Charge distribution near bulk oxygen vacancies in cerium oxides

Abstract

Understanding the electronic charge distribution around oxygen vacancies intransition metal and rare earth oxides is a scientific challenge of considerabletechnological importance. We show how significant information about the chargedistribution around vacancies in cerium oxide can be gained from a study of highresolution crystal structures of higher order oxides which exhibit ordering of oxygenvacancies. Specifically, we consider the implications of a bond valence sum analysis ofCe7O12 andCe11O20. To illuminate our analysis we show alternative representations of the crystal structures in termsof orderly arrays of coordination defects and in terms of fluorite-type modules. We found that inCe7O12, the excess charge resulting from removal of an oxygen atomdelocalizes among all three triclinic Ce sites closest to the O vacancy. InCe11O20, the charge localizes on the next nearest neighbour Ce atoms. Our main result isthat the charge prefers to distribute itself so that it is farthest away from theO vacancies. This contradicts the standard picture of charge localization whichassumes that each of the two excess electrons localizes on one of the cerium ionsnearest to the vacancy. This standard picture is assumed in most calculationsbased on density functional theory (DFT). Based on the known crystal structure ofPr6O11, we also predictthat the charge in Ce6O11 will be found in the second coordination shell of the O vacancy. We alsoextend the analysis to the Magnéli phases of titanium and vanadium oxides (MnO2n − 1, whereM = Ti, V) and consider the problem of metal–insulator transitions (MIT) in these oxides. Wefound that the bond valence analysis may provide a useful predictive tool in structureswhere 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 theMagnéli phases of titanium and vanadium, our approach to characterizing electronicproperties of oxygen vacancies and the physical insights gained should also berelevant to surface defects and to other rare earth and transition metal oxides.