Ionic-thermocurrent study of the dipole relaxation and equilibrium in Gd-doped SrF2

By means of thermal depolarization (ionic thermocurrent) measurements, the population of nearest-neighbor (type-I) and next-nearest-neighbor (type-II) ${\mathrm{Gd}}^{3+}$ interstitial fluoride ion dipoles have been observed as functions of temperature from 180 to 210 K, the region of the type-II dipole relaxation peak. The ratio of type-II to type-I concentration varies exponentially with reciprocal polarization temperature, indicating that at each temperature the two dipole species reach equilibrium within the polarization period, chosen to be at least six times the type-II dipole relaxation time. The enthalpy difference between the two species was determined to be 0.046 eV. The shift in the populations with temperature suggests that reorientation of the type-II dipole proceeds by means of the fluoride passing through a nearest-neighbor (type-I) site, in accordance with the accepted model. By assuming that the total number of dipoles remains constant, the ratio of the dipole moments of the two species is found to be 2.4.