Charge transport in Sr1-xLaxF2+x solid solutions : an ionic thermocurrent study

A comprehensive study of the relaxation of space charges in solid solutions of the type Sr1-xLaxF2+x will be reported. In this investigation we have employed the ionic thermocurrent technique. LaF3 concentrations between 0.01 and 42 mol % have been used. We have paid attention to the resemblance between space-charge relaxation and ionic conductivity in heavily rare-earthdoped fluorite-type solid solutions. We will show that the enhancement of the conductivity and the shift of the space-charge relaxation band to lower temperatures with increasing. rare-earth concentration can be explained with the same mechanism. Charge transport is supported by two kinds of interstitial jumps: free interstitial jumps occurring relatively far from a rare-earth ion and energetically more favorable dipolelike jumps of interstitial fluoride ions in the neighborhood of a rare-earth ion. We will also discuss the influence of the defect concentration on the degree of dissociation of the dipoles present and the effect of the concentration on the defect structure of the Sr1-xLaxF2+x solid solutions. It will be made clear that the charge transport in rare-earth-doped fluorite-type ionic crystals is supported by a percolation mechanism.