ON THE F_2g RAMAN MODE GRÜNEISEN PARAMETER IN Gd DOPED CERIA THIN FILMS

We employed Raman and XRD analysis to study structural changes during heat-induced strain relaxation in thin films of Gd-doped ceria with X=0.05, 0.10 and 0.20. The films were prepared by radio frequency magnetron sputtering. According to the X-ray diffraction pattern acquired as a function of the inclination angle, the as-deposited films have varying in-plane compressive strain of up to 0.5%. Annealing at 400 °C, causes partial or complete strain relaxation. In parallel, we measured the position of the Raman peak corresponding to the F_2g symmetric vibration mode. This measurement was done at defined micrometric areas on the film surface, thus removing film anisotropy effects. The mode Grüneisen parameter, γ_i, calculated from the XRD and Raman data showed a large variance with both location and sample age. This strongly suggests the existence of inelastic relaxation processes in the films. The lattice vibrations in Gd-doped ceria have large anharmonicity and cannot be described by the same γ_i extracted from high-pressure Raman measurements of Ceria. These data are consistent with the recently published EXAFS study stating that the Ce-O bonds adjacent to an oxygen vacancy are strongly anharmonic and Ce_Ce-V_O pairs behave as elastic dipoles capable of rearranging under strain.