Effects of X Irradiation on the Ionic Conductivity of Doped SrF2Crystals

The ionic conductivity of Sr${\mathrm{F}}_2$ doped with various concentrations of ${\mathrm{Nd}}^{3+}$ and ${\mathrm{Tb}}^{3+}$, as well as of nominally pure Sr${\mathrm{F}}_2$ crystals, was measured in the extrinsic region. The conductivity was found to increase with the concentration of the dopant, and was attributed mainly to ${\mathrm{F}}^{-}$ interstitials. The conductivity of the Tb-doped crystals was considerably higher than that of the Nd-doped samples. The difference is attributed to the difference in association between these rare-earth ions and the ${\mathrm{F}}^{-}$ interstitials. The effect of x irradiation at 80 °K on the extrinsic ionic conductivity was investigated. The x irradiation caused a decrease in the conductivity of the pure and the slightly doped samples, but did not notably influence the conductivity of the heavily doped specimens. The activation energies were not affected by the x irradiation. After annealing to about 600 °K, the original conductivity of all the x-rayed samples was restored. It is assumed that, as a result of the x irradiation at low temperature, ${\mathrm{F}}^{-}$ interstitial ions transformed to F° atoms. During the heating of the samples, these atoms recombined with electrons trapped at the sites of rare-earth ions and reformed ${\mathrm{F}}^{-}$ interstitials.