Structural Alterations in Titanate Pyrochlores Induced by Ion Irradiation: Xray Photoelectron Spectrum Interpretation
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Structural Alterations in Titanate Pyrochlores Induced by Ion Irradiation: Xray Photoelectron Spectrum Interpretation J. Chen, J. Lian, L. M. Wang, R. C. Ewing, J. Matt Farmer1, and L. A. Boatner1 Dept. of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109-2104, U.S.A. 1 Oak Ridge National Laboratory, Solid State Division, Oak Ridge, TN 37831-6056, U.S.A. ABSTRACT Titanate pyrochlores are candidate host materials for the disposition of plutonium from dismantled nuclear weapons. These pyrochlores also have potential applications as solid electrolytes and oxygen gas sensors. The radiation-induced microstructural evolution of titanate pyrochlores has been characterized using x-ray photoelectron spectroscopy (XPS). XPS analysis of the Ti 2p and O 1s binding energy shifts of REE2Ti2O7 (REE: rare earth) surface layers before and after irradiation shows that the primary manifestations of amorphization are distortions or changes in the coordination number of the titanium polyhedra. A model based on glass network structure was developed, and predictions of the relative susceptibilities for amorphization of the titanate pyrochlores are obtained that are consistent with the experimental results.
INTRODUCTION Isometric pyrochlore (Fd3m), ideally A2B2O7, is a derivative of the fluorite structure (AX2), but with two cations and one-eighth of the oxygen ions absent. The A- and B-site cations are at equivalent positions, but there are two different types of oxygens. The first type of oxygen (48f) has two A-cation neighbors and two B-cation neighbors. The second type (8b) is on a tetrahedral site that consists entirely of A-cations. Pyrochlore is one of the candidate materials proposed for the immobilization of actinide-rich wastes, and it has recently been selected as a key component in Synroc-based pyrochlore-rich ceramics for the geological immobilization of surplus Pu in the USA [1]. A principal concern with crystalline waste forms is the possibility of radiation-induced amorphization due to the effects of α decay of the actinides [2]. The amorphization of pyrochlore as induced by ion irradiation is of interest because the process of amorphization represents a fundamental microstructural change, and an understanding of the underlying amorphization mechanism can be applied to the design of more radiation-tolerant materials [3]. In a previous study, we found that Gd2Ti2O7 single crystals amorphized under 1.5 MeV Xe+ ions at a fluence of 1.7 × 1014 Xe+/cm2. Cross-sectional transmission electron microscopy revealed a 300 nm-thick amorphous layer at the specimen surface. X-ray photoelectron spectroscopy (XPS) analysis of the Ti 2p and O 1s electron binding energy shifts of Gd2Ti2O7 before and after amorphization showed that the main results of ion-irradiation-induced disorder are a decrease in the coordination number of the titanium polyhedra and a transformation of the Gd-O bond [4]. These features resemble those occurring in titanate glass formation. Here, we use XPS to study the microstruct
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