Ionic conductivity, activation volumes, and frequency-dependent conductivity in crystals with the fluorite structure

Activation volumes were extracted from the effect of pressure on the ionic conductivities of Ca${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, Ba${\mathrm{F}}_2$, and Cd${\mathrm{F}}_2$. The activation volumes for motion of vacancies were measured in oxygen-contaminated samples and used to calculate defect-formation volumes from the activation volumes at high temperatures. The formation volumes are uniformly small and consistent with the dominance of Frenkel defects for temperatures up to 1400 K in the alkaline-earth fluorides. The Cd${\mathrm{F}}_2$ was found to contaminate badly at temperatures around 600 K, and become an electronic conductor. All of the crystals were found to exhibit frequency-dependent conductivities at high temperatures, attributable to electrode effects or bulk dielectric response of the material, or both.