Charge localization in complex oxides: Classical and quantum mechanical concepts

Abstract

In this contribution, we present a brief overview of the classical and quantum mechanical theory of charge localization in solids with application to the nonstoichiometric oxides SrFeO_3−x and WO_3−x. The cubic high-temperature phase of SrFeO_3−x has been studied applying a Monte Carlo procedure with flexible classical charges. Oxygen vacancies exhibit a linear Fe(III)–vacancy–Fe(III) arrangement, a defect that can be described as a quadrupolaron considering its lowest nonvanishing multipole moment. This defect helps to rationalize the broad range of stability of the perovskite and the absence of polaron hopping. From a quantum mechanical perspective, we apply a nonperturbative molecular orbital approach to small polaron formation and hopping in WO_3−x. A tight-binding Hamiltonian is extended by a nonretarded reaction field to account for the dielectric polarizability of the solid. The model can be solved self-consistently, leading to a localized excess charge distribution. The resulting energy barrier for polaron hopping and the impact of dynamic delocalization effects are discussed.