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_1.1Fe_0.95[Mo(CN)₅(NO)]·4H₂O 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.