Transport properties of mixing conduction in CaF₂ nanocrystals under high pressure

We report on the intriguing electrical transport properties of compressed CaF₂ nanocrystals. The diffusion coefficient, grain and grain boundary resistances vary abnormally at about 14.37 GPa and 20.91 GPa, corresponding to the beginning and completion of the Fm3m-Pnma structural transition. Electron conduction and ion conduction coexist in the transport process and the electron conduction is dominant. The electron transference number of the Fm3m and Pnma phases increases with pressure increasing. As the pressure rises, the F^- ion diffusion and electronic transport processes in the Fm3m and Pnma phases become more difficult. Defects at grains play a dominant role in the electronic transport process.