BaCe$_{0.25}$Mn$_{0.75}$O$_{3-\delta}$ (BCM), a non-stoichiometric oxide closely resembling a perovskite crystal structure, has recently emerged as a prospective contender for application in renewable energy harvesting by solar thermochemical hydrogen generation. Using solar energy, oxygen-vacancies can be created in BCM and the reduced crystal so obtained can, in turn, produce H2 by stripping oxygen from H2O. Therefore, a first step toward understanding the working mechanism and optimizing the performance of BCM, is a thorough and comparative analysis of the electronic structure of the pristine and the reduced material. In this paper, we probe the electronic structure of BCM using the combined effort of first-principles calculations and experimental O K-edge x-ray absorption spectroscopy (XAS). The computed projected density-of-states (PDOS) and orbital-plots are used to propose a simplified model for orbital-mixing between the oxygen and the ligand atoms. With the help of state-of-the-art simulations, we are able to find the origins of the XAS peaks and to categorize them on the basis of contribution from Ce and Mn. For the reduced crystal, the calculations show that, as a consequence of dielectric screening, the change in electron-density resulting from the reduction is strongly localized around the oxygen vacancy. Our experimental studies reveal a marked lowering of the first O K-edge peak in the reduced crystal which is shown to result from a diminished O-2p contribution to the frontier unoccupied orbitals, in accordance with the tight-binding scheme. Our study paves the way for investigation of the working-mechanism of BCM and for computational and experimental efforts aimed at design and discovery of efficient water-splitting oxides.

中文翻译：

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研究预期的水分解氧化物 BaCe$_{0.25}$Mn$_{0.75}$O$_{3-δ}$ 在热还原前后的电子结构

BaCe$_{0.25}$Mn$_{0.75}$O$_{3-\delta}$ (BCM) 是一种与钙钛矿晶体结构非常相似的非化学计量氧化物，最近已成为应用在通过太阳能热化学制氢来收集可再生能源。利用太阳能，可以在 BCM 中产生氧空位，这样获得的还原晶体又可以通过从 H2O 中剥离氧气来产生 H2。因此，了解 BCM 的工作机制和优化性能的第一步是对原始材料和还原材料的电子结构进行彻底的比较分析。在本文中，我们利用第一性原理计算和实验 O K 边 X 射线吸收光谱 (XAS) 的联合努力探索了 BCM 的电子结构。计算的投影态密度 (PDOS) 和轨道图用于提出氧和配体原子之间的轨道混合的简化模型。借助最先进的模拟，我们能够找到 XAS 峰的起源，并根据 Ce 和 Mn 的贡献对它们进行分类。对于还原晶体，计算表明，由于介电屏蔽，由还原引起的电子密度变化强烈地集中在氧空位周围。我们的实验研究表明，根据紧束缚方案，还原晶体中的第一个 O K 边缘峰显着降低，这表明这是由于 O-2p 对前沿未占轨道的贡献减少所致。