The electron shuttle: Cerium influences samaraium 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$^−$, SmCeO$_y$$^−$, and Sm2O$_y$$^−$ (y = 1, 2) are reported and compared to previous results on CeO$^−$ [M. Ray et al., J. Chem. Phys. 142, 064305 (2015)] and Ce2O$_y$$^−$ (y = 1, 2) [J. O. Kafader et al., J. Chem. Phys. 145, 154306 (2016)]. Similar to the results on Ce$_x$O$_y$$^−$ clusters, the PE spectra of SmO$^−$, SmCeOy$^−$, and Sm$_2$O$_y$$^−$ (y = 1, 2) all exhibit electronic transitions to the neutral ground state at approximately 1 eV e−BE. The Sm centers in SmO and Sm$_2$O$_2$ neutrals can be described with the 4f56s superconfiguration, which is analogous to CeO and Ce$_2$O$_2$ neutrals in which the Ce centers can be described with the 4f 6s superconfiguration (ZCe = ZSm − 4). The Sm center in CeSmO$_2$, in contrast, has a 4$f$6 occupancy, while the Ce center maintains the 4$f^{6}$6$s$ superconfiguration. The less oxidized Sm centers in both Sm$_2$O and SmCeO have 4$f$6 6$s$ occupancies. The 4$f$6 subshell occupancy results in relatively weak Sm–O bond strengths. If this extra 4$f$ occupancy also occurs in bulk Sm-doped ceria, it may play a role in the enhanced O$^{2−}$ ionic conductivity in Sm-doped ceria. Based on the results of DFT calculations, the heteronuclear Ce–Sm oxides have molecular orbitals that are distinctly localized Sm 4$f$, Sm 6$s$, Ce 4$f$, and Ce 6$s$ orbitals. The relative intensity of two electronic bands in the PE spectrum of Sm$_2$O$^−$ exhibits an unusual photon energy-dependence, and the PE spectrum of Sm$_2$O$_2$$^−$ 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.