Ionic Conductivity of Calcium Fluoride Crystals

Abstract

Transference number measurements on pure and doped crystals show that the predominant defect in CaF₂ crystals in the anti‐Frenkel disorder type involving equal concentrations of negative ion vacancies and negative ion interstitials. The mobilities of the F^— vacancy and F^— interstitial have been determined by measuring the electrical conductivity of crystals doped with NaF and YF₃. The activation energy for mobility of the vacancies varies from 20±1.5 kcal/mole at 200°C to 12±1 kcal/mole at 600°C. For the interstitials, the mobility in the range from 690 to 920°C is given by $${\text{u=(1.34×10}}^7{\mathrm{)T}}^{\text{−1}}\hspace{0.17em}\exp {\text{[−(19±4)×10}}^3\mathrm{/T]\hspace{0.17em}}{\mathrm{cm}}^2/\sec \hspace{0.17em}\mathrm{volt}.$$ The number of anti‐Frenkel defects in the pure crystal in the range from 640 to 920°C is given by $$n_0{\text{=2.96×10}}^{25}\exp {\text{[−(16.3×10}}^3\mathrm{)/T]}{\mathrm{cm}}^{\text{−3}}.$$