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

Abstract

a b s t r a c tPyrochlore-type oxides (A 2 B 2 O 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 2 Ti 2 O 7 (RE = La-Lu), Yb 1.85 Ca 0.15 Ti 2 O 7Àd , and Yb 2 Ti 1.85 Fe 0.15 O 7Àd 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 À 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 À 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 2 Ti 2 O 7 to radiation-induced damage.

A Review of X-Ray Absorption Near-Edge Spectroscopic Studies of Pyrochlore-Type Oxides Proposed for Nuclear Materials Applications

“…Ti K-edge GA-XANES spectra were collected using glancing angles that provided X-ray attenuation depths of 100 nm and 450 nm to investigate the effect of ion implantation on the local coordination environment of Ti. 103 (The change in X-ray attenuation length with glancing angle is demonstrated in Fig. 14a.)…”

“…In a recent study, GA-XANES spectra were collected from ion-implanted RE2Ti2O7 (RE (rare-earth)=La-Lu, and Y), Yb1.85Ca0.15Ti2O7-δ, and Yb2Ti1.85Fe0.15O7-δ materials to study how composition affects the resistance of pyrochlore-type materials to structural damage. 103 The 2 MeV Auions that were implanted in the surface region of the pellets of these materials were calculated to penetrate to a maximum depth of ~450 nm in these materials (Figure 14a) with the largest number of defects being generated at a depth of ~200 nm (Figure 14b).…”

“…All of these changes are a result of a decrease in the Ti CN, as a result of the constituent ions becoming disordered in the near surface region of these materials upon Au ion implantation . 62,63,64,76,77,103 Comparison of the GA-XANES spectra collected from the ion-implanted RE2Ti2O7…”

A study of the electronic structure and structural stability of Gd2Ti2O7 based glass-ceramic composites

“…The spectrum from Gd 2 Ti 2 O 7 , which was reported previously, is also presented in these figures. 41 (Figure 7b). These observations indicate that the FABG glass composition and lower annealing temperature favours the formation of composite materials with a small amount of the pyrochlore crystallites having dissolved in the glass, which is in agreement with the XRD and electron microscopy results.…”

“…The observed decrease in Ti CN indicates that ion implantation damages the structure of Gd 2 Ti 2 O 7 in the composite materials. 40,41 GA-XANES spectra collected at different glancing angles from the ion implanted composite materials annealed at 750 o C or 1100 o C (Figure 12) shows that more damage occurs near the surface compared to deeper into the material. A comparision of the spectra collected at a glancing angles providing the same X-ray attenuation depth from ion-implanted BG-Gd 2 Ti 2 O 7 -750 o C and Gd 2 Ti 2 O 7 (Figure 13) shows that the damage experienced by Gd 2 Ti 2 O 7 in the composite materials was similar to that experienced by the sample that only contained Gd 2 Ti 2 O 7 .…”