An investigation of the electronic structure and structural stability of RE_2Ti_2O_7 by glancing angle and total electron yield XANES

摘要

Pyrochlore-type oxides (A_2B_2O_7) 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. RE_2Ti_2O_7 (RE = La-Lu), Yb_(1.85)Ca_(0.15)Ti_2O_(7-δ), and Yb_2Ti_(1.85)Fe_(0.15)O_(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 Yb~(3+) for Ca~(2+) or Ti~(4+) for Fe~(3+) does not considerably affect the resistance of Yb_2Ti_2O_7 to radiation-induced damage.