Étude du système U–Ca–O par diffractométrie de rayons X à haute température

Abstract

The different solid phases formed from uranium dioxide with lime are studied by high temperature X-ray diffraction (20≤ T (°C)≤1620) under controlled atmosphere (10 5≥ P O 2 (Pa)≥10 −24), with lattice parameter measurements and composition estimations. Thus, with a Ca/U=1 compound, successively we observe the phases: (a) Rhombohedral `CaUO 4' whose reduction (3.67≥O/U≥3.15) is explained by a near 4% increase in volume of the cell. (b) Monoclinic `CaUO 3', thus far unknown; this deformed perovskite shows a constant composition (O/U≈3) and an α-monoclinic, β-orthorhombic transition near 1500°C. (c) Fluorite type U 1− δ Ca δ O 2− δ solid solution whose, contrary to all expectations, with temperature the lattice parameter variations agree with an increasing of the lime content (0.25≤ δ≤0.45 for 20≤ T (°C)≤1390) and with a decreasing above 1390°C. With a Ca/U=3 sample, the monoclinic `Ca 3UO 6' phase remains stable whatever P O 2 until 1100°C, then gradually reduces in `CaUO 3' isomorph oxide under P O 2 ≈10 −24 Pa at 1290°C. A pseudo-binary UO 2–CaO phase diagram is established.