Structure and electrical behavior in air of TiO2-doped stabilized tetragonal zirconia ceramics

From the lattice parameters determined by XRD on TiO2-doped YTZP (3 mol % Y2O3) compositions sintered in the temperature range of 1300 to 1450 degrees C, the tetragonal zirconia solid solutions field for the ZrO2-Y2O3-TiO2 system was established. The solubility of TiO2 in YTZP was found to be about 12-14 mol % at 1450 degrees C. Structural characterization of the Ti-YTZP tetragonal zirconia solid solutions was carried out using X-ray absorption spectroscopy (EXAFS and XANES) to provide information on the environment of Ti stems. The electrical behavior in air of the TiO2-doped tetragonal zirconia solid solutions was studied by impedance spectroscopy in the temperature range of 300 to 800 degrees C, and it was found that the ionic conductivity decreases with increasing titania content. EXAFS and XANES results show that as the Ti4+ ions dissolve into the tetragonal zirconia YTZP matrix, a displacement of Ti ions from the center of symmetry takes place, leading to a non-random substitution of Ti4+ ions on Zr4+ lattice sites. Ti-O bond distances derived from EXAFS indicate that the Ti ion can be in a square-pyramidal arrangement, i.e., fivefold oxygen coordinated. As a consequence two kinds of cation-oxygen vacancy associations are created; the high-mobility oxygen-vacancy-eightfold-coordinated cation (Zr4+) and the low-mobility oxygen-vacancy-fivefold-coordinated cation (Ti4+). This results in a decrease in the global concentration of moving oxygen vacancies and, therefore, in a decrease of the electrical conductivity.