The electron shuffle: Cerium influences samarium 4f orbital occupancy in heteronuclear Ce-Sm oxide clusters.

ABSTRACT

The anion photoelectron (PE) spectra along with supporting results of density functional theory (DFT) calculations on SmO-, SmCeOy-, and Sm2Oy- (y = 1, 2) are reported and compared to previous results on CeO- [M. Ray et al., J. Chem. Phys. 142, 064305 (2015)] and Ce2Oy- (y = 1, 2) [J. O. Kafader et al., J. Chem. Phys. 145, 154306 (2016)]. Similar to the results on CexOy- clusters, the PE spectra of SmO-, SmCeOy-, and Sm2Oy- (y = 1, 2) all exhibit electronic transitions to the neutral ground state at approximately 1 eV e-BE. The Sm centers in SmO and Sm2O2 neutrals can be described with the 4f56s superconfiguration, which is analogous to CeO and Ce2O2 neutrals in which the Ce centers can be described with the 4f 6s superconfiguration (ZCe = ZSm - 4). The Sm center in CeSmO2, in contrast, has a 4f6 occupancy, while the Ce center maintains the 4f 6s superconfiguration. The less oxidized Sm centers in both Sm2O and SmCeO have 4f6 6s occupancies. The 4f6 subshell occupancy results in relatively weak Sm-O bond strengths. If this extra 4f occupancy also occurs in bulk Sm-doped ceria, it may play a role in the enhanced O2- ionic conductivity in Sm-doped ceria. Based on the results of DFT calculations, the heteronuclear Ce-Sm oxides have molecular orbitals that are distinctly localized Sm 4f, Sm 6s, Ce 4f, and Ce 6s orbitals. The relative intensity of two electronic bands in the PE spectrum of Sm2O- exhibits an unusual photon energy-dependence, and the PE spectrum of Sm2O2- exhibits a photon energy-dependent continuum signal between two electronic transitions. Several explanations, including the high magnetic moment of these suboxide species and the presence of low-lying quasi-bound anion states, are considered.