The behavior of mixed-metal oxides: Structural and electronic properties of Ce1−xCaxO2 and Ce1−xCaxO2−x

Abstract

Synchrotron-based time-resolved x-ray diffraction (TR-XRD), x-ray absorption near edge spectroscopy (XANES), Raman spectroscopy (RS), and first-principles density functional (DF) calculations were used to study the structural and electronic properties of Ce–Ca mixed-metal oxides. The XRD results and DF calculations show that doping with calcium induces relatively minor variations (<0.05 Å) in the cell dimensions of ceria. However, the presence of Ca leads to slightly distorted tetragonal structures, a substantial strain in the lattice of the oxide and a tendency to form O vacancies in an ideal Ce1−xCaxO2 solid solution. The two latter effects can be a consequence of the large number of oxygen neighbors that Ca is forced to have in Ce1−xCaxO2 and differences in the electronic charges of calcium and cerium cations. The Ce1−xCaxO2−x systems are not fully ionic. Cation charges derived from the DF calculations indicate that these systems obey the Barr model for charge redistribution in mixed-metal oxides. The Ca atoms in Ce1−xCaxO2−x are more electropositive than the cations in CaO, while the Ce cations of Ce1−xCaxO2−x are less electropositive than those of CeO2. These trends are consistent with XANES measurements at the Ca K- and Ce LIII-edges. The cation charge redistributions should be taken into consideration when explaining or predicting the chemical and catalytic properties of Ce1−xCaxO2−x. Ca induces structural and electronic perturbations on ceria quite different from those found after doping with Zr. The behavior of Ce1−xCaxO2−x illustrates the drastic effects that doping with an electropositive element can have on the properties of ceria.