Defects and Minor Phases in O⁺ and Zr⁺ Ion Co-implanted SrTiO₃

Abstract

This article reports on a comprehensive experimental study of 16O+ and 90Zr+ ion co-implanted SrTiO3 (STO) single crystals. In this study, STO was 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 β– particles. Sequential 16O+ and 90Zr+ ion implantation at 550 K was performed and Zr and O atomic concentrations of up to 1.5 atom % each in STO were achieved. A number of analytical methods were employed to characterize the implanted STO, including secondary-ion mass spectroscopy, multiaxial ion-channeling analysis, and high-resolution X-ray diffraction. The 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. Thermal annealing at 1273 K leads to a significant defect recovery at the surface with little recovery occurring at the damage peak, where a modest recovery occurs upon further annealing at 1423 K. Some of the implanted Zr species are well aligned with the STO ⟨001⟩ axis, but nearly all are barely located at the substitutional sites due to structural distortion. A minor phase with a tetragonal structure is observed in the Zr distributed region, which has the ⟨001⟩ axis parallel to that of the STO host. The tetragonal phase survived annealing at 1423 K with only a small decrease in the c parameter. A general assessment of the model waste form is also provided in this report.

Superlattice Structure and Precipitates in O+ and Zr+ Ion Coimplanted SrTiO3: A Model Waste Form for 90Sr

“…The substitutional Zr has seven chemical bonds with neighboring O atoms, including the extra one introduced into the structure. Although Zr is well aligned with the [001] axis (Figure 3c), in agreement with ionchanneling observations, 10 neither Zr nor O is located exactly at the original Sr site, which is again consistent with our previous observations. 10 There are several local minima for this defect, but the differences consist of minor rearrangements of the nearest oxygen atoms.…”

“…Detailed experimental procedures and parameters were described elsewhere. 10 A cross-sectional thin specimen was prepared using a FEI Helios 600 Nanolab system of focused ion beam (FIB) and scanning electron microscope (SEM). Ga + ions at 30 and 5 keV were used for cutting and thinning the specimen, respectively, and Ar + ions were used at 1 keV (Fischione Nanomill) for removal of the damage layer produced by the Ga + ions.…”

Chemical and Charge Imbalance Induced by Radionuclide Decay: Effects on Waste Form Structure

“…The data from the experimental study reported here may help understand various effects induced by 90 Sr decay on waste form properties and allow us to validate theoretical models and simulations. A part of the experimental data (Jiang 2011) reported here has been submitted for publication and the most recent ones, including TEM results, will be published in a later date.…”

Nanoparticle Precipitation in Irradiated and Annealed Ceria Doped with Metals for Emulation of Spent Fuels

“…The microstructure of the irradiated fuel is complex due to various factors, including the presence of various fission product precipitates, β-decay induced changes in charge imbalance, − increase in oxygen potential, temperature gradient, irradiation-enhanced GB migration, and lattice disorder produced by elastic collisions with high-energy neutrons and energetic daughter nuclide recoils in the damage cascade process. There is a long history of effort devoted to characterization of spent nuclear fuels.…”