Dielectric Properties of Alkaline Earth Fluoride Single Crystals

Dielectric constant κ′ and dielectric loss κ″ of CaF2, SrF2, and BaF2 single crystals were determined as a function of frequency from 102 to 106 (partly to 1010) cps, in the temperature range from –193° to 500°C. κ′ in all three crystals is frequency independent at 25°C. The dielectric loss κ″ varies only slightly with frequency, having the greatest values at lowest and highest frequencies. The temperature has a striking effect on κ′ and κ″. κ′ increases continuously from 6.4, 6.1, and 6.9 (extrapolated values at absolute zero) to 8.07, 7.70, and 8.46 at 500°C at frequencies above 106 cps for CaF2, SrF2, and BaF2, respectively. Below about 25°C, κ′ is frequency-independent; at higher temperatures it increases with decreasing frequency, κ″ increases with temperature exponentially and decreases with frequency. A detailed interpretation of the frequency and temperature influence on κ′ and κ″ is given. Using expansion and density variation with temperature, a separation of electronic from atomic and dipolar contributions was made. The total dielectric constant is subdivided into six terms (three electronic and three atomic).