Ion Beam Irradiation-induced Amorphization in Nano-sized K x Ln y Ta 2 O 7-v Tantalate Pyrochlore
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Ion Beam Irradiation-induced Amorphization in Nano-sized KxLnyTa2O7-v Tantalate Pyrochlore Fengyuan Lu1, May Nyman2, Yiqiang Shen1,3, Zhili Dong3, Gongkai Wang1,4, Fuxiang Zhang5, Rodney Ewing5, and Jie Lian1 1
Department of Mechanical, Aerospace & Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, U.S.A. 2 Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM 87185, U.S.A. 3 School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798 4 Key Laboratory for Anisotropy and Texture of Materials of Ministry of Education, Northeastern University, Shenyang, Liaoning 110004, China 5 Departments of Geological Sciences and Materials Science & Engineering, University of Michigan, Ann Arbor, MI 48109-1005, U.S.A.
ABSTRACT The radiation response of nano-sized tantalate pyrochlores, KxLnyTa2O7-v (Ln = Gd, Y, and Lu) with average grain sizes of ~ 10 nm was investigated using 1 MeV Kr2+ ion beam irradiations. EDS measurements and XRD refinement reveal that the Y3+ and Lu3+-doped samples consist of two pyrochlore phases as K0.8YTa2O6.9/K0.4Y0.8Ta2O6.4 and KLuTa2O7/K0.4Lu0.8Ta2O6.4 respectively; whereas a single phase of K0.8GdTa2O7 only exists in the Gd3+-doped tantalate pyrochlore. In situ TEM observation confirms ion beam-induced amorphization occurring in all of the nano-sized KxLnyTa2O7-v. At elevated temperatures, both K0.8GdTa2O7 and K0.8YTa2O6.9/K0.4Y0.8Ta2O6.4 exhibit higher radiation tolerance than KLuTa2O7/K0.4Lu0.8Ta2O6.4, and the critical temperatures of K0.8GdTa2O7 and K0.8YTa2O6.9/K0.4Y0.8Ta2O6.4 are estimated to be 1167 ± 41 K and 1165 ± 34 K, respectively, lower than that of KLuTa2O7/K0.4Lu0.8Ta2O6.4 (~ 1291 K). The K0.8GdTa2O7, K0.8YTa2O6.9 and KLuTa2O7 phases have less structural deviation from the parent fluorite structure and thus may be responsible for the overall radiation tolerance. The high K+ occupancy at pyrochlore A sites in KLuTa2O7 is believed to contribute to the decrease of radiation tolerance, consistent with the large ionic radius ratio of K+/Ta5+. These results highlight that the radiation tolerance of nanostructured materials is highly compositional dependent, and nano-sized tantalate pyrochlores are sensitive to radiation damage. INTRODUCTION The pyrochlore A2B2O7 compounds, where A and B sites are occupied by metal cations, exhibit chemical durability, high compatibility with actinides, and tunable radiation tolerance by controlling their cationic compositions and electronic bonding, and therefore are considered as important host materials as nuclear waste forms and fuel matrix [1, 2]. Cationic compositions of pyrochlores greatly affect their radiation stability. By substituting the Ti4+ cations in the Gd2Ti2O7 pyrochlore with Zr4+ cations, resulting in a smaller cation ionic radius ratio (rA/rB), the material’s radiation resistance can be improved dramatically, and no amorphization can be achieved in Gd2Zr2O7 at an extremely high damage level up to 100 dpa at room temperature [3]. The discovery of the radiation-resistant materials
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