Pressure-directed mixed ionic–electronic to pure electronic conduction transition and enhanced grain boundary conductivity in solid electrolyte CdMoO4

Scheelite ABO4-type solid electrolytes have attracted much attention for potential applications as oxygen ionic conductors of solid oxide fuel cells. Herein, a systematic study was carried out on the electrical transport properties of CdMoO4 under high pressure by impedance spectroscopy measurements and theoretical calculations. The sequence of structural phase transitions at pressures was determined as I41/a → C2/c → P21/c by the Crystal structure AnaLYsis by Particle Swarm Optimization (CALYPSO) method. A pressure-induced conduction transition from mixed ionic–electronic to pure electronic conduction was observed. Below 25.6 GPa, O2− ions play a major role in the electrical transport process. The microscopic transport mechanism was analyzed with grain boundary energies and migration energy barriers. Above 26.9 GPa, the grain boundary response was weakened significantly after a pressure cycle, and the grain boundary conductivity increased by about three times due to pressure. These results provide guidelines for the optimization and application of scheelite ABO4-based oxygen ionic conductors in solid oxide fuel cells.