Microstructure of ZrO2–CeO2 hetero-multi-layer films grown on YSZ substrate

Multi-layer films of pure ZrO2 and CeO2 were grown using oxygen plasma assisted molecular beam epitaxy on yttria stabilized zirconia substrates. The microstructure of the film was analyzed using X-ray diffraction, conventional and high-resolution transmission electron microscopy, electron energy-loss spectroscopy, energy dispersive X-ray elemental mapping, selected area electron diffraction, and dynamical electron diffraction calculations. The deposited pure CeO2 layers exist in the cubic fluorite structure, and the ZrO2 layers show a good epitaxial orientation with respect to the CeO2 layers. However, distinctive forbidden diffraction spots of (odd, odd, even) type were observed on the selected area electron diffraction patterns of the film. Dark-field imaging clearly reveals that these forbidden diffraction spots were contributed solely by the ZrO2 layers. Dynamical electron diffraction calculation based on the tetragonal phase of unity tetragonality (space group P42/nmc) with oxygen displaced along the c  -axis does not match with the experimental pattern. Instead, a diffraction pattern calculated based on a cubic structure (space group P4¯3m) for which the oxygen sub-lattice was displaced along the 〈1 1 1〉 matches with the experimental data. It is further suggested that the displacement of the oxygen from the ideal (1/4, 1/4, 1/4) position was introduced during the film growth process.