Ionic thermocurrents in sodium fluoride thin film

Ionic thermocurrent (ITC) in NaF thin film deposited between gold electrodes has been investigated in situ in the temperature region 300–500 K. The thermally stimulated polarization current (TSPC) and thermally stimulated depolarization current (TSDC) obtained under low electric field pf polarization (E_p) at a lower temperature of polarization (T_p) exhibited current-peak temperatures (T_M) about 420 K and 450 K. The activation energies associated with the first and the second current peaks were 1.15 + 0.05 eV and 0.65 + 0.02 eV, which are assigned to cation vacancy blocking at the grain boundary barrier and the electrode contact interface barrier respectively. Polarization under a strong electric field (E_p > MV m₋₁) at T_p > 480 K causes a quasi-stable shift of the ITC peak positions to higher temperature. The current peak positions and magnitudes then depend upon the polarity of the electrode metal during polarization, and activation energies associated with the first peak positions are 0.90 + 0.02 eV and 0.79 + 0.02 eV for positive and negative bias at the counter electrode respectively, which may be attributed to the clustering of the cation vacancies and the dislocation networks at the interfaces.