A comprehensive study of 8 mol% yttria-stabilised zirconia has been made between 150 and 1000 degrees C, using ac impedance spectroscopy and high-temperature neutron powder diffraction. It has been demonstrated that the conductivity anomaly, which occurs at ca. 650 degrees C, is structural in origin. A sharp decrease in the activation energy for conduction of ca. 0.2 eV was observed at ca. 650 degrees C. Additional broad, diffuse scattering peaks were observed below 600 degrees C in the neutron diffraction patterns; above 650 degrees C, the diffuse scattering peaks disappeared. A deviation from linearity was observed at a similar temperature in the plots of both Y/Zr and O isotropic temperature factors vs. temperature. The low-temperature behaviour can be explained in terms of ordering of oxygen vacancy-(dopant) cation clusters to form microdomains, which are evidenced by the presence of diffuse scattering peaks. At high temperature, the association of vacancies with defects breaks down, or at least becomes randomised, allowing vacancies to move more freely as indicated by the decrease in activation energy for conduction. A discontinuity in thermal expansion coefficient (from neutron diffraction data) confirms the second-order nature of the transition.
- defect structure
- fluorite oxides