Dynamics of the Ionic Space-Charge Electret State in CaF2

The space-charge electret state is strongly correlated with the bulk ionic conduction in Ca${\mathrm{F}}_2$. Its thermogram is a narrow symmetric peak with a characteristic depolarization temperature which varies around 100 °C depending on logarithmically on the impurity doping concentration. The electret-state polarization-depolarization dynamics show dominant bilinear behavior with a simple thermally activated rate function with a single activation energy. The equilibrium electret polarization is first directly proportional to the polarizing voltage and then for voltages greather than about 250 V follows a $V^{\frac{1}{2}}$ dependence. A typical charge release for a polarizing voltage of 1000 V is 0.05 × ${10}^{-6\mathrm{}}$ C ${\mathrm{cm}}^{-2\mathrm{}}$ independent of both the sample thickness and impurity concentration.