In this work, the $A$- and $B$-site cation migration pathways involving defect complexes in bulk ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{MnO}}_{3\pm \delta }$ (LSM) at $x=0.0–0.25$ are investigated based on density-functional-theory modeling for solid-oxide fuel-cell (SOFC) cathode applications. We propose a dominant $A$-site cation migration mechanism which involves an $A$-site cation (e.g., ${\mathrm{La}}_A^x$) hop into a $V_A^{\prime \prime \prime }$ of a $V_A^{\prime \prime \prime }\text{−}{V}_B^{\prime \prime \prime }$ cluster, where ${\mathrm{La}}_A^x$, $V_A^{\prime \prime \prime }$, and, $V_B^{\prime \prime \prime }$ are ${\mathrm{La}}^{3+}$, $A$-site vacancy, and $B$-site vacancy in bulk LSM, respectively, and $V_A^{\prime \prime \prime }\text{−}{V}_B^{\prime \prime \prime }$ is the first nearest-neighbor $V_A^{\prime \prime \prime }$ and $V_B^{\prime \prime \prime }$ pair. This hop exhibits an approximately 1.6-eV migration barrier as compared to approximately 2.9 eV of the ${\mathrm{La}}_A^x$ hop into a $V_A^{\prime \prime \prime }$. This decrease in the cation migration barrier is attributed to the presence of the $V_B^{\prime \prime \prime }$ relieving the electrostatic repulsion and steric constraints to the migrating $A$-site cations in the transition-state image configurations. The $V_A^{\prime \prime \prime }\text{−}{V}_B^{\prime \prime \prime }$ interaction energy is predicted to be weakly repulsive (0.2–0.3 eV) in bulk LSM, which enables the $V_A^{\prime \prime \prime }\text{−}{V}_B^{\prime \prime \prime }$ cluster to readily form. The predicted apparent activation energy of $D_{\mathrm{La}}^{*}$ in ${\mathrm{LaMnO}}_{3\pm \delta }$ (LMO) for the $A$-site migration pathway is about 1.4 eV, in good agreement with the experimental $A$-site cation impurity diffusivity measurements. By examining the $A$-site cation migration barriers among different metal cations (${\mathrm{Zr}}^{4+}$, ${\mathrm{Y}}^{3+}$, ${\mathrm{Gd}}^{3+}$) relevant for SOFC applications, it is demonstrated that migration barriers of the cation impurity in bulk LSM correlate with the ionic charge and ionic radius at a given formal cationic charge. The $B$-site cation migration barrier takes place by an analogous mechanism that involves a ${\mathrm{Mn}}_B^x$ (${\mathrm{Mn}}^{3+}$ on the $B$ site) hop into a $V_B^{\prime \prime \prime }$ via the ${\mathrm{Mn}}_B^x\text{−}{V}_A^{\prime \prime \prime }\to V_B^{\prime \prime \prime }$ path with the same cation transport carrier of $V_A^{\prime \prime \prime }\text{−}{V}_B^{\prime \prime \prime }$. This diffusion pathway is found to have a barrier of approximately 1.6 eV, similar to the analogous $A$-site hop. However, hopping of the ${\mathrm{Mn}}_A^x$ antisite defect (${\mathrm{Mn}}^{3+}$ on the $A$ site) to a nearest-neighbor $V_A^{\prime \prime \prime }$ [${\mathrm{Mn}}_A^x$ ($V_A^{\prime \prime \prime }$) mechanism] has a barrier of only 0.5 eV. Such a low ${\mathrm{Mn}}_A^x$ ($V_A^{\prime \prime \prime }$) migration barrier opens the possibility to activate Mn transport in bulk LSM through the diffusion of the antisite ${\mathrm{Mn}}_A^x$ ($V_A^{\prime \prime \prime }$) pathway on the $A$-site lattice, particularly when the concentration of the Mn antisite defect can be altered upon varying the $A/B$ ratio and the activity of ${\mathrm{MnO}}_y$. The increase in ${\mathrm{Sr}}_A^{\prime }$ doping concentration in bulk ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{MnO}}_{3\pm \delta }$ ($x=0.0–0.25$) is found to influence primarily the formation energies of cation transport carriers (cation vacancies), whereas the cation migration barriers exhibit only a weak dependence on the ${\mathrm{Sr}}_A^{\prime }$ concentration.

• Received 11 March 2017

DOI:https://doi.org/10.1103/PhysRevApplied.8.044001