Anharmonic vibrations in UO2 as determined by EXAFS

Anharmonicity in the lattice vibrations of oxygens in UO2 has been examined by using EXAFS of the ULIII absorption edge. Whilst the EXAFS-determined U-U distance of 3.89 AA closely matches the crystallographic separation, the U-O distance of 2.43 AA is 0.06 AA longer. This shift is interpreted on the basis of an anharmonic lattice model in which oxygens occupy anti-tetrahedral sites.