A polaron cloud of correlated electron states in ceria

TL;DR

This paper investigates the electronic structure of cerium oxide, revealing how 4f electron localization varies with oxidation state and proposing that oxygen vacancy diffusion involves polaron hopping with a correlated 4f electron cloud.

Contribution

It introduces a combined ab initio and many-body physics approach to analyze 4f electron localization and models oxygen vacancy diffusion as polaron hopping involving correlated 4f electrons.

Findings

CeO₂ has itinerant 4f states.

Ce₂O₃ exhibits nearly localized 4f states.

Oxygen vacancy diffusion involves polaron hopping with a correlated 4f electron cloud.

Abstract

The electronic structure of cerium oxide is investigated here using a combination of ab initio one-electron theory and many-body physics, with emphasis on the nature of the 4f electron shell of cerium ions. We propose to use the hybridization function as a convenient measure for the degree of localization of the 4f shell of this material, and observe that changing the oxidation state is related to distinct changes in the hybridization between the 4f shell and ligand states. The theory reveals that CeO$_2$ has essentially itinerant 4f states, and that in the least oxidized form of ceria, Ce$_2$O$_3$, the 4f states are almost (but not fully) localized. Most importantly, our model points to that diffusion of oxygen vacancies in cerium oxide may be seen as polaron hopping, involving a correlated 4f electron cloud, which is located primarily on Ce ions of atomic shells surrounding the vacancy.