John Stuart McCloy

Brannerite (UTi2O6) is a mineral analogue that is both flexible to a wide range of actinide/lanthanide substitutions and is also highly refractory; thus, it has generated interest as a waste form for used nuclear fuel (UNF). Given the wide range of elements in UNF, systematically tracking how compositional changes map onto changes in stability will guide the rational design of waste forms like brannerite. Changes in valence state, not just cation substitutions, are particularly important for actinide-containing materials, and the brannerite structure is flexible to a variety of charge-balancing substitutions. To isolate the impact of valence contributions to (de)stabilization of brannerite materials, a series of Ce(III/IV)Ti2O6–x ceramics were synthesized for study. Their compositions and structures were rigorously characterized by X-ray absorption spectroscopy, X-ray diffraction, and thermogravimetric analysis. High temperature oxide melt drop solution calorimetry was used to investigate thermodynamic properties: 20% coupled Ce(III)-VO resulted in increased stability driven by enthalpic contributions. These findings highlight that valence-driven changes in stability are factors that must be considered when designing waste forms or other ceramic materials with longevity in mind.

Optical basicity (OB) values have been compiled, reviewed, and computed for 95 binary oxides using electronegativity, ionic–covalent parameter, and optical properties methods. The merits of the various scales are discussed. A current, self‐consistent set of values are recommended for use. These oxide optical basicity values should prove useful for assessing complex compounds and glasses such as those encountered in nuclear waste vitrification. Extensions of the oxide scale to sulfides, halides, and nitrides are discussed, as well as micro‐basicity and group basicity concepts. A few examples are given of the use of optical basicity for understanding the uranium speciation in glasses, the inclusion of halides in oxide glasses, and the prediction of crystallization of nepheline. Further application of OB to broad problems in glass science is enabled given the robust set of coefficients compiled for this work.

As part of an effort to establish the principles and application of F K -edge X-ray absorption spectra to complex inorganic systems, the F K -edge spectra of a range of glasses, minerals and synthetic crystalline compounds are presented and discussed in terms of the F environments present. Crystalline fluorides often display richly detailed spectra as a result of the small core-hole broadening at the F K -edge, and the analysis of the spectra is presented by way of examining trends in material structure and bonding, spectral fingerprinting, and linear combination fitting. The benefits and drawbacks of each method of analysis are discussed, and recommendations for collection of high quality spectra are presented. The speciation of F in three oxide glasses is established by comparison of the observed F K -edge spectra with the spectra of suitable crystalline reference material. The F environments determined this way are in excellent agreement with previous works that utilized other methods of probing the F speciation, though in one case ambiguity in the observed spectrum does prevent definitive identification. F K -edge spectra were also calculated using the FEFF software package, with the aim of calculating spectra that reproduce the experimental spectra accurately with a minimum of prior knowledge and assumptions. The spectrum of CaF ₂ was calculated with a wide range of simulation parameters, and comparisons with experimental data are made to facilitate a simple computational workflow, that is validated by application to the simulation of the spectrum of Ca ₄ Si ₂ O ₇ F ₂ . Observations and recommendations are made that will assist other experimenters in the collection and analysis of future high-quality F K -edge spectra. The data and analyses presented establish F K -edge X-ray absorption spectroscopy as a powerful tool for the characterization of F speciation and environments in complex inorganic materials.