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The Characterisation and Ion-Irradiation Tolerance of the Complex Ceramic Oxides

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The Characterisation and Ion-Irradiation Tolerance of the Complex Ceramic Oxides The Characterisation and Ion-Irradiation Tolerance of the Complex Ceramic Oxides Ln2TiO5 (Ln = lanthanides) A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy Robert D. Aughterson Primary Supervisor: Julie M. Cairney Associate Supervisors: Baptiste Gault & Gregory R. Lumpkin Australian Centre for Microscopy and Microanalysis School of Aeronautical, Mechanical and Mechatronics Engineering October 2018 The characterisation and ion-irradiation tolerance of the complex ceramic oxides Ln2TiO5 (Ln = lanthanides) Abstract Ceramics are a promising candidate for nuclear waste-form matrices, as part of the nuclear fuel matrix, and as cladding for fission or diverters for fusion reactors. This is due to their chemical durability, the ability to incorporate high loadings of radionuclides with minimal leaching, and tolerance to exposure to high-energy particles. It is the radiation tolerance that is the main focus of this study. There are two main parts to this study: the characterisation of the crystal structure of a range of compounds, all with the Ln2TiO5 (Ln = lanthanides) stoichiometry, and the testing of their ion-irradiation response. The most extensively-studied ceramic-based waste-form material is the polyphase, titanate- based ceramic known as SYNROC. This PhD project looks closely at Ln2TiO5 compounds that make up part of the SYNROC matrix and their radiation response. Ion-irradiation is used throughout this study to simulate the damage effects of high-energy particles such as those from alpha-decay. In particular, in-situ ion-irradiation coupled with transmission electron microscopy (TEM) and electron diffraction are used for radiation damage measurement. The Ln2TiO5 series of compounds consist of four major crystallographic structure types; orthorhombic (Pnma), hexagonal (P63/mmc), cubic (Fm-3m), and cubic (Fd-3m). The final structure type is dependent on the ionic radius of the lanthanide used, the fabrication regime (temperature and pressure), and whether combinations of lanthanides are used. In this study all these variables, except pressure, are investigated. The full range of structure types was fabricated and their crystal structure characterised. An initial systematic study of the orthorhombic Ln2TiO5 series (Ln = La, Pr, Nd, Sm, Gd, Tb and Dy) using synchrotron x-ray diffraction provided greater detail on the crystal structures than the current literature, plus new data on the Sm2TiO5 and Pr2TiO5 structures. By using a systematic approach to investigate the orthorhombic series, lattice parameters, atomic positions, and valency trends could be inspected. From Dy to La, the lattice parameters for both a and b increased systematically by approximately 6 %, whilst c only increased by ~ 1.5 %. For decreasing lanthanide size, from La to Gd, there was little Page i The characterisation and ion-irradiation tolerance of the complex ceramic oxides Ln2TiO5 (Ln = lanthanides) variation in Ti-O bond lengths, while there was significant variation from this trend for the Tb2TiO5 and Dy2TiO5 compounds. This system of orthorhombic Ln2TiO5 compounds, including Eu2TiO5, was subsequently tested for ion-irradiation response using the in-situ TEM approach. For this particular study, the series of Ln2TiO5 compounds were exposed to 1 MeV krypton ions and ion- irradiation response monitored via bright field images and selected area electron diffraction patterns (SADPs). The SADPs were used to determine the critical dose of irradiating ions required for the transition from crystalline to amorphous. This critical dose of amorphisation gave a quantity allowing radiation response of the various ceramics to be compared. The critical dose was also monitored at a variety of temperatures allowing a critical temperature for maintaining crystallinity to be quantified, with lower critical temperatures being the desired outcome. The critical temperatures for the La to Sm compounds were all high before sequentially decreasing from Eu to Dy. The source of improved radiation tolerance was further investigated for Dy2TiO5 by using bulk sample ion-irradiation, 12 MeV gold ions, and characterising the structure transitions using grazing incidence x-ray diffraction. Having determined that crystal structure is a significant influence on radiation response, a systematic series of compounds SmxYb2-xTiO5 was used as a controlled means of fabricating all of the major structure types for the Ln2TiO5 compounds. Various approaches were used, synchrotron powder x-ray diffraction, neutron diffraction, single-crystal x-ray diffraction, and transmission electron microscopy, to characterise the structures. The largest