A photoswitchable polar crystal that exhibits superionic conduction

Area of Science:

• Materials Science
• Solid-State Chemistry
• Crystallography

Background:

• Ionic conductors are crucial for energy storage (batteries, fuel cells) but are typically distinct from polar crystals used in electronics.
• Existing polar materials are usually insulators, limiting their application in electrochemical devices.
• A material combining both superionic conductivity and polar properties at room temperature is highly desirable.

Purpose of the Study:

• To synthesize and characterize a novel material exhibiting both superionic conductivity and a polar crystal structure.
• To investigate the relationship between the material's structure, its polar properties, and its ionic conductivity.
• To explore the influence of light on the material's ionic conductivity.

Main Methods:

• Synthesis of a novel three-dimensional anionic network material, Cs$_{1.1}$Fe$_{0.95}$[Mo(CN)$_{5}$(NO)]·4H$_{2}$O.
• Structural analysis to confirm the polar crystal structure with non-symmetrically shifted charges.
• Measurement of ionic conductivity at room temperature and its response to 532 nm light irradiation.

Main Results:

• The synthesized material exhibits spontaneous electric polarization and second harmonic generation (SHG), confirming its polar nature.
• It demonstrates superionic conductivity with a value of 4 × 10^-3 S cm^-1 at 318 K.
• Ionic conductivity significantly decreases under 532 nm light irradiation (from 1 × 10^-3 S cm^-1 to 6 × 10^-5 S cm^-1) and recovers within ~1 hour after irradiation stops.

Conclusions:

• A novel material simultaneously possessing superionic conductivity and a polar crystal structure at room temperature has been discovered.
• The material's polar properties are linked to asymmetric charge distribution within its anionic framework.
• The light-responsive ionic conductivity opens possibilities for optoelectronic applications in solid electrolytes.