F K-edge XAS as a tool to examine F environments in complex inorganic systems

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 CaF₂ 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 Ca₄Si₂O₇F₂. 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.