In a solid oxide fuel cell whose cathode is composed of Sr-doped LaMnO3 (Sr-doped LaMnO3, LSM) and Y2O3-stabilized ZrO2, the reduction of oxygen on the surfaces of the LSM is critical to the cell performance. As a first step toward identifying the key LSM surfaces that are present in the cell, the energetics of cubic LaMnO3 (cubic LaMnO3, c-LM) surfaces are appropriate to examine. Previously, the surface energies of the first six c-LM high-index surfaces, (210), (211), (221), (310), (311), and (320), were computed at the density-functional theory level by the authors of this work. Four of the surfaces, (210), (320), (211), and (221), were found to be more stable than the c-LM low-index surfaces (011) and (111), while the (310) and (311) surfaces were difficult to characterize due to a structural transformation or phase change in the surface models of the surfaces away from c-LM. Here, a detailed structural and atomic charge analysis of the first six c-LM high-index surfaces is given. All six surfaces are shown to undergo a significant restructuring upon relaxation, explaining the relatively small surface energies obtained for four of the surfaces. Also, by performing a Bader charge analysis, all six surfaces are shown to be polar and compensated by an electronic charge redistribution. The fact that the c-LM low-index surfaces and high-index surfaces examined here are all polar and compensated by the same mechanism is consistent with the surface energy results. Next, the surface energies of the c-LM low-index surfaces, previously computed by one of the authors of this work, are revisited to discuss small oscillations observed in the unrelaxed surface energy versus model thickness for the (111) surface. An argument is made that the oscillations are a quantum-size effect arising from the finite thicknesses of the surface models.

Structures and atomic charges of LaMnO3 high-index surfaces and surface energies of LaMnO3 low-index surfaces, revisited

ACS Spring 2022

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