Structural and Chemical Insights on the Incorporation of Americium into Monoclinic Zirconia (m-ZrO₂)

Monoclinic zirconia (m-ZrO₂) has been long of fundamental chemical interest and crucially it serves as a key barrier to radionuclide release at the fuel–zircaloy interface in spent nuclear fuel. However, the incorporation of transplutonic elements like americium in m-ZrO₂ remains poorly understood. Using a combination of microscopy, diffraction and high resolution X-ray spectroscopic techniques we have examined the doping of m-ZrO₂ with 5 mol % Am. We show Am enters m-ZrO₂ tetravalently, where its solubility is less than 1.05 mol %, m-(Am^4 + _0.0105Zr_0.9895)O₂, attributed to the large Am⁴⁺ cation coupled with its preference under the synthesis conditions to revert to its trivalent state, where excess Am adopts a C-type (Am⁴⁺/³⁺_1−xZr_x)₂O_3+x phase in space group Ia-3. The known reversible high temperature phase transformation of m-ZrO₂ to tetragonal is further shown to be reduced from 1150 ^oC to 1050 ^oC via Am⁴⁺ incorporation. The investigation provides critical insight into behaviour of transplutonic elements with m-ZrO₂.