Abstract
The microscopic origin of chemical expansion in perovskite oxides, due to formation of oxygen vacancies accompanied by formal reduction of a 3d transition metal, is studied by first-principles calculations. We compare the II–IV manganite and titanate series, having Ca, Sr, or Ba on the A site. In particular, the effect of electron localization is elucidated by systematically varying the Hubbard U, and we find that the localization behavior is significantly different in the manganites and titanates. The chemical expansion is explicitly calculated for all compounds, and we demonstrate that increasing on-site repulsion (Hubbard U) on the B site in the lattice yields increased chemical expansion in the manganites and reduced chemical expansion in the titanates. The opposite behavior of the manganites and titanates arises from different electrostatic screenings of oxygen vacancies. We show that this can be attributed to differences in electronic energy levels, specifically that Mn–O bonds are more covalent than Ti–O bonds. Fundamental understanding of electronic and crystal chemical origins of the important phenomenon of chemical expansion is required for rational design of oxide materials for energy technology, sensors, and actuators. We hope our analysis will inspire further fundamental studies of other oxides for solid state ionics applications.
Highlights
Ever since Stuart Adler introduced chemical expansion as a term for a physical phenomenon in electroceramics in 2001,1 there have been many efforts[2−4] to provide an understanding of the underlying microscopic mechanisms
We first consider structural distortions caused by the formation of an oxygen vacancy in a charge neutral supercell corresponding to an oxygen deficiency of δ = 0.0414 and with fixed lattice parameters to simulate a low concentration
Since the oxygen vacancy has a relative charge of +2, positively charged nearest neighbor ions are displaced away from the vacancy whereas the negatively charged oxygen ions displace toward the vacancy as expected from electrostatic considerations
Summary
Ever since Stuart Adler introduced chemical expansion as a term for a physical phenomenon in electroceramics in 2001,1 there have been many efforts[2−4] to provide an understanding of the underlying microscopic mechanisms. Chemical expansion is defined as the spatial dilation of a material upon compositional changes, e.g., point defects such as cation or oxygen vacancies or interstitials. Stoichiometric expansion ε (in one dimension) resulting from a compositional change, δ, is given by ε = a − a0 a0 αCδ (1). In applications based on ionic conductivity, i.e., solid oxide fuel cells[5] or battery electrodes,[6] chemical expansion can lead to large mechanical stresses and cause device deterioration or failure.[7,8] Oxygen vacancies have attracted particular attention, being unavoidable in transition metal oxide ceramics at finite temperatures, and tunable by controlling the oxygen partial pressure. Chemical expansion by formation of oxygen vacancies can serve as a route to new functionalities, e.g., making electrochemical actuators,[9] or serve as a strain mediation mechanism under tensile epitaxial strain in thin films.[10−12] Understanding the mechanisms driving chemical expansion is highly important for the selection or design of new materials for various electrochemical applications
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