XAFS study of local structure with picometer accuracy: Th _1-xU_xO₂ and Th_1-xPu _xO₂ solid solutions

XAFS spectroscopy using synchrotron radiation is an extremely suitable technique to study local atomic and electronic structure of mixed Th _1-xU_xO₂ and Th_1-xPu _xO₂ oxides. Despite of XAFS technique overall success, a pico-meter barrier (10^-2?Å) exists in XAFS data analysis. Here we present the dependence of the mixed oxide structure on composition, probed by XAFS with picometer accuracy. Complimentary XAFS spectra were measured at the Th, U, and Pu L-edges. We found that opposite to the lattice parameter obtained by XRD, the distances given by XAFS for the first and second shells do not follow completely neither Vegard's law nor the virtual crystal approximation (VCA). The Th-O, U-O and Pu-O distances obtained vary slightly upon dilution. These values are close to the ones expected from Vegard's law but are always smaller than the ones expected in VCA. The U-U(Th) and Th-Th(U) distances vary strongly upon dilution and the values are close to the ones expected from the VCA model but are always smaller than the ones expected by the Vegard law. The average lattice parameter calculated from XAFS data agrees well with a random distribution of metal (Th, U, Pu) and with one calculated from XRD data. With this complementary XAFS data on the local structure around two selected metal ions it is then possible to give a better view on the mixed oxides structure, the distribution of the two metal ions and local distortions in such medium crystal structure studied by XRD.