The crystal structure was evaluated for U1-yLnyO2-x (Ln = Gd, Er; y = 0−0.4) samples sintered at 1973 K for 8 h under Ar and Ar-10% H2 atmospheres. The effect of LnO1.5 doping on the crystal structure was investigated by X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS). The structures were refined by Rietveld analysis of the XRD patterns. LnO1.5 doping into UO2 reduced the lattice parameter of UO2−LnO1.5 solid solutions up to 40 mol % LnO1.5. The lattice parameters of these samples were comparable to those of stoichiometric (U,Ln)O2.00 solid solutions, that is, the O/M ratios were close to 2.00. XAFS spectra in the X-ray absorption near edge structure region showed that higher U oxidation states of +5 or +6 formed, in addition to +4. Extended XAFS indicated that the interatomic distances of U−O and Gd−O decreased with increasing y, whereas those of Er−O may not decrease monotonically.
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