Studies of relaxation processes in BaF2:La3+crystals by ionic-thermocurrent techniques

Ionic thermocurrent (ITC) experiments have been performed on Ba${\mathrm{F}}_2$ single crystals doped with ${\mathrm{La}}^{3+}$ with concentrations ranging from ${10}^{-4\mathrm{}}$ to ${10}^{-2\mathrm{}}$ (molar fractions). For crystals with a concentration lower than ${10}^{-2\mathrm{}}$, the spectrum consists of three peaks, $A$, $B$, and $C$. For the most-doped crystal there is an additional peak attributed to polarizable clusters. To calculate the dipole concentrations from the depolarization curves, a new method is presented. Based on the results of thermal treatments and concentration-dependent studies, peaks $A$ and $B$ are assigned to nearest-neighbor and next-nearest-neighbor dipoles, respectively. The relaxation parameters are $E_A=0.39\mathrm{}$ eV, ${\tau }_0=4.9\mathrm{}\times {10}^{-13\mathrm{}}$ sec, $E_B=0.50\mathrm{}$ eV, and ${\tau }_0=3.4\mathrm{}\times {10}^{-12\mathrm{}}$ sec. The effect of thermal and mechanical treatments on the highest-temperature peak is studied. It is proposed that the dislocations present in the crystal are responsible for this relaxation.