A fundamental scientific question of technological importance concerning oxides of transition metals and rare earths is:
When an oxygen atom is removed from a bulk crystal of the oxide where do the two excess electrons go?
Elvis Shoko, Michael Smith, and I recently finished a review article which answers this question for the case of cerium oxide.
The approach we took was to consider high resolution crystal structures of
Ce11O20 and Ce7O12 and see how they could be viewed as ordered arrays of oxygen vacancies in an underlying CeO2 crystal. The charge distribution in the local environments of the O vacancies can then be deduced from the bond valence model.
An important finding we make is that the results are incompatible with the widely accepted standard picture of charge localization on two cerium ions next to the vacancy. Instead, we found that the charge distributes itself predominantly in the second coordination shell of cerium ions. Furthermore, one excess electron can be delocalised over more than one cerium ion.
Our conclusions concerning the charge distribution near oxygen vacancies are significant for several reasons.
First, they contradict many (but not all) atomistic simulations based on density functional theory.
Second, the actual charge distribution around the defect has important implications for the other questions we posed at the beginning of the review. For example,
1. the charge distribution has a significant effect on the relative stability of surface and subsurface vacancies.
2. the charge around oxygen surface and subsurface vacancies is not simply localised on Ce ions next to the vacancy this could change our understanding of the catalytic activity of these surfaces since it has been claimed or assumed that it is associated with Ce3+ ions at the surface.
3. the charge distribution around the vacancies has implications for the relative importance of electronic and ionic conduction, a subject we have discussed in this preprint.
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