Microstructure of ZrO 2–CeO 2 hetero-multi-layer films grown on YSZ substrate

Abstract

Multi-layer films of pure ZrO 2 and CeO 2 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 CeO 2 layers exist in the cubic fluorite structure, and the ZrO 2 layers show a good epitaxial orientation with respect to the CeO 2 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 ZrO 2 layers. Dynamical electron diffraction calculation based on the tetragonal phase of unity tetragonality (space group P4 2/ 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 P 4 ¯ 3 m ) 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.