lanthanide, samarium, gave an orthorhombic symmetry, a slight increase in ytterbium content, Sm1.4Yb0.6TiO5 and SmYbTiO5, gave hexagonal symmetry, and the higher ytterbium content, Sm0.6Yb1.4TiO5 and Yb2TiO5, gave cubic symmetry. The hexagonal and cubic symmetry compounds showed modulated structures. For the cubic materials the structure consisted of long range fluorite-type, space group Fm-3m, and related but slightly incommensurate short range pyrochlore-type, space group Fd-3m, nano-domains. The SmxYb2-xTiO5 series of compounds were also tested for ion-irradiation response using the in-situ TEM approach. The critical temperatures for both the orthorhombic and hexagonal symmetry compounds were high, with significantly lower temperatures found for the cubic symmetry compounds. This was the first study to look at the radiation Page ii The characterisation and ion-irradiation tolerance of the complex ceramic oxides Ln2TiO5 (Ln = lanthanides) tolerance of a hexagonal Ln2TiO5 compound, which gave an unexpectedly poor radiation response. After the success of using the SmxYb2-xTiO5 series to give a controlled means of fabricating all the major Ln2TiO5 structures, a further TbxYb2-xTiO5 systematic series was investigated. In this study, the fabrication of Tb2TiO5 in two different polymorphs, orthorhombic, and hexagonal as bulk single phase materials allowed the crystal structure details of a mono- lanthanide polymorph compound to be determined for the first time. This also created the unique opportunity to test the ion-irradiation response of a single stoichiometry, Tb2TiO5, in two different structure types. The hexagonal structured Tb2TiO5 performed worst, having a higher critical temperature and a lower critical dose of amorphisation compared to the orthorhombic form. The TbxYb2-xTiO5 series again showed a marked improvement in radiation response for compounds based on cubic symmetry. With all in-situ ion-irradiation studies carried out thus far indicating that Ln2TiO5 compounds with cubic symmetry have improved radiation tolerance, the final study of HoxYb2-xTiO5 and Er2TiO5 compounds focussed exclusively on the cubic system. There was sequential improvement in ion-irradiation response, lowering of critical temperature, from Ho2TiO5 to Er2TiO5 to HoYbTiO5 to Yb2TiO5. This trend corresponded with a decreasing lanthanide ionic radius and a greater tendency toward the fluorite-type structure. The in-situ, 1 MeV Kr ion-irradiation results were compared with bulk material ex-situ, 1 MeV Se ion-irradiation. The ion-induced damage in bulk samples was characterised using cross-sectional TEM and critical temperatures for maintaining crystallinity were determined. Whilst critical temperature values determined using these two different techniques differed, the general trend of improved response from Ho2TiO5 to Yb2TiO5 still remained. This gives validation to the in-situ approach being used to determine radiation response trends for given systems of ceramics. This PhD study provides systematic and comprehensive ion-irradiation experimental data and analysis for a large range of Ln2TiO5 compounds coupled with crystal structure detail. The effect of crystal structure on ion-irradiation response shown here means Ln2TiO5 compounds can be tailored to give the best possible properties for application. This is a first step in the process of validation for nuclear-based applications. Page iii The characterisation and ion-irradiation tolerance of the complex ceramic oxides Ln2TiO5 (Ln = lanthanides) Certificate of Originality I hereby certify that this thesis does not contain, without the appropriate acknowledgement, any materials previously submitted for a degree at The University of Sydney or any other university. I also certify that this thesis does not contain, without the appropriate acknowledgement, any materials previously published or written by another person. Any contribution made to the research by others is explicitly acknowledged in the thesis. Signature ………………………………………….. Robert D. Aughterson Page iv The characterisation and ion-irradiation tolerance of the complex ceramic oxides Ln2TiO5 (Ln = lanthanides) Author and Co-author Contributions and Signatures The following lists the journal publications in the order they appear through the thesis with the associated contributions to each detailed: Chapter 6.1: Crystal Chemistry of the Orthorhombic Ln2TiO5 compounds with Ln = La, Pr, Nd, Sm, Gd, Tb and Dy, Journal of Solid State Chemistry, 227: 60-67 (2015) DOI: 10.1016/j.jssc.2015.03.003 Authors: R. D. Aughterson, G. R. Lumpkin, G. J. Thorogood, Z. Zhang, B. Gault, and J. M. Cairney Fabrication of samples was carried out by R. D. Aughterson. All synchrotron powder XRD data was collected by R. D. Aughterson and G. J. Thorogood, except synchrotron powder XRD for Sm2TiO5, which was collected by Z. Zhang. Rietveld refinements of
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