Cluster polarization inSr1−xCexF2+x: Hopping transport in fractal structures

Abstract

In this paper we present results obtained with ionic thermocurrent and ac-impedance measurements performed on single crystals of ${\mathrm{Sr}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ce}}_{\mathrm{x}}$${\mathrm{F}}_{2+\mathrm{x}}$ (0.0001<x<0.4). Attention has been focused on the dielectric polarization observed in these crystals occurring just before the onset of dc ionic conductivity. In addition, the activation energy (E) for the dc ionic conductivity has been determined. Both the polarization and ionic conductivity are caused by hopping of interstitial ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ ions. For x=0.05, a maximum in the dielectric constant is observed; at this same concentration, E decreases rapidly. These results have been explained by considering the regions in the crystal near ${\mathrm{Ce}}^{3+}$ impurities as a well-${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$-conducting phase which is dispersed randomly in the poorly-${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$-conducting ${\mathrm{SrF}}_{2}$ host lattice. Computer simulations have revealed a percolation threshold of the well-conducting phase in the poorly conducting host crystal at Ce concentrations near 3 mol %.