An EXAFS study of nanocrystalline yttrium stabilized cubic zirconia films and pure zirconia powders

Detailed EXAFS (extended X-ray absorption fine structure spectroscopy) measurements have been collected for two nanocrystalline forms of zirconia, namely, dense films of yttria-stabilized cubic zirconia (YSZ) and tetragonal phase powders of pure ZrO2. Zr and Y K edge EXAFS spectra for the YSZ films with grain sizes of 6, 15, and 240 nm showed no major differences with the corresponding spectra of the bulk counterpart. This is clear proof that these nanocrystalline films exhibit similar levels of disorder to that of large crystals. In particular, there is no support for the view that the intergrain regions are highly disordered, and the present work is consistent with recent EXAFS studies of other nanocrystalline oxides (SnO2 and ZnO) and metals (Cu). The pure nanocrystalline ZrO2 powders were produced by calcining zirconium hydroxide, a widely used method of synthesising ZrO2. The Zr K edge EXAFS of the powders, with grain sizes of 10 and 20 nm, yielded spectra in which the signal was strongly attenuated in comparison to the EXAFS bulk of ZrO2. A significant feature is the dramatically reduced amplitude of the second peak in the Fourier transform, which is due to the Zr-Zr correlation. This feature is often interpreted as evidence of high levels of disorder in nanocrystalline materials. However, using the results from other techniques, notably, NMR measurements, it is argued that the samples contained amorphous material due to an incomplete conversion of the hydroxide precursor. Overall, the studies of the two types of nanocrystalline zirconia emphasize the need for careful characterization of the materials prior to the application of techniques such as EXAFS, which provide an average picture of the local structure.