Influence of H-D isotopic substitution on the protonic conductivity ofLiNbO3

The electric dc dark conductivity in congruent ${\mathrm{LiNbO}}_3$ has been studied in the temperature range 80–600 °C as a function of the proton and deuteron concentration and the driving electric field. The conductivity was found to follow an Arrhenius-like behavior with an activation energy equal for protons and deuterons, ${\mathrm{\varepsilon }}_{\mathrm{act}}$=1.23 eV. The pre-exponential factor ${\mathrm{\sigma }}_0$ shows a linear concentration dependence for both H and D, but with different slopes. The ratio of the slopes yields (∂${\mathrm{\sigma }}_0$/∂c${)}_{\mathrm{H}}$/(∂${\mathrm{\sigma }}_0$/∂c${)}_{\mathrm{D}}$=1.36. This agrees with the mass dependence of the attempt frequencies in a hopping model of protons and deuterons and rules out models of OH-OD migration. The conductivity was found to be independent of the electric field up to values of ${10}^5$ V/cm, which is comparable to inner electric fields in volume holograms in ${\mathrm{LiNbO}}_3$. In the temperature range accessible to both methods, the dc and the holographically measured values of the conductivity agree very well, displaying the same temperature and concentration dependence. The charge-carrier concentration was determined from the integrated absorption strength of the ir OH-OD stretch-mode absorption of proton- and deuteron-exchanged waveguides with a known rate of exchange of Li by H-D, yielding an oscillator strength ${\mathit{f}}_{\mathrm{H}}$=1.7×${10}^{\mathrm{-}2}$.