An investigation of the electronic structure and structural stability of RE₂Ti₂O₇ by glancing angle and total electron yield XANES

Highlights: • Structural stability of ion-implanted pyrochlore-type oxides investigated by XANES. • Glancing angle and total electron yield XANES spectra are sensitive to changes in structure. • RE₂Ti₂O₇ (RE = rare-earth) oxides less susceptible to damage as RE radius decreases. • Presence of oxygen vacancies not found to affect structural stability of Yb₂Ti₂O₇. - Abstract: Pyrochlore-type oxides (A₂B₂O₇) have been investigated for nuclear waste sequestration applications due to their overall resistance to radiation-induced structural damage. This damage can be simulated by implanting the materials with high-energy ions to aid in understanding how the structure of a crystalline material responds to radioactive decay of incorporated nuclear waste elements. RE₂Ti₂O₇ (RE = La–Lu), Yb_1.85Ca_0.15Ti₂O_7−δ, and Yb₂Ti_1.85Fe_0.15O_7−δ were synthesized by the ceramic method and investigated by Ti K-edge X-ray absorption near edge spectroscopy (XANES) and powder X-ray diffraction before and after being implanted by 2 MeV Au⁻ ions. Surface sensitive total electron yield (TEY) XANES spectra and glancing-angle (GA) XANES spectra were collected to study the ion-implanted materials. It was necessary to use these surface sensitive detection methods because the 2 MeV Au⁻ ions only implant to ∼450 nm into the surface of the materials. An effective sample mounting procedure has been developed that uses conductive silver paint to collect high quality TEY-XANES spectra from the damaged surface layer of the implanted pellets so as to minimize charging effects. This investigation has shown that the local coordination number of Ti decreased after implantation, and that the degree of damage observed decreases with decreasing radius of the RE cation. The study has also shown that the oxygen vacancies generated by a slight substitution of Yb³⁺ for Ca²⁺ or Ti⁴⁺ for Fe³⁺ does not considerably affect the resistance of Yb₂Ti₂O₇ to radiation-induced damage