The origin of elastic anomalies in thin films of oxygen deficient ceria, CeO_{2 - X}

Self-supported films of CeO_1.95 display time-scale dependent elastic moduli, a phenomenon which has been termed the chemical strain effect. In order to probe the possible structural origins of this behavior, extended X-ray absorption fine structure spectroscopy and X-ray diffraction were used. Evidence was found that, although this oxygen deficient ceria appears to maintain the fluorite structure on average, the mean Ce-O bond length is shorter than the mean Ce-oxygen vacancy distance. This finding is consistent with crystallographic data from more strongly reduced ceria in which the oxygen vacancies are ordered. By studying strain induced structural changes, we show that it is possible to relate this lattice distortion to the chemical strain effect. Similar conclusions were previously reached for films of Ce _0.8Gd_0.2O_1.9. Since the ionic radii of both Gd³⁺ and Ce³⁺ are larger than that of Ce⁴⁺, we suggest that when cation dopants are larger than the host, ceria compounds containing a high concentration of oxygen vacancies may exhibit elastic anomalies.