Ionic thermal current of concentrated cubic solid solutions of SrF2: LaF3and BaF2: LaF3

In this paper we present new results of ionic-thermal-current (ITC) experiments on concentrated solutions of Sr${\mathrm{F}}_2$ and Ba${\mathrm{F}}_2$ on one hand and La${\mathrm{F}}_3$ on the other. The concentration range of the trivalent La impurities is 0-20 mol%. In earlier work we have shown that even for moderate La concentrations there are significant effects of defect-defect interactions on the ITC results for dipole reorientation. The present work is concerned with polarization effects in doped alkaline-earth fluorides, which are not associated with dipolar complexes but with space charges. In contrast with the results for dipoles, where the ITC peak is located at a fixed temperature as a function of the concentration of the dipoles, we find that the depolarization peak associated with space charges shifts towards lower temperatures with increasing La concentrations. In order to explain this observation we have investigated a new relaxation process, which is in fact a combination of two processes. It appears that with this model we can understand the important features of the experimental results obtained in this investigation.