Chem. Mater., ASAP Article 10.1021/cm0715388 S0897-4756(07)01538-4 Web Release Date: October 19, 2007 Copyright © 2007 American Chemical Society
Vacancy and Cation Distribution in Yttria-Doped Ceria: An 89Y and 17O MAS NMR Study
Namjun Kim* and Jonathan F. Stebbins
The local structure and dynamics of oxide ions in yttria-doped ceria (YDC, 5 to 30% Y2O3) were studied using high-resolution 89Y and 17O MAS NMR spectroscopy at ambient temperature and high temperatures to 500 C. Eight-, seven-, and six-coordinated yttrium cations are clearly resolved in 89Y MAS NMR spectra, and their relative populations were measured. The derived average coordination number of yttrium is smaller than that for a random distribution of oxygen vacancies, suggesting that there is strong association between yttrium cations and vacancies and there is the possibility of pairing of two yttrium cations with one vacancy. In the 17O MAS NMR spectra, resonances for oxygens with different coordination environments are resolved and are assigned to oxygens with different numbers of yttrium cations in the first coordination sphere. The relative intensities of the 17O resonances also deviate from those expected from a random distribution, again indicating possible pairing of yttrium cations. High-temperature 17O MAS NMR shows coalescence among various peaks. The exchange between oxygen sites with yttrium neighbors appears to occur at a lower temperature (about 100 C) than the exchange that involves the sites without yttrium neighbors; rapid exchange of all oxide anions throughout the structure occurs above 400 C.