Charge transport inSr1−xLaxF2+xsolid solutions. An ionic thermocurrent study

Abstract

A comprehensive study of the relaxation of space charges in solid solutions of the type ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{F}}_{2+x}$ will be reported. In this investigation we have employed the ionic thermocurrent technique. La${\mathrm{F}}_{3}$ 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-earth-doped 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 ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{F}}_{2+x}$ 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.