Local Structure and Strain‐Induced Distortion in Ce0.8Gd0.2O1.9

Abstract

Cerium oxide, in both pure and doped forms, is one of the most important and extensively studied oxygen ion conductors. It exhibits a number of interesting properties including ionic conductivity due to the high mobility of oxygen vacancies, a series of different phases formed upon reduction, dependence of the lattice parameter and electrical properties on grain size, and non-linear elastic effects, which have been named ‘‘chemical stress’’ and ‘‘chemical strain’’. Recent structural studies, both theoretical and experimental, have been aimed at understanding the mechanism of interaction between the cations and the oxygen vacancies. These interactions are thought to be directly responsible for a number of effects such as resistance to radiation damage, vacancy ordering leading to phase transformations, dependence of ionic conductivity on the ionic radius of the dopant, and the non-linear elastic effects. Furthermore, in addition to the fluorites, cation-vacancy interactions are observed in a number of other solids with a large concentration of oxygen vacancies, for instance, perovskites. Therefore, characterizing the details of cation-vacancy interactions in Ce0.8Gd0.2O1.9 may have implications for a wider range of materials. One property in which the cation-vacancy interaction is thought to be directly implicated is the chemical strain effect, which is the ability of thin films of Ce0.8Gd0.2O1.9 to exhibit two different elastic moduli at temperatures below 200 8C. This effect is accompanied by an absolute change in volume of 0.2% even when the external stress is homogeneous, which distinguishes it from the Gorsky, Snoek or Zener effects. The chemical strain effect has been tentatively attributed to a change in specific volume due to the interaction of the Gd3þ ions with oxygen vacancies (5% of all oxygen sites in Ce0.8Gd0.2O1.9 ). However, no evidence has been presented that the rearrangement of vacancies in Ce0.8Gd0.2O1.9 can be stress-induced, or even takes place at all. The present study uses extended X-ray absorption fine-structure (EXAFS) spectroscopy to evaluate the local structure of strain-free nanocrystalline films of Ce0.8Gd0.2O1.9 and to compare it with that of Ce0.8Gd0.2O1.9 films with in-plane compressive strain of 0.3% 0.1%. Since the lattice parameter of Ce0.8Gd0.2O1.9 varies by a few tenths of a percent, depending on the preparation route and sample history, the X-ray diffraction (XRD) and EXAFS measurements were performed on the same samples, thus permitting direct comparison of the local ion arrangement with the long-range structure. We report that even in strain-free Ce0.8Gd0.2O1.9, interaction of oxygen vacancies with Ce4þ ion neighbors is favored, rather than interaction with Gd3þ ion neighbors. As a result, Ce4þ ions are shifted away from the oxygen vacancies. Furthermore, compressive strain of 0.3% 0.1% causes the Ce O bond to contract by 1.0% 0.5%, whereas other bonds remain much less affected. This anomalous Ce O bond contraction potentially offers a microscopic explanation for the chemical strain effect observed in Ce0.8Gd0.2O1.9. Films of Ce0.8Gd0.2O1.9 (200–450 nm) were deposited on a (001) Si substrate by RF sputtering and then annealed as described in refs. . The films were kept at room temperature for > 4 months to achieve a constant value of the lattice parameter, indicating that the low-temperature equilibrium of the point defects had been reached. After annealing, the in-plane stress in the Ce0.8Gd0.2O1.9 films was found to be less than 30MPa. The sputtering conditions were adjusted so that the annealed films comprised two groups: (1) those films that were strain free after annealing and (2) those that had a residual compressive strain of 0.3% 0.1%. The presence of strain was evident from the anisotropy of the d-spacings as measured by XRD in the iso-inclination mode. According to XRD data, acquired in the Q–2Q mode (with Q offset of 38 for thin films in order to suppress Si single-crystal reflections from substrate), all films and the reference CeO2 and Ce0.8Gd0.2O1.9 powders were in a single fluorite phase (Fig. 2 in this work and Fig. 2 in Ref. [6]). The lattice parameters of the powders (milled parts of the sputtering targets, grain size > 1mm) determined from highangle (2Q> 808) measurements were 5.411 A and 5.425 A for CeO2 and Ce0.8Gd0.2O1.9, respectively. [6,20]