In-situ X-ray diffraction study of phase transformations in the Am-O system

► We realize an in-situ high-temperature X-ray diffraction study on an AmO2 sample. ► Fluorite AmO2 transforms to cubic Am2O3+δ and then to hexagonal Am2O3. ► Little-known monoclinic Am2O3 is observed during the cubic-to-hexagonal transition. ► Lattice parameter thermal expansion of hexagonal Am2O3 is given up to 1840 K. ► We give additional data on AmO2 lattice parameter expansion under self-irradiation.