Mn-doped UO2 is considered a potential advanced nuclear fuel due to ameliorated microstructural grain growth compared to non-doped variants. However, recent experimental investigations have highlighted limitations in grain growth apparently arising from misunderstandings of its redox-structural chemistry. To resolve this, we use synchrotron X-ray diffraction and spectroscopy measurements supported by ab initio calculations to cross-examine the redox and structural chemistry of Mn-doped UO2 single crystal grains and ceramic specimens. Measurements reveal Mn enters the UO2 matrix divalently as (Mnx+2U1-x+4)O2-x with the additional formation of fluorite Mn+2O in the bulk material. Extended X-ray absorption near edge structure measurements unveil that during sintering, the isostructural relationship between fluorite UO2 and Mn+2O results in inadvertent interaction and subsequent incorporation of diffusing U species within MnO, rather than neighbouring UO2 grains, inhibiting grain growth. The investigation consequently highlights the significance of considering total redox-structural chemistry of main and minor phases in advanced ceramic material design.
