Ion Conduction and Complicated Phase Transition Behaviors in an Organic Ion Plastic Crystal, [Tetra‑n‑butylammonium]2[Ni(maleonitriledithiolate)2]

Plastic crystals show diverse functional properties, for example, resemblance to a liquid with internal rotation degrees of freedom, the colossal barocaloric effect (BCE), excellent ion conduction, huge enthalpy changes between phase transition of the crystal and plastic crystal, and so forth. These unique natures endow plastic crystals with a range of applications in the material areas of the BCE and solid-state thermal energy storage, solid electrolytes, and solid-state “solvent” for ion doping. Herein, we present the study of DSC, temperature-dependent powder X-ray diffraction, and ion conduction for an organic ion plastic crystal, TBA2[Ni­(mnt)2] (1; TBA+ = tetra-n-butylammonium, mnt2– = maleonitriledithiolate). Upon heating, 1 undergoes a crystal-to-plastic crystal transition at ∼398 K and a plastic crystal-to-plastic crystal transformation at ∼415 K. The annealing process cooled from the plastic crystal state induces a series of reversible solid–solid phase transitions, related to the conformation transformation of alkyl chains in cations; amongst them, an exothermic solid–solid phase transition occurs at ∼324 K, releasing huge latent heat during heating process, which is in contrast to most observations, and such a material releasing large latent heat near room temperature has promising application in thermal energy storage. Additionally, two plastic crystal phases in 1 extend from 398 to 473 K at least (1 starts to decompose around 500 K), with a wide temperature window and high ion conductivity of 10–3 to 10–2 S·cm–1, indicating that 1 is a good ion conductor.