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

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), Yb1.85Ca0.15Ti2O7−δ, and Yb2Ti1.85Fe0.15O7−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 2MeV 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 2MeV Au− ions only implant to ∼450nm 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.