Effect of the ${\mathrm{Ce}}^{3+}$ concentration on the reorientation of dipoles in Sr${\mathrm{F}}_2$:${\mathrm{Ce}}^{3+}$

The effect of the impurity concentration on the reorientation of dipolar complexes in Sr${\mathrm{F}}_2$, consisting of a trivalent cerium ion and a neighboring interstitial fluorine ion, has been investigated. It was found in an earlier paper that the width of the reorientation band as measured by the ionic-thermocurrents technique (ITC) increases with increasing concentration. This can be understood when it is assumed that the activation energy associated with the reorienting dipoles is modified by dipole-dipole interaction. In the present paper we propose that there is a Gaussian distribution of values for the activation energy. The width $p$ of this distribution has been calculated on the basis of a crude electrostatic interaction model. It will be shown in this paper that $p$ increases with increasing concentration of ${\mathrm{Ce}}^{3+}$. In addition, we shall see that in the interpretation given in this paper the average activation energy is independent of the ${\mathrm{Ce}}^{3+}$ concentration, whereas in earlier treatments of the ITC bands several authors have found that the activation energy decreases with increasing concentration.