Another point of interest beyond point defects is the clustering of oxygen interstitials. Indeed, oxygen interstitial clustering has been deduced from diffrac­tion experiments174 many years ago. However, a debate remains on the exact shape of such clusters. Two configurations are contemplated: the so-called Willis clusters174,175 or cubo-octahedral clusters that have been observed by neutron diffraction in U4O9176 and U3O7.177 These clusters are made of 12 oxygen and 8 uranium atoms and amount for 4 oxygen inter­stitials. An additional oxygen interstitial may reside in the center of the cluster, forming a so-called filled cube-octahedral cluster (with five interstitials).

Recent calculations have proved that Willis clus­ters are in fact unstable and transform upon re­laxation into assemblies of three or four interstitials surrounding a central vacancy cluster (Figure 13).178 The three interstitial-1 vacancy cluster has been found independently by other authors173,180 who refer to it as split di-interstitials. These clusters prove in fact to have a formation energy higher than the cube-octahedral cluster (Figure 14), especially the

filled one.178,179