DOI: 10.1038/NMAT3393
原文链接：
https://www.nature.com/articles/nmat3393
分享一篇12年前的文章！
对于SOFC的阴极材料，氧空穴非常重要，与材料的导电性、氧扩散系数、催化性能等密切相关。而对于氧空穴的表征工具，本公众号里分享过综述文献，感兴趣可以自行查阅。
本论文的研究目的在于了解晶胞尺度上的氧空穴分布，进而明确SOFC运行过程中的原子机制。作者开发了一种基于扫描透射电镜局部晶格参数测量直接映射氧空穴浓度的策略。化学膨胀性的概念被证明适用于亚晶胞水平：局部化学计量变化会产生可以量化的局部晶格膨胀。该方法已经成功应用于在不同对称性基底上外延生长的镧锶钴铁薄膜，其中极化种子反射法显示磁性存在很大差异。在两层薄膜中发现的不同氧空穴含量表明氧化学势的变化是不同磁性的来源，为氧空穴浓度及其梯度的结构调整开辟了途径。
a,b, ADF images of [110]c-oriented LSCO thin films grown on different substrates, NGO (a) and LSAT (b). c, A representative STEM–EELS result for a bulk region of the LSCO film grown on NGO substrate. The alternating dark contrast (marked as red arrows) in every other Co–O plane results from the structural relaxation due to oxygen vacancy ordering in the planes (green bar graph shows O K edge intensity oscillation); an intensity modulation can also be seen in the overlaid line trace of the ADF signal (teal graph). The Co L3/L2 ratio (yellow circles) does not show significant modulations. Scale bars, 2 nm.
a,b, Top-most images on both sides are portions of ADF STEM images of LSCO on NGO (a) and LSCO on LSAT (b) taken in the pseudo-cubic [110] zone axis orientation. The graphs at the bottom of each image show atomic spacings along the out-of-plane (solid circles) and in-plane (solid squares) directions, averaged over vertical atomic rows of the images. The error bars show the standard deviation with respect to averaging for each (vertical) atomic layer in the image. The position of the interface can be tracked by the B-site cation ADF intensities across the interface (cyan bar graphs). Before quantification, STEM images were de-noised using Wiener filtering (HREM-Filters software package, HREM Research).
a, Interatomic spacing changes of A-site cations in the two respective systems, LSCO on NGO (red) and LSCO on LSAT (green), along the c axis. The spacing changes for oxygen-deficient layers and stoichiometric layers in the structures are represented by solid circles and triangles, respectively. The error bars show the standard deviation with respect to averaging along the interface in multiple images (~10 for each data set). b, Schematic of a brownmillerite structure La0.5Sr0.5CoO2.5, where green spheres represent La/Sr atoms, blue spheres represent octahedral Co sites, orange spheres represent tetrahedral Co sites and small red spheres represent O atoms. Note the vertical shifts of Co in the tetrahedral layers. c, ADF STEM image of the [100]b (subscript b denotes brownmillerite)-oriented LSCO grown on NGO substrate showing the characteristic in-plane (vertical in the figure coordinates) pairwise shift of Co ions in the oxygen-depleted planes (solid arrows). The corresponding simulated image for the chemical composition of La0.5Sr0.5CoO2.5 is given in the inset. d, Plot of the Co pairwise shift averaged over atomic rows of the LSCO film on NGO. The error bars show the standard deviation with respect to averaging for each (vertical) atomic layer in the image.
a, Lattice expansivity as a function of oxygen deficiency (x in CoOx). A linear fit (solid blue line) was obtained from DFT calculations (marked as solid green circles) for five models with different oxygen contents from x = 2 (a stoichiometric Co–O layer in cubic perovskite) to x = 1 (for an oxygen-deficient layer in brownmillerite); variability for LSCO/LSAT layer spacing is due to both instrumental error (black error bars) and intrinsic inhomogeneity (red cross-hatched box). b, Average standard deviations of different lattice spacing measurements can be used to estimate the measurement error (see text) and separate the contribution due to inhomogeneity for LSCO/LSAT. c,d, Bird’s eye views of out-of-plane lattice spacing mapping results for the respective LSCO/NGO and LSCO/LSAT systems. The local oxygen content for each system is directly visualized over the whole region imaged by STEM; note the higher inhomogeneity of the oxygen-deficient layer composition for LSCO/LSAT (Fig. 4d). The oxygen-deficient layers in both LSCO structures are marked as solid arrows. e,f, Results of the PNR data showing the reduction in the NSLD for the films on NGO, indicating an agreement with STEM observations. The PNR also shows the impact of the observed effects on magnetism, which is approximately twice as large for the films on NGO.
总而言之，作者展示了一种新范式，可以高精度高效地映射有序和无序钙钛矿中的氧空位浓度，将化学膨胀性的概念扩展到亚晶胞水平。此外，还证明了基底对称性产生的倾斜效应可以影响薄膜中的整体氧化学计量。这些研究对于固体氧化物燃料电池材料和设备的探索以及对氧化物材料和界面的一般物理性质的理解具有直接意义。大家感兴趣可以仔细阅读原文，参考借鉴相关知识。
