PNNL

Abstract

This article reports on a comprehensive experimental study of 16O+ and 90Zr+ ion co-implanted SrTiO3 (STO) single crystals. In this study, STO is used as a model material to simulate a waste form for disposal of radioactive 90Sr that decays to 90Y and subsequently to 90Zr by emission of beta particles. Sequential implantation of 16O+ and 90Zr+ ions was performed at 550 K to balance the charge and to avoid full amorphization of STO. A number of experimental methods have been employed to characterize the implanted sample, including multiaxial ion channeling analysis and high-resolution x-ray diffraction. Results show that a high defect concentration was generated and accumulated in STO during the ion implantation, but the crystal structure was not rendered fully amorphous even for an atomic percentage of implanted O and Zr up to 1.5 at.% each. Thermal annealing at 1273 K leads to significant defect recovery at the surface with little recovery occurring at the damage peak, where a modest recovery is observed upon further annealing at 1423 K. Some of the implanted Zr species are well aligned with the STO axis, but all are barely located at the substitutional sites. A minor phase with a tetragonal structure is observed in the Zr distributed region, which has the axis parallel to that of the STO host. The tetragonal phase survives annealing at 1423 K with only a small decrease in the c value. A general assessment of this model waste form is also provided in this report.