Ionic Conductivity of Calcium Fluoride Crystals

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 u=(1.34×107)T−1 exp[−(19±4)×103/T] cm2/sec volt.The number of anti-Frenkel defects in the pure crystal in the range from 640 to 920°C is given by n0=2.96×1025exp[−(16.3×103)/T] cm−3.