Fluorite structure oxygen diffusion

Other refractory oxides that can be deposited by CVD have excellent thermal stability and oxidation resistance. Some, like alumina and yttria, are also good barriers to oxygen diffusion providing that they are free of pores and cracks. Many however are not, such as zirconia, hafnia, thoria, and ceria. These oxides have a fluorite structure, which is a simple open cubic structure and is particularly susceptible to oxygen diffusion through ionic conductivity. The diffusion rate of oxygen in these materials can be considerable. 

Structure and Oxygen Diffusion in Fluorite Structure Oxides... 

A number of oxides with the fluorite structure are used in solid-state electrochemical systems. They have formulas A02 xCaO or A02 xM203, where A is typically Zr, Hf, and Th, and M is usually La, Sm, Y, Yb, or Sc. Calcia-stabilized zirconia, ZrC)2.xCaO, typifies the group. The technological importance of these materials lies in the fact that they are fast ion conductors for oxygen ions at moderate temperatures and are stable to high temperatures. This property is enhanced by the fact that there is negligible cation diffusion or electronic conductivity in these materials, which makes them ideal for use in a diverse variety of batteries and sensors. 

Ce02 crystallizes in the fluorite (fee) structure with octahedral coordination of the cations. This structure is maintained upon reduction to at least CeOu and sometimes lower. Metastable phases of varying compositions, all with defective fluorite structures, have been observed, suggesting that the diffusion of oxygen vacancies into the bulk is rapid at reduction temperatures this is a reason why the surface and bulk reduction peaks in the TPR spectrum overlap. The (defective) fluorite structure is retained even to high degrees of reduction in CeOx/ZrOx. ... 

P.S. Manning, J.D. Sirman, J.A. Kilner, Oxygen Self-Diffusion and Surface Exchange Studies of Oxide Electrolytes Having the Fluorite Structure . Solid State Ionics 93, 125-132 (1996)... 

Oxides with close-packed oxygen lattices and only partially filled tetrahedral and octahedral sites may also facilitate diffusion of metal ions in the unoccupied, interstitial positions. Finally, even large anions may diffuse interstitially if the anion sublattice contains structurally empty sites in lines or planes which may serve as pathways for interstitial defects. Examples are rare earth sesquioxides (e.g. Y2O3) and pyrochlore-type oxides (e.g. La2Zr207) with fluorite-derived structures and brownmillerite-type oxides (e.g. Ca2Fe205) with perovskite-derived structure. 

Fig. 5.8 Arrhenius plot of the oxygen ion diffusivity in ionic conductors with fluorite (9.5 mol% YSZ [63]) and perovskite (Lao 9Sro i YO3 [38], LSGM 9182 [40], and LSGM 8282 [40]) structures...

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