The crystal structure, phase stability, and oxideion 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 Ce 0.5 Zr 0.5 O 2 . Contrary to the previous work, we have observed no tetragonal-to-cubic phase transition in the nanocrystalline Ce 0.5 Zr 0.5 O 2 up to 1176 K. The axial ratio c/a F and oxygen displacement along the c-axis from the fluorite regular 8c 1/4,1/4,1/4 position of the nanocrystalline Ce 0.5 Zr 0.5 O 2 are almost independent of temperature. It was found that the c/a F ratio and oxygen displacement in nanocrystalline Ce 0.5 Zr 0.5 O 2 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 Ce 0.5 Zr 0.5 O 2 are larger than those in nanocrystalline CeO 2 , which are factors of the high bulk oxygen diffusivity and catalytic activity in nanocrystalline Ce 0.5 Zr 0.5 O 2 . 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 Ce 0.5 Zr 0.5 O 2 at 1023 K. The diffusion path is not straight but curved and forms a three-dimensional network.

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