The Role of Sn, Zr and Hf in the Radiation Damage in II, III, IV and V Pyrochlores
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The Role of Sn, Zr and Hf in the Radiation Damage in II, III, IV and V Pyrochlores Karl R. Whittle1, Massey de los Reyes1, Mark G. Blackford1, Nestor J. Zaluzec2 and Gregory R. Lumpkin1 1 Institute of Materials Engineering, ANSTO, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia 2 Materials Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA ABSTRACT Ceramics based on the general compositions CaLnXNbO7 (where Ln = La, Nd and Sm, and X=Zr and Sn) have been prepared, and irradiated with 1 MeV Kr ions at the IVEM-TANDEM user facility. The radiation tolerance of these materials has been found to be less than Zr and Hf equivalents. The results also suggest that the amorphisation cross section for these materials is related to the Ln component, and is similar to those observed for Zr and Hf equivalents. INTRODUCTION How materials behave while experiencing high levels of radiation damage is key to the development of both future waste form and nuclear materials development[1-3]. Material behaviour under such conditions is not readily predictable - two factors influencing behaviour are structure and composition. Pyrochlores provide a means by which compositional effects can be understood with minimal change in structure[4, 5]. Pyrochlore[6] (A2B2O7 - typically A=2+/3+ and B=5+/4+) is a 2x2x2 superstructural relative of fluorite with ordered cation/anion arrays, and ordered anion vacancies. Such vacancies allow the surrounding anions to relax, from the ideal position in fluorite, which gives rise to an observable superstructure within the anion array. The cations, A and B, are ordered in chains parallel to [110], with cation species separated by (½ ½ ½), giving rise to superstructure within the cation array. The vacancy in the anion array changes the local co-ordination from cubic (8) in fluorite to scalenohedral (A-6+2) and octahedral (B-6). The combination of both superstructures gives rise to a change in symmetry from Fm-3m to Fd-3m, an example representation is shown in figure 1.
Figure 1. Representative image of fluorite (left) and pyrochlore (right) showing the change in coordination and formation of chains within the cation array.
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EXPERIMENTAL Samples were prepared by the calcination of stoichiometric mixed metal oxides, i.e. CaCO3, La2O3, Nd2O3, Sm2O3, ZrO2, SnO2 and Nb2O5. The oxides were preheated at 800°C for 12 hrs to both decarbonate and dehydrate (except CaCO3), mixed within a ball mill, dried and pressed into pellets. These pellets were heated at 1500°C for 168 hrs, with a heating/cooling rate of 5K min-1. Once cold the pellets were analysed using X-ray powder diffraction in a Bruker D8 diffractometer coupled with a SolX energy selective detector, an angular range of 5-85°, a step size of 0.01° and a counting time of 5 s per point. Sample irradiations were undertaken at the IVEM-TANDEM user facility[7], using 1 MeV Kr ions irradiated at 15° normal to the sample, using temperature controlled stages and powdered samples loaded onto ‘holey’ carbon film. The fluence
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