Dielectric properties of manganese-or sodium-doped CaF 2 single crystals

Abstract

The dielectric constant ( K), loss (tan δ) and conductivity (σ) of single crystals of CaF 2:Mn (with 0.2, 1 and 5 mol % of manganese content) and CaF 2:Na (with 0.2 and 1 mol% sodium content) have been measured as a function of frequency in the region 10 2−10 7 Hz and temperature 30°C−450°C. K of all these samples at 30°C (room temperature) is independent of frequency. The dielectric constant of CaF 2 does not appreciably change with Mn or Na doping. All these samples are found to exhibit dipolar relaxation effects at room temperature. The dielectric constant as a function of temperature (above 30°C) exhibits two stages of increase: (i) a slow frequency-independent rise which is attributed to an increase in ionic polarization and (ii) a fast frequency-dependent rise at higher temperatures which is ascribed to space-charge polarization. The dipole in Na-doped samples (negative-ion vacancy attached to a substitutional Na + ion) is found to have an activation energy of 0.43 eV. The nature of the dipole in Mn-doped CaF 2 is not clear. log σ versus 1/ T plots for these solids show that the activation energy for conduction at the high-temperature region is found to decrease considerably with Mn or Na doping of CaF 2 (compared to the value in pure CaF 2). An attempt has been made to understand these results.