Structures of UO2, UO2+x andU4O9 by neutron diffraction

Abstract

Single crystals have been examined by neutron diffraction in the range of temperatures 20 °C to 1 100 °C and of compositions UO2 to U4O9. The investigations divide into three groups, relating to UO2, UO2+x (where 0 < x < 0.25) and U4O9. UO2 has the fluorite structure at room temperature, with isotropic Debye-Waller factors. At high temperatures the atomic arrangement can no longer be described exactly in terms of the ideal fluorite structure with harmonic thermal vibrations. Better agreement between observed and calculated intensities is obtained by allowing the oxygen atoms to relax to positions with coordinates 1/4 + δ, 1/4 + δ, 1/4 + δ ... in the Fm3m space group, where δ is 0.016 at 1 000 °C. The relaxation effect can be interpreted in terms of either disorder or anharmonic thermal motion. On oxidising UO2 to UO 2+x a solid solution is formed, containing three kinds of oxygen atom, O, O' and O". The O atoms occupy the fluorite-type sites of UO2, but a proportion of these sites is empty in UO2+x. The O' and O" atoms occupy interstitial positions, which are displaced along < 110 > (for O') and along < 111 > (for O") from the large interstices in the fluorite structure. The interstices themselves are not occupied. At the composition UO 2.25 the oxygen atoms are ordered to form the new phase U4O9. The " main " neutron reflexions of U4O9, corresponding to the fluorite-type peaks of UO2, are much stronger than the superlattice reflexions, which arise from the ordering of the interstitial oxygen atoms within the fluorite framework. The analysis of the intensities of the main reflexions shows that there are two kinds of interstitial sites, as in UO2+x : the 0' atoms are located 0.85 Å along the < 110 > directions from the centres of the large interstices, and the O" atoms 1.05 A along < 111 >. Vacancies occur in the fluorite-type oxygen sites (0) but not in the uranium sites, and the chemical formula UO2.25 is expressed more fully as U1.00 O1.77 O'0.29 O"0.19. It is concluded that oxygen atoms cannot enter the fluorite structure individually but are incorporated in small groups containing O' and O" atoms and O vacancies. A complete structure determination of U4O9 must await the analysis of the superlattice reflexions