Crystallographic studies of anion-excess uranium oxides

Abstract

At room temperature UO₂ assumes the fluorite structure with both uranium and oxygen atoms vibrating isotropically about their equilibrium positions. At higher temperatures (> 400 K) this model must be modified to account for the anharmonic, anisotropic contribution of the oxygen atoms to the Bragg intensities. Up to 1300 K the uranium atoms continue to vibrate isotropically. On oxidation of UO₂ to UO_2+x, defect clusters are formed in the oxygen sublattice. At x= 0.12 diffraction studies indicate that the configuration of the cluster is 2 : 2 : 2, i.e. consisting of two vacant oxygen sites, two interstitial oxygens displaced away from the cubic-coordinated interstitial sites along 〈110〉 and two oxygens displaced along 〈111〉. The uranium sublattice is not disturbed by the rearrangement of atoms in the oxygen sublattice. At the limiting composition of UO_2.25, or U₄O₉, the oxygen clusters link together into an ordered superlattice structure. Many attempts have been made (using X.r.d., n.d., e.d. and X.p.s.) to solve this structure, and several incorrect solutions have been published. There are three closely related phases of U₄O₉, α, β and γ, and of these the crystal structure of the beta phase only is known, which was determined from neutron-diffraction data. The clusters in β-U₄O₉ are not like those in UO_2.12. Each cluster consists of an octahedral arrangement of six UO₈ square antiprisms, which share corners to enclose a cubo-octahedron of anions. The composition of such a cluster with an oxygen at the centre is U₆O₃₇, and the overall composition is U₄O₉–y where y= 0.0625. Further progress in understanding the nature of the defect clusters in UO_2+x and U₄O₉ requires a combined attack with the methods of neutron scattering and of computer simulation.