Microstructures In The Iron-based Superconductors Studied By Transmission Electron Microscopy

In this dissertation we applied multiple methods of transmission electron microscopy to study the microstructures of the iron-based superconducting and nonsuperconducting samples.Main points are summaried as follows:1. We succesfully preprared a set of TEM samples by multiple methods to obtain high-quality images, including dispersing onto a micro-grid, embedding, cleavage and Ar-ion thinning, and focused ion beam milling.2. In the superconducting K0.7Fe1.77Se2, K0.8Fe1.7SeS, Rb0.8Fe1.7S2, we found that there are obvious modulations and phase separations. We applied the selected-area electron diffraction (SAED) to characterize the structure modulations in these three samples; at lease four types of the modulations induced by the Fe-vacancy ordering had been identified. Meanwhile, the modulation induced by the K-vacancy ordering had also been revealed, which usually correspond to parent regions outside the superconducting domains or accompany the situation involving the Fe-vacancy sometimes.3. We had carefully investigated the microstructure and morpholoy of the superconducting K0.8Fe1.7SeS, two kinds of the morphologies and three-dimensional distribution of the superconducting domains were uncovered. High-angle annular dark-field (HAADF) images and dark-field images revealed that there are two kinds of superconducting morphologies along the [001] direction:one is the coexistence of the superconducting phase and an antiferromagnetic phase; another is the superconducting domain composed of micro-domains from several nanometer to a hundred nanometer. STEM-EDS results indicate that it is a Fe-rich and K-poor region inside domains and Fe-poor region outside domains, which is in agreement with the electron enery-loss spectra (EELS) experiments. In addtion, we analysed the morphologies from both the [001] and [100] directions and concluded that the superconducting domains are connected with each other, promising its superconducting behavior in this phase-separated system.4. For the nonsuperconducting Rb0.8Fe1.7S2, Fe-vacancy-related modulations are dominated and their modulated diffractions are obviously stronger than that of superconducting samples mentioned above. The moduations involved K-vacancy are always accompanied by the Fe-vacancy. Many stripes had been observed along the [110] direction. High-resolution transmission electron microscope (HRTEM) images revealed that stripes are amorphous, while regions between stripes are good crystalline lattices.