Cation-heterogeneity in internally gelated U_1-zCe_zO_2-x, 0.15 ≤ z ≤ 0.3 microspheres

Internal gelation of aqueous mixtures of metal (M = Ln, An) nitrate with Uranyl Nitrate is generally assumed to yield cation homogeneity and a fluorite type single phase U_1-zM_zO_2±x solid solution. As-sintered (U,Ce)O₂ internally gelated microspheres, manufactured with target z values up to 0.3 using Ce(NO₃)₃, were observed to exhibit systematic peak broadening and splitting at higher 2θ angles in their X-Ray diffraction (XRD) patterns, correlating with increasing z≥0.15. This was interpreted as an unexpected departure from a single phase material. Thermogravimetry was used to make an initial assessment whether these peak anomalies were caused by an oxygen hypostoichiometry. Results indicated global oxygen stoichiometry for all compositions. The subsequent detailed characterization study via Electron Probe Micro Analysis of cross-sections of the as-sintered microspheres revealed the systematic presence of spherical Ce concentration gradients, as well as µm-sized highly Ce-enriched features. EDS and TEM studies on focused ion beam lamellae extracted from the cross-sections of as-sintered microspheres revealed a hexagonal Ce_4.67(SiO₄)₃O minor phase manifesting as single grain precipitates and clusters uncovering the presence and critical role of Silicon as an unexpected contaminant and Ce-scavenger from surrounding (U,Ce)O₂ grains. Characterization at intermediate heat treatment steps revealed that the systematic U/Ce heterogeneity features are already present post-gelation and are independent of the superimposed trace Ce-Si-O phase. This work constitutes the first systematic cation distribution study on cross-sections of (U,Ce)O_2±x microspheres, executed on a series of compositions, using a combination of elemental mapping techniques.