First-principles calculations are performed by taking into account the strong correlation effects on ceria. To obtain an accurate description including f electrons, the authors optimized the Coulomb U parameter for use in Local-Density Approximation (LDA) and Generalized Gradient Approximation (GGA) calculation. A good agreement with experimental data is obtained within the GGA+U (Wu-Cohen scheme). Elastic stiffness constants are found in correct agreement with the available experimental results. Born effective charge, dielectric permittivity, and the phonon-dispersion curves are computed using density functional perturbation theory. The origin of magnetism in undoped ceria with intrinsic defects is investigated. The authors show that both of Ce and O vacancies induce local moments and ferromagnetism without doping ceria by magnetic impurities in this chapter.
TopIntroduction
Cerium dioxide or ceria has gained a large interest for technological and industrial applications. For instance, it is widely applied in automobile exhaust catalysts as an oxygen storage material, due to its ability to take and release oxygen under oxidizing and reducing conditions. Properties of ceria-based materials, such as electrical conductivity and diffusivity, have been reviewed for its use as an electrolyte in solid oxide fuel cells (Dresselhaus, 2001, Skorodumova, 2001). CeO2 is one of the most studied of the fluorite oxides (Wang, 2003) because of their particular physical properties. Mainly, the lattice mismatch to Si and high dielectric constant, make ceria a potential material for use in microelectronic applications, high-quality epitaxy on Si and buffer layers of high-temperature superconductors (Luo,1991).
Because of its wide technological usage, several studies have already been conducted for CeO2 compound. The hydrostatic pressure dependence of structural properties of CeO2 were investigated by high-pressure Raman (Kourouklis, 1988) and x-ray diffraction (Duclos 1988, Gerward,1993) studies and it has been found that bulk CeO2 has a phase transition from fluorite-type to α-PbCl2-type structure at about 31 GPa pressure. For nanocrystalline CeO2 the transition occurs at significantly lower pressures 26.5 or 22.3 GPa (Wang, 2001). Indeed, the experimental elastic constants were derived from the sound velocity for each acoustic phonon mode estimated from the frequency shift of Brillouin scattering lines (Nakajima,1994).
On the theoretical side, many calculations have been reported by various methods such as periodic Hartree-Fock (Gennard,1999), self-interaction-corrected local-spin-density approximation (Gerward,2005), local-density approximation (LDA), and generalized gradient approximation within density functional theory (DFT) (Skorodumova, 2001, Koelling, 1983 - Da Silva, 2007). More generally, it is well known that LDA and GGA functional usually lead to qualitatively wrong results when they are applied to lanthanide oxide materials containing localized electrons (Perdew, 1981). The main reason for this shortcoming is the self-interaction error (the electrostatic interaction of an electron with itself) that is contained in these approximate functionals and their associated potentials. Corrections to these approximations, LDA+U and GGA+U methods (Anisimov,1997) are usually applied to CeO2 (Gürel, 2006- Sevik, 2009) where Hartree-Fock type interactions were parameterized with Coulomb U and exchange J terms.