Mapping chemical defects in oxide materials via X-ray diffraction microscopy

Research output: Book/ReportPh.D. thesis

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Abstract

Despite the critical role of point defects in determining the properties and functionality of metal oxides, characterizing their type and distribution within bulk materials remains a significant challenge. Current techniques mainly rely on spectroscopic methods, which provide indirect and volume-averaged data, limiting the ability to visualize the local influence of the defects. Alternatively, microscopic methods offer localized information but do so only from either the surface of the material or from thin foils, neither of which represent the three-dimensional boundary conditions representative of macroscopic materials and devices.
To study point defects locally and directly in the bulk requires the development of a new experimental technique that can visualize atomic-sized features within macroscopic volumes. Given that point defects generate strain fields that extend far beyond the defect itself, we believe it is possible to measure such defects directly in the bulk of metal oxides using Dark-field X-ray Microscopy (DFXM); a strain-mapping technique based on X-ray diffraction from a synchrotron source. This technique enables real-space imaging of strain and misorientation fields with sub-micrometer spatial resolution and angular resolution in the order of 10-4 rad, making it a suitable candidate for the measurement of the weak strain fields generated by typical point defects in oxide materials.
In this thesis, we describe the development of a forward modelling framework to achieve both qualitative and quantitative characterization of point defects in metal oxides by relating the expected strain fields generated by known populations of defects to the local reciprocal and real space intensity measurements obtained by DFXM. Our model spans multiple length scales by combining Density Functional Theory (DFT) computations to predict the strain tensor associated with individual point defects, with Finite Element Modelling (FEM) to predict the interactions between these defects across larger volumes. The simulated elastic displacement fields may then be directly compared with the DFXM maps of strain and misorientation, as well as other intensity-derived metrics (e.g. peak width). This approach was demonstrated via study of oxygen vacancies within the prototypical metal oxide, Strontium titanate (SrTiO3).
This thesis also presents the first application of DFXM to study structural changes within the bulk in nominally undoped SrTiO3 samples subjected to external stimuli, along with a framework for analyzing DFXM data in terms of small defects such as dislocations or point defects. Demonstrating the effectiveness of the developed method for studying small defects with DFXM, we revealed the internal structure of an indented crystal of SrTiO3. In-situ electrical experiments on this sample showed the local behavior of the internal structure under an electric field, revealed by acquiring rocking maps.
Additionally, this work includes the first application of DFXM to in-situ reoxidation of nominally undoped SrTiO₃. We followed the evolution of the internal structure of undoped SrTiO₃ samples during the reoxidation process using reciprocal space and strain maps, confirming the internal changes. During high-temperature reoxidation, the internal structure showed a redistribution of dislocations in the bulk, with the formation of a dislocation structure near the sample surface.
Finally, DFXM was used to study larger chemical defects, such as growth striations, in an Yttrium Aluminum Garnet (Y3Al5O12 ) sample doped with Chromium (Cr), Thulium (Tm) and Holmium (Ho) elements. The sample's behavior under an in-situ electric field experiment was examined, and 3D model reconstructions of the strain and orientation confirmed the nature of the striation along the sample.
Original languageEnglish
PublisherDepartment of Physics, Technical University of Denmark
Number of pages129
Publication statusPublished - 2024

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