The structural-chemistry of monoclinic zirconia (m-ZrO2) has long attracted fundamental interest, whilst crucially it serves as a key barrier to radionuclide release at the fuel–zircaloy cladding interface in spent nuclear fuel. However, the incorporation of transuranic elements like americium with m-ZrO₂ remains poorly understood. Using a combination of diffraction, microscopy and spectroscopic techniques we have examined the doping of 5 mol % Am in m-ZrO2. We find that Am enters m-ZrO2 in its tetravalent oxidation state where its solubility is less than 1.05 mol %, Am4+0.0105Zr0.9895O2, attributed to the large sized Am4+ cation coupled with its preference under the synthesis conditions to adopt its trivalent form, where excess Am adopts an Am rich cubic structure consistent with C-type (Am4+/3+1-xZrx)2O3+x in space group Ia-3. Despite the limited solubility of Am4+ within m-ZrO2, the reversible high temperature phase transformation to tetragonal-ZrO2 (t-ZrO2) can be reduced from 1150 oC to 1050 oC via Am4+ incorporation. The investigation provides key insight into chemical behavior of Am and its interactions with m-ZrO₂.

The structural-chemistry of monoclinic zirconia (m-ZrO2) has long attracted fundamental interest, whilst crucially it serves as a key barrier to radionuclide release at the fuel–zircaloy cladding interface in spent nuclear fuel. However, the incorporation of transuranic elements like americium with m-ZrO₂ remains poorly understood. Using a combination of diffraction, microscopy and spectroscopic techniques we have examined the doping of 5 mol % Am in m-ZrO2. We find that Am enters m-ZrO2 in its tetravalent oxidation state where its solubility is less than 1.05 mol %, Am4+0.0105Zr0.9895O2, attributed to the large sized Am4+ cation coupled with its preference under the synthesis conditions to adopt its trivalent form, where excess Am adopts an Am rich cubic structure consistent with C-type (Am4+/3+1-xZrx)2O3+x in space group Ia-3. Despite the limited solubility of Am4+ within m-ZrO2, the reversible high temperature phase transformation to tetragonal-ZrO2 (t-ZrO2) can be reduced from 1150 oC to 1050 oC via Am4+ incorporation. The investigation provides key insight into chemical behavior of Am and its interactions with m-ZrO₂.