Magnetic bistable organic ionic plastic crystal with room temperature ion conductivity comparable to NASICON and superionic conduction in a broad temperature window†

Organic ion plastic crystals (OIPCs) show promising applications in various all solid-state electrochemical energy storage devices. In this regard, superionic conducting OIPCs with a broad temperature window are highly in demand. Herein, we report an OIPC, [DEIm][Ni(mnt)₂] (1; DEIm⁺ = 1,3-diethyl-1H-imidazolium and mnt²⁻ = maleonitriledithiolate), showing crystal-crystal and crystal-plastic crystal transformations prior to melting. 1 exhibits magnetic bistability and negative thermal expansion, which are coupled with crystal-crystal transformation, the recorded room temperature ionic conductivity (σ) at the 10⁻⁴ S cm⁻¹ level for the as-prepared crystals, comparable to the room temperature fast ion conductor NASICON (Na₃Zr₂Si₂PO₁₂), and the superionic conduction beyond 10⁻¹ S cm⁻¹ with a broad temperature window (σ = 0.19-0.94 S cm⁻¹ in 318-368 K for the annealed crystals and σ ≈ 1.77 S cm⁻¹ in 408-453 K in the plastic crystal phase). These results will allow for expanding the scope of OIPCs, providing an ideal platform for seeking multifunctional superionic conducting OIPCs.