Thermally stimulated depolarization spectra of CaF2containing Gd and hydrogen

Thermally stimulated depolarization measurements on Ca${\mathrm{F}}_2$ containing Gd and hydrogen reveal the presence of a relaxation of 0.24-eV activation energy whose intensity correlates well with the population of $C_{4\nu }$ ${\mathrm{Gd}}^{3+}$-${\mathrm{H}}^{-}$ dipolar centers as assayed by infrared spectroscopy. The 0.38-eV activation energy reported from EPR line-broadening data appears to be another relaxation mode of the same complex. A mechanism is proposed for the 0.24-eV relaxation that involves tunneling of the ${\mathrm{H}}^{-}$ ion between equivalent interstitial sites about the ${\mathrm{Gd}}^{3+}$.