Structural and Compositional Characterization of Yttria-Stabilized Zirconia: Evidence of Surface-Stabilized, Low-Valence Metal Species

The structural properties of polycrystalline yttria-stabilized zirconia (YSZ) have been studied using FT-Raman spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy (XPS). Yttria content was varied between 8 and 15% (by mole fraction) to determine compositional effects on YSZ phonon structure, lattice parameter, and oxidation state. The dominant feature in the low-frequency Raman spectrum correlates quite closely with the material's sole (cubic) lattice parameter. XPS measurements of typical YSZ samples show only a single species of both Y and Zr. After exposing YSZ to a reducing environment (H2) at elevated temperatures (1000 degrees C), however, the XPS spectra of YSZ show new features at lower binding energy for both Y and Zr. Angle-resolved XPS measurements suggest that these reduced forms of Y and Zr exist only within the first few molecular layers of the sample. This treatment does not effect the XRD pattern, nor does it change the low-frequency phonon structure observed in the Raman spectrum, although the Raman spectrum does experience approximately 50% reduction in overall signal intensity. These disparities are reconciled with each other based on differences in each technique's sampling depth. The impact that surface-reduced YSZ may have on the chemistry occurring within solid oxide fuel cells is discussed briefly.