Measurements of dipole relaxation of two types of $R^{3+}$-interstitial complexes in $\mathrm{Sr}{\mathrm{F}}_2:R^{3+}$ single crystals using the ionic-thermocurrents method are reported. For the light elements in the series of lanthanides we have found relaxations due to tetragonal complexes, whereas in crystals doped with heavy $R^{3+}$ ions such as ${\mathrm{Dy}}^{3+}$, ${\mathrm{Ho}}^{3+}$, etc., the predominant defects have trigonal symmetry. The variation of the activation energy as a function of the $R^{3+}$ radius is discussed. In addition, the effect of the impurity concentration on the relaxation parameters is studied.

• Received 8 March 1976

DOI:https://doi.org/10.1103/PhysRevB.14.1811