科研之友
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摘要
In this paper, we perform a comparative study based on ab initio modeling for perovskite ABO(3) (001) surfaces and surface defect energetics in order to understand the influence of polarity and redox activeMn in the LaMnO3 system. We consider LaMnO3, LaAlO3, SrTiO3, and briefly LaFeO3 systems for comparison, which illustrate the interplay between properties of polar surfaces and the varying d-electron shell of transition metals. We are motivated by the need to understand the surfaces of mixed electronic and ionic conductors typically used in solid oxide fuel cell cathodes and other ion conducting technologies, which are represented here by the LaMnO3 system. We focus on the influence of the metal character and surface polarity on the surface and surface defect chemistry in these selected systems. We demonstrate that the facile redox of the TM (3d(4)) in LaMnO3 with partial e(g) orbital occupation (or specifically e(g) occupancy close to 1) allows the polar surfaces to be compensated by changes in charge density over relatively short length scales (3 to 4 unit cells or similar to 1.5 nm) near the surface as compared to LaAlO3. In contrast to LaAlO3, this low-energy and short-range screening mechanism leads to low surface energies without any additional reconstruction, rapidly converging surface properties with film thickness (by similar to 8 unit cells), bulklike defect chemistry more than similar to 1.5 nm from the surface, and surface defect energetics that are primarily governed by the local charge doping or the created electric field near the polar surfaces. We show that LaMnO3 exhibits very different surface properties from LaAlO3 and SrTiO3, thereby demonstrating that these properties are due to the presence of the redox active transition metal with partial e(g) orbital occupation and a polar surface, respectively. These understandings can help guide qualitative analysis, computational study, and design of surfaces of mixed electronic and ionic conductors.
- 出版日期2015-5-21
- 单位MIT
