal hysteresis (∼55 K). Additionally, LTP and HTP coexist in a temperature region near T _C but not at T _C in this 1D spin system. The large hysteresis is related to the huge deformation of anion stack, which needs high activation energy for the structure transformation and magnetic transition between LTP and HTP. This study would not only provide new insight into the relationship of spin-Peierls-type transition and structure phase transition but also offer a roadmap for searching molecular-scale magnetic bistable materials, which are in huge demand in future electronic, magnetic, and photonic technologies.