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 equivalent to C-Mo- units, with Cs cations hosted in every other pore. In the resulting Cs1.1Fe0.95[Mo(CN)(5)(NO)]center dot 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 x 10(-3) S cm(-1) at 318 K). Furthermore, the ionic conductivity significantly decreases under 532 nm light irradiation (from 1 x 10(-3) S cm(-1) to 6 x 10(-5) S cm(-1) at room temperature) and, when irradiation stops, returns to its original value within similar to 1 h.