A photoswitchable polar crystal that exhibits superionic conduction

Ionic conductors serve as solid electrolytes for fuel cells and batteries, whereas polar crystals such as ferroelectrics and pyroelectrics—which are typically insulating materials—are used in electronic devices. Here we show a material that combines superionic conductivity with a polar crystal structure at room temperature. This three-dimensional anionic network is based on –Fe–N≡C–Mo– units, with Cs cations hosted in every other pore. In the resulting Cs 1.1 Fe 0.95 [Mo(CN) 5 (NO)]·4H 2 O material, the negative and positive charges of the framework and Cs + ions, respectively, are non-symmetrically shifted in the c -axis direction of the unit cell, and spontaneous electric polarization is generated, in turn leading to second harmonic generation (SHG). Additionally, this material is a superionic conductor (with an ionic conductivity value of 4 × 10 −3 S cm −1 at 318 K). Furthermore, the ionic conductivity significantly decreases under 532 nm light irradiation (from 1 × 10 −3 S cm −1 to 6 × 10 −5 S cm −1 at room temperature) and, when irradiation stops, returns to its original value within ~1 h. A material based on a three-dimensional –Fe–N≡C–Mo– anionic framework that hosts a Cs + cation in every other pore has been shown to exhibit superionic conductivity, despite its polar crystal structure. It also exhibits second harmonic generation (SHG)—usually observed in insulators—and its ionic conductivity was reversibly altered under light irradiation.