Mechanochemical α to β phase transition of U3O8

Abstract

Recognized as the most stable phase of uranium oxide, α-U₃O₈ is widely used throughout the nuclear fuel cycle for safe storage and transportation. There is, however, a lack of understanding of the thermodynamic relationship between α-U₃O₈ and its polymorph, β-U₃O₈. This research contributes new knowledge regarding the kinetics and mechanism of a previously described mechanochemical phase transition between the α- and β-U₃O₈ polymorphs to improve the understanding of their thermodynamic relationship. In this work, tumble milling of α-U₃O₈ using different milling media shows an ingrowth of the β-phase over time, with an observed correlation between media density, percent conversion and lattice strain. Anisotropic peak broadening observed within collected X-ray diffraction powder patterns suggests the preservation of uranium polyhedral layered sheets throughout the milling process. Preservation of the sheets implies a shear-induced slip mechanism occurring through in-plane shifting of uranium polyhedra along lattice planes perpendicular to the polyhedral sheets.