The thermal and mechanical properties of cerium dioxide (CeO2) were assessed using a range of experimental techniques. The oxygen potential of CeO2 was measured by the thermogravimetric technique, and a numerical fit for the oxygen potential of CeO2 is derived based on defect chemistry. Mechanical properties of CeO2 were obtained using sound velocity measurement, resonant ultrasound spectroscopy and nanoindentation. The obtained mechanical properties of CeO2 are then used to evaluate the Debye temperature and Grüneisen constant. The heat capacity and thermal conductivity of CeO2 were also calculated using the Debye temperature and the Grüneisen constant. Finally, the thermal conductivity was calculated based upon laser flash analysis measurements performed on pellets fabricated using a range of feedstock purities to resolve discrepancies in the existing literature.

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X‐ray diffraction and X‐ray absorption spectroscopy have shown that a material composed of two (Pu,Am)O2–x substoichiometric fluorite‐type structures is obtained by sintering in moisturized Ar/H2. The Pu oxidation state is strictly +IV, whereas Am exhibits a +III/+IV mixed valence. Thus, the Am–O system and particularly the Am oxidation state play a major role in the sintering behavior of the Pu–Am–O system. The heating of this compound in air leads to the formation of a fluorite Pu0.80Am0.20O1.98 solid solution. The transition from a one‐phase to a two‐phase structure takes place at a temperature lower than 800 °C. The results were compared with previous studies on Am‐bearing UO2 and ThO2, which have evidenced different behaviors.

The electronical and structural properties of Th(0.80)Am(0.20)O(2-x) materials have been studied by the coupling of X-ray diffraction and X-ray absorption spectroscopy techniques. A substoichiometric fluorite Th(IV)(0.80)Am(III)(0.20)O(1.90) solid solution is found following sintering in moisturized Ar-H(2). In contrast, heating of this sample in air leads to a nondefective fluorite Th(IV)(0.80)Am(IV)(0.20)O(2.00) solid solution. The structures of these solid solution compounds were fully characterized by assessing the interatomic distances, the coordination numbers, and the structural disorder. The effect of the sintering atmosphere on these crystallographical parameters and on the cation valences has been determined and the capability of ThO(2) to accommodate tri- and tetravalent actinides in the fluorite structure assessed.

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