Resolving and quantifying nanoscaled phases in amorphous FeF₃ by pair distribution function and mössbauer spectroscopy

Probing the atomic structure of materials displaying a lack of long-range order has been a continuous challenge for the material science's community. X-ray amorphous FeF₃ has been shown to be a promising electrode material in Li and Na ion batteries. Providing structural information on this class of compounds is therefore of interest as it can help rationalize the material's properties and further enabled its optimization. Herein, we used the pair distribution function and Mössbauer spectroscopy to provide unique insights into the atomic structure of amorphous FeF₃. The results showed that amorphous FeF₃ contained two phases built from corner-sharing of FeF₆ octahedra. According to X-ray diffraction data, the PDF was successfully modeled based on two structural models related to the distorted ReO₃ and the hexagonal-tungsten-bronze networks of FeF₃. The lack of long-range order shown by conventional XRD data and PDF analysis was shown to arise mostly from disorder. This study provides detailed atomic structure with corresponding spectroscopic signature of amorphous phases. Quantitative analysis of both techniques indicated similar trends. This showed that our approach can be employed to determine the structure of other complex materials.