ELNES investigation of spinels

Docherty, Frances Therse (2001) ELNES investigation of spinels. PhD thesis, University of Glasgow.

Abstract

Electron energy loss spectroscopy (EELS) provides information about the energy loss suffered by primary electrons when they interact with a specimen. The resulting energy loss spectrum contains edges due to the excitation of core electrons to unoccupied states in the conduction band. These edges exhibit fine structure, known as energy-loss near-edge structure (ELNES), which is dependent on the local chemical and structural environment of the absorbing atom. The interactions that cause ELNES are complex and not well understood, making the interpretation of the fine structure from complex materials very difficult. The understanding of this technique can be improved by examining data collected from a series of compounds with the same crystal structure, to investigate the effect on the fine structure of changing the element in a specific crystallographic site. Further understanding can also be obtained by modelling the fine structure and investigating how changes in the model modify the spectral features. The aim of this project is to improve the understanding of the fundamental interactions which cause ELNES by collecting and modelling the near-edge structure from a series of spinels, which have the general formula AB2O4. Chapter 1 provides an introduction to the theory of ELNES and reviews studies of this type which have been carried out previously. It also gives an introduction to the structure and chemistry of spinels and details the experiments planned in this study. In chapter 2 a detailed description is given of the key features of the HB5 scanning transmission electron microscope (STEM) used to acquire EELS data, and the mode of operation used. This chapter finishes with an account of how the data was acquired, and the processing routine employed. A discussion of the three main methods used to model the fine structure is given at the start of chapter 3. It is followed by a more detailed description of the multiple scattering technique, and the FEFF code, which was exclusively used to simulate the fine structure. Chapter 4 describes the syntheses of most of the spinels used in this study. Since these materials are used as standards in the EELS experiments, it is crucial that they are well characterised. This chapter therefore also describes the characterisation techniques employed, including X-ray and neutron diffraction, energy-dispersive X-ray analysis (EDX) and nuclear magnetic resonance (NMR). Information on structural parameters and purity was obtained from these techniques and where possible the results from different methods are compared and found to be consistent. The ELNES data recorded is first presented in chapters 5 and 6. In both chapters attempts have been made to correlate the observed features in the ELNES with the structural information obtained in chapter 4. Chapter 5 explores the oxygen K-edges from all of the spinels synthesised. Extra fine structure in the region up to lOeV beyond the edge onset is observed for the chromium and iron-containing compounds, and is assigned to transitions to states created by mixing of oxygen 2p and metal 3d orbitals. The possible fingerprints in the oxygen K-edge ELNES corresponding to a high degree of inversion in the spinel structure and for tetragonal distortions of the cubic structure are discussed. Simulations of the experimental data using the multiple scattering code FEFF8 are also presented in this chapter. Good agreement was obtained in the case of magnesium aluminate, but relatively poor agreement was obtained in the cases of the chromites and ferrites. It is though that this may be due to the influence of factors such as magnetic effects which are not considered in the calculations. For a full interpretation of the ELNES present on the oxygen K-edges, the cation edges must also be considered. These are addressed in chapter 6. The first part of this chapter compares the L3 and La-edges recorded in this study with simulations from the literature. The relationship between the intensity of these edges and the number of d-holes in the metal, and the pairing of the d-electrons is then explored. The methods available to obtain the intensities of these edges, and the difficulties encountered are discussed. The second part of this chapter presents the magnesium and aluminium K-edges acquired. The fine structure recorded in this study is compared to examples from the literature recorded from materials in which the magnesium and aluminium ions are in similar coordination environments to these ions in the spinel structure. Chapter 7 compares some of the oxygen edges and metal edges recorded to investigate the consistency of the information obtained from each type of site. The findings of the study are summarised in chapter 8 and suggestions for further work that could undertaken are made.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: Professor A J Craven
• Keywords: Analytical chemistry
• Colleges/Schools: College of Science and Engineering
• Supervisor's Name: Supervisor, not known
• Date of Award: 2001
• Depositing User: Enlighten Team
• Unique ID: glathesis:2001-73245
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 14 Jun 2019 08:56
• Last Modified: 28 Oct 2021 14:16
• URI: https://theses.gla.ac.uk/id/eprint/73245