Controlled disorder for the Yb2Ti2-xO7-2x (x = 0 to 1) series and corresponding radiation tolerance

The use of ceramics as radionuclide containment matrices for the safe long-term storage and disposal of high-level nuclear waste is studied with emphasis on chemistry flexibility, structural stability, and radiation tolerance. Here we investigate the flexibility of a particular series of compounds Yb2Ti2-xO7-2x (x = 0, 0.2, 0.52, 0.8, and 1) to form a solid solution between the end members Yb2Ti2O7 of pyrochlore structure through to Yb2TiO5 of fluorite structure. Whilst it is shown that cation disorder and corresponding transition from the ordered pyrochlore to disordered fluorite is facilitated via increasing the ytterbium to titanium ratio, it is also shown that sintering conditions may be controlled in such a way as to encourage the growth of the pyrochlore structure even in Yb2TiO5. The radiation response of these materials was tested in situ via 1 MeV krypton irradiation coupled with transmission electron microscopy characterisation. The critical fluence of ions required to transition the crystalline phase to amorphous was found to increase with increasing disorder from Yb2Ti2O7 to Yb2TiO5. However, of the two forms (pyrochlore and fluorite) of Yb2TiO5 studied via ion irradiation that with the greater pyrochlore structure and so more ordered also had the lower critical temperature, 434 K, for maintaining crystallinity during irradiation.