Comparison of the bulk and surface properties of ceria and zirconia by ab initio investigations

In this paper, we present quantum mechanical (QM) calculations, at a period ic Hartee-Fock (HF) level, on the bulk and surface properties of cubic CeO2 and ZrO2. We have investigated the M-O bending features, and established t he high degree of ionicity of both materials, which is greater for CeO2 tha n ZrO2. The calculated values for the C-11, C-12, and C-44 elastic constant s, are in close agreement with experiment; an extended oxygen basis set, co ntaining d-orbital polarization functions, is essential to model accurately the symmetry lowering during the C-44 distortion. In the surface studies, we have calculated the surface energies of the {011} and {111} faces of bot h ceria and zirconia. QM results are compared with interatomic potential-ba sed (IP) methods to assess the accuracy of the latter. We found that IP met hods provide a correct estimate of the surface relaxations and the correct order of stability of the two faces examined, with the energy difference be tween the {011} and the {111} surfaces being approximately 1 J/m(2), as in the QM study, However, IP methods do not always discriminate adequately bet ween the properties of the two materials. Finally, we found that geometric and electronic relaxations in the {111} surface are confined to the outermo st oxygen ions, while in the {011} slabs they are more important and extend to the subsurface layers in a columnar way, The unsaturation of the surfac e ions in the {011} face may have important implications for the catalytic activity of the materials.