Spontaneous Strain-Spin Transition Coupling Molecular Crystal with Thermal Magnetic Memory Effect, Anisotropic High-κ and Switchable Dielectric Permittivity.

Materials that exhibit controllable changes in electrical, magnetic, or spontaneous strain properties, particularly those that couple these functionalities simultaneously, hold significant potential for technological applications. In this study, a 1D phase transition ion-pair compound is investigated, triethylmethylammonium bis(1,2-maleonitriledithiolato)nickelate (abbr. [EtMeN][Ni(mnt)], 1), composed of flexible EtMeN cation and planar radical [Ni(mnt)] anion. This salt undergoes a paraelastic-ferroelastic phase transition at ≈233/224 K (on heating/cooling), driven by spin-lattice interactions. Importantly, the phase transition couples spontaneous strain, bistable magnetism with switchable dielectric properties. Another distinctive feature of 1 is its pronounced dielectric anisotropy and high dielectric permittivity, which arise due to a barrier layer capacitor effect due to cation displacement polarization and significant electron polarization of the highly conjugated anions. These findings provide a versatile molecular design strategy for developing magnetoelectric and mechanically multifunctional materials, with promising applications in next-generation electronic and smart devices that leverage coupled physical properties.