Hopping ionic conductivity in Ce-doped SrF2. II. Results obtained from impedance measurements.

Abstract

The complex dielectric constant of ${\mathrm{Sr}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ce}}_{\mathrm{x}}$${\mathrm{F}}_{2+\mathrm{x}}$ single crystals has been measured at frequencies between 90 Hz and 30 kHz with the temperature ranging from 100 to 550 K and 0.0001lxl0.40. We have paid special attention to the frequency and temperature dependence of the ionic conductivity. For Ce concentrations lower than 0.5 mol?frequency-independent Arrhenius-type ionic conductivity has been observed, which is attributed to the mobility of interstitial ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ ions dissociated from ${\mathrm{Ce}}^{3+}$-${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ dipolar complexes present in the crystals. At higher Ce concentration an increase of the ionic conductivity by several orders of magnitude has been observed; in addition the ionic conductivity depends upon the frequency. Employing a model in which the mobility of the interstitial ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ ions is relatively large in the vicinity of the Ce impurities and the ${\mathrm{Ce}}^{3+}$ ions are randomly distributed over the ${\mathrm{SrF}}_{2}$ lattice the results have been interpreted. Essential in the interpretation is the random potential energy barrier structure in the crystals experienced by a hopping interstitial ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ ion.