Charge distribution near bulk oxygen vacancies in cerium oxides

Abstract

Understanding the electronic charge distribution around oxygen vacancies in transition metal and rare earth oxides is a scientific challenge of considerable technological importance. We show how significant information about the charge distribution around vacancies in cerium oxide can be gained from a study of high resolution crystal structures of higher order oxides which exhibit ordering of oxygen vacancies. Specifically, we consider the implications of a bond valence sum analysis of Ce₇O₁₂ and Ce₁₁O₂₀. To illuminate our analysis we show alternative representations of the crystal structures in terms of orderly arrays of coordination defects and in terms of fluorite-type modules. We found that in Ce₇O₁₂, the excess charge resulting from removal of an oxygen atom delocalizes among all three triclinic Ce sites closest to the O vacancy. In Ce₁₁O₂₀, the charge localizes on the next nearest neighbour Ce atoms. Our main result is that the charge prefers to distribute itself so that it is farthest away from the O vacancies. This contradicts the standard picture of charge localization which assumes that each of the two excess electrons localizes on one of the cerium ions nearest to the vacancy. This standard picture is assumed in most calculations based on density functional theory (DFT). Based on the known crystal structure of Pr₆O₁₁, we also predict that the charge in Ce₆O₁₁ will be found in the second coordination shell of the O vacancy. We also extend the analysis to the Magnéli phases of titanium and vanadium oxides (M(n)O(₂n₋₁), where M = Ti, V) and consider the problem of metal-insulator transitions (MIT) in these oxides. We found that the bond valence analysis may provide a useful predictive tool in structures where the MIT is accompanied by significant changes in the metal-oxygen bond lengths. Although this review focuses mainly on bulk cerium oxides with some extension to the Magnéli phases of titanium and vanadium, our approach to characterizing electronic properties of oxygen vacancies and the physical insights gained should also be relevant to surface defects and to other rare earth and transition metal oxides.

Occupation matrix control of d- and f-electron localisations using DFT + U

“…The two electrons are typically modelled as being localised at NN sites to the vacancy, 50,[67][68][69][70] although recent studies have reported them to be located at next-nearest neighbour (NNN) sites. 71,72 To assess the application of the occupation matrix control methodology to this kind of defective f-element system, the SOD code 58 was again used to identify all symmetry inequivalent combinations of arranging the two Ce(III) ions within the O-deficient cell. Due to the high symmetry of the fluorite lattice, the number of possible arrangements was only 33, full details of which are provided in the ESI.…”

Toxicity and Protective Effects of Cerium Oxide Nanoparticles (Nanoceria) Depending on Their Preparation Method, Particle Size, Cell Type, and Exposure Route

“…[15,16] In fact, ceria normally contains many defects, that is, oxygen vacancies, which enable it to rapidly give up or take up oxygen atoms in a reversible manner. When an oxygen atom is released according to Equation (1), two of the four neighboring Ce 4+ atoms get reduced to their +3 oxidation state in order to equilibrate the charge, as depicted in Figure 1.…”

“…When an oxygen atom is released according to Equation (1), two of the four neighboring Ce 4+ atoms get reduced to their +3 oxidation state in order to equilibrate the charge, as depicted in Figure 1. [16,17] Figure 1. Simplified schematic representation of the charge redistribution in CeO 2 upon the occurrence of an oxygen vacancy.…”

“…Adapted with permission from ref. [16] The remarkable oxygen-storage capacity and antioxidative property of ceria have also been attributed to its radical scavenging, [18] protection against irradiation, [19] and UV-filtering properties. [20] This leads to the protection of eukaryotic cells against oxidative stress, as it was previously reported in many studies.…”

Human macrophage responses to metal-oxide nanoparticles: a review

“…Interestingly, some metal oxide nanoparticles present this property, as is the case of cerium oxide nanoparticles. Cerium oxide exhibit catalytic properties since they present a reversibility of the oxidation states Ce þ3 and Ce þ4 that leads to oxygen vacancies [81]. Lord et al [82] investigated the antioxidant properties of cerium oxide nanoparticles in U-937 derived macrophages (PMA-differentiated) exposing the cells for up to 72 h to the nanoparticle.…”