The generation of electricity from nuclear power sources, whilst proving a low carbon emission alternative to fossil fuel based sources, produces large amounts of actinide containing radioactive wastes as a by-product. As these radioactive wastes pose a significant potential risk to both the environment and public health, a means of safely disposing of them is required. The UK government, and indeed many others across the globe, consider disposal of these wastes in geological repository, using a multi-barrier approach, as the safest method. A principal component of the multi-barrier approach is to either vitrify waste products in glass mediums, such as borosilicate, or immobilise them in ceramic systems. Ceramics are of particular interest due to their high aqueous durability, favourable physical qualities, tolerance of radiation damage and high potential waste loading.
The ability of a ceramic wasteform to resist and have predictable responses to radiation damage is key to their implementation. The development of novel Ln2TiO5 ceramics as nuclear wasteforms has recently been undertaken due, in large part, to their reportedly high radiation tolerance. This study aims to characterise the local atomic arrangements of structures associated with Ln2TiO5 series of ceramics and their response to radiation damage by utilising multiple X-ray absorption techniques. The Ln2TiO5 series of ceramics form the end member of the Ln2(Ti2-xLnx)O7-x/2 (x = 0.667) pyrochlore solid solution and as such are often referred to as stuffed pyrochlores due to the additional lanthanide cations ‘stuffed’ onto the Ti-site. Dependent on ionic radius of the lanthanide used, pressure and temperature, different structure types can be obtained. The structures that Ln2TiO5 stuffed pyrochlores have been commonly observed to form include orthorhombic Pnma, hexagonal P63/mmc and cubic Fd-3m and Fm-3m symmetries. The radiation tolerance of these materials is highly dependent upon the crystal structure used.
Whilst the long-range order of these materials is relatively well described there is a significant lack in understanding of the short-range order. Various studies have reported conflicting conclusions about the short-range structure of cubic Ln2TiO5. The long-range structures have been observed to be defectfluorite Fm-3m and the short-range order to be possibly consist of a series of Fd-3m pyrochlore nanodomains or to have orthorhombic Pnma symmetry. Through a combination of X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) analyses, the local structure is shown to be well represented by a model based off of a Fd-3m structure where the Ti-O coordination is 5-fold (reduced from an initial 6-fold coordination of the pyrochlore structure).
Examination of Ln2TiO5 stuffed pyrochlores, that adopt either an orthorhombic or hexagonal symmetries through X-ray diffraction (XRD), using XAS techniques concludes that they are well represented by models based upon the same structure assigned to their long-range ordering. Furthermore, it is shown that the TiO5 polyhedra associated with both orthorhombic and hexagonal structures are found to have different degrees of centro-symmetry.
Understanding the local structure of these materials is vital to predicting how they will behave under extreme environmental conditions, such as in high intensity radiation fields. The successful design of XAS models allowed for further study of the local structural environments of Ln2TiO5 stuffed III pyrochlores through the application of glancing angle X-ray absorption spectroscopy (GAXAS). This technique was used to probe the damaged surface region of ion beam irradiated (Au2+ and Kr+ ) bulk monoliths of orthorhombic Gd2TiO5, hexagonal Dy2TiO5 and cubic Yb2TiO5. The results of this study gave key insights into the structural responses of these materials to radiation damage and allowed for the development of an understanding of mechanisms that drive their responses.
The comprehensive study of the structures and radiation response of Ln2TiO5 stuffed pyrochlores provided in this PhD study will facilitate future tailoring of their applications for roles within the nuclear industry and beyond.
