Mixed (oxygen ion and n-type) conductivity and structural characterization of titania-doped stabilized tetragonal zirconia

By using X-ray diffraction lattice parameter measurements and Raman spectroscopy studies, both the solid solubility limit of titania in yttria tetragonal zirconia polycrystalline solid solutions (Y-TZP, 3 mol % Y2O3) and the TiO2-YTZP tetragonal solid solution field in the ZrO2-Y2O3-TiO2 system have been established. Valence state, site symmetry, and changes in local structures of Ti ions in Y-TZP with 5 and 10 mol % TiO2 are studied for the first time using EXAFS (extended X-ray absorption fine structure), XANES (X-ray absorption near edge structure), and X-ray photoelectron spectroscopy. The total electrical conductivity in air of the TiO2-Y-TZP tetragonal solid solution decreases with increasing titania content. XANES results show that as the TiO2 dissolves into the tetragonal zirconia Y-TZP matrix, a displacement of Ti4+ ions from the center of symmetry seem to take place which leads to a nonrandom substitution of Ti4+ ions on Zr4+ lattice sites. Ti-O bond distances derived from EXAFS results indicate that Ti4+ ion can be in a square-pyramidal arrangement, i.e., fivefold oxygen-coordinated. As a consequence, two kind of cation-oxygen vacancy associations (Zr-V-(O) double over dot and Ti-V-(O) double over dot) with different,diffusion dynamics are created. This results in a decrease of the global concentration of moving oxygen vacancies and therefore, a decrease of ionic conductivity. Electronic conductivity, n-type, only appeared at oxygen partial pressure lower than 10(-15) atm and above 800 degrees C in Y-TZP containing 10 mol % titania. Such an n-type electronic conduction was attributed to a hopping of electrons between Ti4+ and Ti3+ cations by a small polaron hopping mechanism. (C) 1999 The Electrochemical Society. S0013-4651(98)09-052-1. All rights reserved.