CORRELATIONS BETWEEN STRUCTURAL AND ELECTRICAL-PROPERTIES OF BACEO3 STUDIED BY COUPLED IN-SITU RAMAN-SCATTERING AND IMPEDANCE SPECTROSCOPY

Coupled impedance and Raman measurements were achieved on a ceramic of undoped BaCeO3 in the 25-600 degrees C temperature range under air, a rgon, nitrogen and carbon dioxide. The sample was studied both in micr o and normal Raman spectroscopy. At 600 degrees C, in air, the conduct ivity of the compound decreases from 3 X 10(-7) to 1.5 X 10(-7) S . cm (-1) after a thermal cycle. The nature of the charge carriers is only briefly discussed. The simultaneous measurement of the Raman spectra a nd of the impedance diagrams has allowed to show that a large part of the variations of the conductivity is due to the structural changes in duced by the temperature variation. Reciprocally, this coupled study c ontributes to the understanding of the phase transition mechanism. The resistance of the compound and the intensity of the 133-110 cm(-1) Ra man band vary in the same way when the temperature is raised or lowere d between 400 and 600 degrees C. These observations could be related t o the growth of the domains of the tetragonal phase. Their kinetics of growth is favored at 600 degrees C but their thermodynamic stability should be maximum at about 450 degrees C: actually, as it can be deduc ed from the reduction of the fullwidth of the Raman bands and from the ir intensity, the long range ordering takes place at about 450 degrees C, while the nucleation process would start at a lower temperature. T his process should start between 180 and 350 degrees C and is characte rized by the variation of the frequencies of the modes at low energy. Thus, the increase of the resistance when the sample is maintained at 600 degrees C should be related to the ordering of the tetragonal phas e and the large fluctuations in the electric signal which are observed between 200 and 450 degrees C could be related to the fluctuation of the sizes of domains. Finally, some preliminary results related to the influence of water and of carbon dioxide are mentioned.