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

In the frame of minor actinides recycling, americium can be transmuted by adding it in UO{sub 2} or (U, Pu)O{sub 2} fuels. Americium oxides exhibiting a higher oxygen potential than U or Pu oxides, its addition alters the fuel properties. To comprehend its influence, a thorough knowledge of the Am-O phase equilibria diagram and of thermal expansion behavior is of main interest. Due to americium scarcity and high radiotoxicity, few experimental reports on this topic are available. Here we present in-situ high-temperature XRD results on the reduction from AmO{sub 2} to Am{sub 2}O{sub 3}. We show that fluorite (Fm-3m) AmO{sub 2} is reduced to cubic (Ia-3) C Prime -type Am{sub 2}O{sub 3+{delta}}, and then into hexagonal (P6{sub 3}/mmc) A-type Am{sub 2}O{sub 3}, which remains stable up to 1840 K. We also demonstrate the transitional existence of the monoclinic (C2/m) B-type Am{sub 2}O{sub 3}. At last, we describe, for the first time, the thermal expansion behavior of the hexagonal Am{sub 2}O{sub 3} between room temperature and 1840 K. - Graphical abstract: Americium dioxide was in situ studied by high-temperature X-ray diffraction. First, fluorite AmO{sub 2} is reduced to cubic C Prime -type Am{sub 2}O{sub 3+{delta}} and then transforms into hexagonal A-typemore » Am{sub 2}O{sub 3}, which remains stable up to 1840 K. Then, we demonstrate the transitional existence of monoclinic B-type Am{sub 2}O{sub 3}. At last, we describe, for the first time, the thermal expansion of A-type Am{sub 2}O{sub 3} between room temperature and 1840 K. This work may contribute to a better understanding of Am oxide behavior. Highlights: Black-Right-Pointing-Pointer We realize an in-situ high-temperature X-ray diffraction study on an AmO{sub 2} sample. Black-Right-Pointing-Pointer Fluorite AmO{sub 2} transforms to cubic Am{sub 2}O{sub 3+{delta}} and then to hexagonal Am{sub 2}O{sub 3}. Black-Right-Pointing-Pointer Little-known monoclinic Am{sub 2}O{sub 3} is observed during the cubic-to-hexagonal transition. Black-Right-Pointing-Pointer Lattice parameter thermal expansion of hexagonal Am{sub 2}O{sub 3} is given up to 1840 K. Black-Right-Pointing-Pointer We give additional data on AmO{sub 2} lattice parameter expansion under self-irradiation.« less

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