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首页> 外文期刊>The journal of physical chemistry, C. Nanomaterials and interfaces >Dynamical Scattering and Electron Channeling in Orthorhombic and Tetragonal LaFeAsO
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Dynamical Scattering and Electron Channeling in Orthorhombic and Tetragonal LaFeAsO

机译：正交和四方LaFeAsO中的动态散射和电子沟道

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In the study of LaFeAsO and doped compounds thereof, high-resolution transmission electron microscopy (TEM) has been used to characterize the structural and morphological properties, while cryo-TEM has been used to purportedly observe the structural phase transition occurring at 160 K. Often, the appearance and disappearance of Bragg spots in diffraction patterns, as well as changes in diffraction contrast in bright-field images, have been pointed to as indicators of the phase transition. Here we show that effects not related to the transition can produce signatures reminiscent of those typically associated with the symmetry change. In particular, we demonstrate the effects of electron channeling and multiple scattering on intensity modulation of atomic columns and Bragg spots in high-angle annular dark-field scanning TEM (HAADF-STEM) images and parallel-beam electron diffraction (PBED) patterns, respectively, from both tetragonal and orthorhombic LaFeAsO. From electron-transparent lamellae, we quantify the spatially varying thickness and, via atomic-resolution HAADF-STEM imaging, demonstrate the thickness-dependent modulation of intensities within the (Fe,O) and (La, As) columns at 300 K. From PBED patterns and Fourier-filtered high-resolution bright field images acquired both above and below the structural phase-transition temperature, we show how intensities of forbidden reflections are modulated by moving to regions of differing thickness, independent of a symmetry change (i.e., at a fixed temperature). The experimental results are supported with multislice simulations of thickness-dependent atomic-column contrast and Bragg-spot intensities.
机译：在LaFeAsO及其掺杂化合物的研究中，高分辨率透射电子显微镜（TEM）被用于表征结构和形态学特性，而冷冻TEM被用于观察在160 K下发生的结构相变。 ，衍射图谱中布拉格点的出现和消失，以及明场图像中衍射对比度的变化已被指示为相变的指标。在这里，我们显示与过渡无关的效果可以产生使人联想到通常与对称性变化相关的签名的特征。特别地，我们分别证明了电子通道化和多重散射对高角度环形暗场扫描TEM（HAADF-STEM）图像和平行束电子衍射（PBED）模式中原子列和布拉格点强度调制的影响。 ，来自四方和正交LaFeAsO。从电子透明薄片中，我们量化了空间变化的厚度，并通过原子分辨率HAADF-STEM成像，展示了在300 K下（Fe，O）和（La，As）列中强度的厚度依赖性调制。在结构相变温度之上和之下获取的PBED图案和经傅立叶滤波的高分辨率明场图像，我们展示了如何通过移动到不同厚度的区域来调制禁止反射的强度，而与对称性变化无关（即固定温度）。实验结果得到了与厚度有关的原子-柱对比和布拉格斑点强度的多层模拟的支持。

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