Local electronic structure and dynamics of hydrogen in ${\text{CeO}}_2$

Abstract

The local electronic states of muon (Mu) as an isotope of hydrogen (H) in high-quality single-crystalline ceria (CeO${}_2$) are investigated using muon spin rotation/relaxation ($\hat{I}¼\text{SR}$) and first-principles density functional theory (DFT) calculations. Upon positive muon implantation, both paramagnetic (Mu${}^0$) and diamagnetic (Mu${}^{\ast }$) states are observed below $\approx$10 K. Magnetic field dependence of the Mu${}^0$ signals combined with DFT results provides evidence for the formation of a polaron state, consisting of Mu bonded to a ligand oxygen and a $4f$ electron localized on a nearby Ce site. The crystal-orientation dependence of the Mu${}^0$ signal suggests strong anisotropy of the $4f$ electron due to the spin-orbit coupling with lifted degeneracy. Furthermore, the Mu${}^0$ state exhibits transition to the Mu${}^{\ast }$ state that corresponds to another Mu${}^0$ state (exhibiting a diamagnetic response due to fast spin/charge fluctuations) at temperatures above $\approx$10 K, before disappearing above $\approx$30 K. These findings suggest rapid diffusive motion of the $4f$ electrons and/or Mu (as well as H) at higher temperatures.

7 pages, 5 figures, with supplemental material