We have investigated the formation of intrinsic defects in CeO2 using density functional theory with the generalized gradient approximation (GGA) corrected for on-site Coulombic interactions (GGA+U). We employed an ab initio fitting procedure to determine a U{O2p} value that satisfies a Koopmans-like condition and obtained a value of U{O2p} = 5.5 eV. We subsequently demonstrated that by applying GGA+U to the O2p states, in addition to the Ce4f states, we were able to model localized holes in addition to localized electrons, thus improving the description of p-type defects in CeO2. Our results show that under oxygen-poor conditions the defects with the lowest formation energy are oxygen vacancies, while oxygen interstitials, which form peroxide ions, will be more favorable under oxygen-rich conditions. We carried out temperature and pressure dependence analyses to determine the relative abundance of intrinsic defects under real-world conditions and determined that oxygen vacancies will always be the dominant defect. Furthermore, we determined that at the dilute limit none of the defects studied can account for the intrinsic ferromagnetism that has been observed in nanosized CeO2.

Intrinsic ferromagnetism in CeO(2) is a source of controversy in the literature and has been linked to the excess electrons left over upon oxygen vacancy formation on Ce sites neighbouring the vacancy. A recent theoretical study (Han et al 2009 Phys. Rev. B 79 100403) concluded that increased vacancy concentration changes the localization behaviour of CeO(2), resulting in some degree of charge localization in the vacancy site itself, which leads to superexchange and polarization effects that enhance the stability of ferromagnetism. In this report, we show conclusively that oxygen vacancy concentrations of up to 12.5% do not cause localization in the vacancy site, and that this is not responsible for any enhanced ferromagnetism. Investigation of oxygen vacancies on the (111), (110) and (100) low index surfaces also show no evidence for ferromagnetic preference.

Due to its high dielectric constant, large band gap, and very small lattice mismatch with Si, CeO 2 has been proposed as a promising candidate high-k dielectric material. The performance of CeO 2 as a dielectric material, however, is severely limited due its propensity for facile reduction (oxygen vacancy formation), which causes a high interface state density, and subsequent decreased drain currents. In this article we use density functional theory (DFT) to screen for trivalent dopants which could decrease the concentration of defects in CeO 2 samples. We demonstrate that La and Y are the most soluble trivalent dopants in CeO 2 , and can reduce the number of the electrons in the system both ionically (formation of [M Ce -V O -M Ce ] clusters) or to a lesser extent electronically (hole formation). La doping also increases the lattice constant of CeO 2 , improving the lattice match with Si.

The doping of CeO2 with trivalent cations is a common technique for enhancing ionic conductivity in electrolytes for solid oxide fuel cell applications. However, the local defect structure in these materials is yet to be fully explored. Furthermore, many studies have overlooked the effect of the dopants on the reducibility of CeO2, which is important as electronic conductivity can short-circuit the fuel cell. Density functional theory (DFT)+U calculations have been performed on a series of CeO2 systems doped with trivalent cations. The most stable configuration and the relative attraction between dopant cations and oxygen vacancies were determined, and it was found that the defect structure is principally dependent on the ionic radius of the dopant cations. The reduction energy was found to be dependent on the structure around the dopants but did not vary significantly between dopants of similar ionic radii. From these results, it is possible to suggest which trivalent cations would be most suitable to enhance ionic conductivity without increasing electronic conductivity in solid oxide fuel cell electrolytes.

The oxygen states on CeO 2 surfaces were investigated with DFT+U calculations. The results reveal the variable nature of the oxygen states, including the never before modelled intrinsic peroxide surface defect. Under O-rich conditions, the peroxide defects on the (100) and (110) surfaces is more stable than oxygen vacancies. On surfaces doped with La(III) it is found that under O-rich conditions the (100) and (110) surface will preferentially form peroxide ions in response to the presence of the dopants while the (111) surface prefers oxygen vacancies. Calculated shifts in core levels match experimental binding energies, further suggesting the presence of peroxide species.