IONIC-CONDUCTIVITY OF NA2S SINGLE-CRYSTALS BETWEEN 295 AND 1350 K EXPERIMENTAL SETUP AND FIRST RESULTS

We study the ionic conductivity versus temperature and frequency of la rge (NaS)-S-2 single crystals by using a calibrated impedance apparatu s. The experimental setup used for the ionic conductivity measurements up to 1350 K and its calibration are described. The apparatus allows to measure complex impedances between 0.1 Omega and 10 G Omega. The hi gh temperature conductivity data were analyzed in terms of the convent ional Frenkel defect model. We assume that cation vacancies and cation interstitials are the dominant intrinsic defects. The energy of motio n was found to be 0.61 +/- 0.05 eV for a cation vacancy. The energy of formation of a Frenkel defect pair is 2.51 +/- 0.05 eV. Results are g iven that show clear evidence of a superionic behaviour close to the m elting point, similar to the one found in alkaline earth fluorides and several halides. Furthermore, X-ray diffraction experiments on a high optical quality single crystal were performed. The cell parameter and the population parameter of Na+ were accurately determined (6.5373 An gstrom and 0.988, respectively). (C) 1997 Elsevier Science Ltd.