Transmission Electron Microscopy Investigation Of Transition Metal Compounds Synthesized Under High Pressure And Theoretical Study Of Electron Energy-Loss Spectroscopy

Transmission electron microscope(TEM)is a powerful scientific instrument which can be used for small size imaging.In recent decades,it has helped scientists to make important discoveries such as carbon nanotube and quasicrystal,and has been widely used in materials science,condensed matter physics,structural biology and other fields.The principle of transmission electron microscope is similar to that of optical microscope,but its resolution is far superior to that of optical microscope.Compared with the resolution of optical microscope about 300 nm,the resolution of transmission electron microscope can reach 0.039 nm at present.In addition to excellent spatial resolution,transmission electron microscope has many functions such as structural analysis,elemental composition analysis and electronic structure analysis.TEM can be also used to carry out in-situ observation and time resolution observation.Because of its several advantages mentioned above,transmission electron microscopy attracts much attention since its birth in the early 1930s,and new methods are constantly appearing.In this dissertation,we have performed the following four parts including both experiments and theoretical calculations:1.Perovskite chromate Sr Cr O₃ containing Cr⁴⁺ions shows anomalous electronic states and physical properties,which are believed to be related to the bonding instability,but this has not been confirmed directly in experiments up to now.In order to address this issue,the crystal structure and electronic structure of Sr Cr O₃ are investigated by using transmission electron microscopy and first-principles calculations.The results demonstrate that there is no four-fold rotational symmetry in the selected area electron diffraction patterns along the three main zone axes of Sr Cr O₃,which is inconsistent with a reported cubic structure.Moreover,the orientation-dependent electron energy-loss spectra demonstrate a clear correlation between the dissimilar spectra and the anisotropic effects derived from the crystal structure,and first-principles calculations support the experimental results.All these strongly support the bonding instability of ambient pressure and room temperature phase of Sr Cr O₃ and highlight the important influence of bonding fluctuation on its electronic structure and transport property.The influence of the core-hole effect and Hubbard potential on the theoretical spectra is further investigated.2.The rare high-oxidation Cr⁴⁺of Ruddlesdene-Popper chromate?-Sr₂Cr O₄ with3d² electronic configuration is considered as a new candidate to investigate spin and orbital physics.We have studied?-Sr₂Cr O₄ synthesized under high pressure by transmission electron microscopy at variable temperatures,and found the change of bond length and electronic structure at low temperatures.At first,we studied the crystal structure of?-Sr₂Cr O₄ at room temperature by SAED and ABF and HAADF images,and then studied the orientation-dependent EELS of?-Sr₂Cr O₄ at low temperature(89 K).The results show that although the octahedral distortion of Cr O₆and the change of bond length of Cr-O bond caused by temperature decrease are difficult to be detected by electron microscopy image,the orientation-dependent EELS gives strong evidence of the change of crystal structure and electronic structure for?-Sr₂Cr O₄ at low temperature because of its high sensitivity to bond length and atomic local environment.First-principles calculations confirm the experimental results of EELS.3.Electron energy-loss spectroscopy is widely applied combining with transmission electron microscopes with high spatial resolution,but its interpretation is a challenging task.One of the reasons is that the factors affecting EELS are very complicated.In this dissertation,we focus on several factors involved in density functional theory calculations.The sensitivity of calculated energy-loss near-edge structure to magnetic structure,lattice compression and expansion,and on-site Coulomb energy has been discussed.Since EELS technique detects the local environment of atoms,the influence of magnetic structures cannot be ignored.The chemical shift and peak intensity of EELS are also closely related to corresponding lattice parameters.The correlation effects are very important for transition metal compounds and play an important role in EELS simulations.Our work helps to understand how these factors affect EELS and to explain and predict the experimental EELS spectra.Through the discussion to these factors,we provide a useful guidance for more precise EELS simulations.4.Hexagonal boron nitride has attracted considerable interest over the past decade due to its excellent properties and potential applications.As a means of manipulating physical properties,the effect of antisite defects and their density on monolayer hexagonal boron nitride is discussed in detail in this dissertation.We set up different supercell sizes to simulate different defect densities.All the structures containing antisites are fully optimized.It is indicated that higher density of antisite defects leads to the instability of the B-B bond.The influence of supercell size on lattice structure is also summarized.Like vacancies and dopant atoms,the antisite defects also lead to the appearance of the defect energy band.Different antisite defect densities have different effects on different orbitals.We use electron energy-loss spectroscopy to analyze the effect on the electronic structure.It is shown that the high density of antisite defects promotes some transition processes,weakens some other transition processes.The concentration of antisite defects plays a key role in manipulating the physical properties of monolayer hexagonal boron nitride and will be helpful to expand potential application scenarios.