Mixed Ionic Electronic Conducting (MIEC) transition metal oxide (ABO₃) perovskites are utilized for catalytic,¹ energy conversion,^2,3 and other applications. The oxygen ionic conductivity of these MIECs depends on their oxygen vacancy concentration and ionic mobility. In turn, the oxygen ion concentration and mobility both depend on the charge of the B-site atom,⁴ which itself depends on factors such as the A site La/Sr ratio, the crystal structure, etc.

Previous calculations demonstrated that the Fe atom charge state distribution observed in strontium ferrite (SrFeO_3-δ) is caused by differences in the d-orbital splitting of octahedral (Oh) versus square pyramidal (SP) Fe-O coordination (Figure 1).⁵ This d-orbital splitting ensures that the two electrons left behind when a charge-neutral oxygen vacancy forms in cubic SrFeO_3-δ are transferred to the second nearest Fe neighbors,⁵ resulting in a large oxygen vacancy polaron size that produces strong oxygen vacancy interactions at high oxygen nonstoichiometry. As a result, the oxygen vacancy formation energy increases with oxygen nonstoichiometry in cubic SrFeO_3-δ.⁵ Previous calculations also demonstrated that in orthorhombic lanthanum ferrite (LaFeO_3-δ) the formation of a neutral oxygen vacancy produces a polaron that is localized to the oxygen-vacancy-adjacent Fe atoms. Hence, in orthorhombic LaFeO_3-δ the oxygen vacancies do not interact with increasing oxygen nonstoichiometry.⁶

The objective of the present work was to examine whether d-orbital splitting could also explain the oxygen vacancy behavior of other MIEC perovskites. Hence, the impact of d-orbital splitting on oxygen vacancy polaron size and formation energies in lanthanum strontium manganite and lanthanum strontium cobaltite were modeled for the first time.

Here, the charge redistribution caused by oxygen vacancy formation was studied for six different compositions: LaFeO_3-δ, SrFeO_3-δ, LaMnO_3-δ, SrMnO_3-δ, LaCoO_3-δ, and SrCoO_3-δ. The GGA+U method with PAW potentials implemented in VASP was utilized for all the structural energy calculations. First, the Hubbard-U parameter was calibrated to describe the charge on the B-site atom (U = 3 was selected for Fe, as done previously).⁶ The oxygen vacancy formation energy as a function of oxygen non-stoichiometry (δ) was then calculated at 0 K in vacuum by varying the size of the supercell, with oxygen vacancy interactions being enabled by the periodic boundary conditions.

The calculated polaron size for neutral-oxygen-vacancy–containing ABO₃ structures are summarized in Table 1.In LaMnO_3-δ, Mn³⁺ has an [Ar] 3d⁴ electronic configuration in Oh coordination. After the formation of a neutral oxygen vacancy, the two Mn atoms adjacent to the oxygen vacancy go from Oh to SP coordination and the excess electrons left behind by the removed oxygen are pushed to the a_1g level of the second nearest neighboring Oh-Mn (instead of remaining localized to the b_1g level of the SP-Fe). This long range charge transfer results in large polaron size. This behavior is in contrast with that of LaFeO_3-δ. However, LaMnO_3-δ and SrFeO_3-δ exhibit comparable polaron sizes since Fe⁴⁺ has an [Ar] 3d⁴ electronic- configuration. In SrMnO_3-δ, Mn⁴⁺ has a [Ar] 3d³ electronic configuration in Oh coordination. Therefore, the formation of a neutral oxygen vacancy localizes the electrons to the oxygen vacancy adjacent Mn atoms, producing a small polaron, as explained by the d-orgbial splitting in Figure 1. This d-orbital splitting analysis indicates that the polaron size in LSM > LSF > LSC. Additional work aimed at calculating the oxygen vacancy formation energies of other ABO₃ compositions are in progress.

References

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3. Jason D. Nicholas, Electrochem. Soc. Interface, 49 (2013).

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6. T. Das, J. D. Nicholas, and Y. Qi, In Preparation (2017).