Crystal Structure and Oxide-Ion Diffusion of Nanocrystalline, Compositionally Homogeneous Ceria–Zirconia Ce0.5Zr0.5O2 up to 1176 K

The crystal structure, phase stability, and oxide-ion diffusion of nanocrystalline ceria–zirconia materials are unresolved important issues, in particular at high temperatures, where the ceria–zirconia catalysts work efficiently. Here, we report a high-temperature neutron diffraction study of nanocrystalline [10.1(7) nm], compositionally homogeneous, tetragonal Ce0.5Zr0.5O2. Contrary to the previous work, we have observed no tetragonal-to-cubic phase transition in the nanocrystalline Ce0.5Zr0.5O2 up to 1176 K. The axial ratio c/aF and oxygen displacement along the c-axis from the fluorite regular 8c 1/4,1/4,1/4 position of the nanocrystalline Ce0.5Zr0.5O2 are almost independent of temperature. It was found that the c/aF ratio and oxygen displacement in nanocrystalline Ce0.5Zr0.5O2 are smaller than those of the bulk sample. It was shown that the refined atomic displacement parameter and spatial (maximum entropy method nuclear density) distribution of oxygen atom in nanocrystalline Ce0.5Zr0.5O2 are larger than those in nanocrystalline CeO2, which are factors of the high bulk oxygen diffusivity and catalytic activity in nanocrystalline Ce0.5Zr0.5O2. Possible diffusion pathways of oxide ions along the fluorite ⟨100⟩ and ⟨110⟩ directions were visualized in the spatial distribution of bond valence sums calculated using the present refined crystal structure of nanocrystalline Ce0.5Zr0.5O2 at 1023 K. The diffusion path is not straight but curved and forms a three-dimensional network.