Radiation Tolerance of A 2 Ti 2 O 7 Materials - A Question of Bonding?
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Radiation Tolerance of A2Ti2O7 Materials - A Question of Bonding? Karl R. Whittle1, Gregory R. Lumpkin2, Katherine L. Smith2, Mark G. Blackford2, Elizabeth J. Harvey3, and Nestor J. Zaluzec4 1 University of Sheffield, Sheffield, S1 3JD, United Kingdom 2 Australian Nuclear Science and Technology Organisation, Sydney, NSW 2234, Australia 3 University of Cambridge, Cambridge, CB2 3EQ, United Kingdom 4 Argonne National Laboratory, Chicago, IL, 60439 ABSTRACT The resistance of Ln2Ti2O7 (Ln = lanthanide) compounds to radiation damage is an important topic in the understanding and development of new materials by which radioactive nuclear waste can safely be immobilised. A model has been developed, from previously published density functional theory and molecular orbital theory simulations of the band structure for Ln2Ti2O7 materials. This model provides a chemical interpretation of radiation stability. INTRODUCTION Ceramic waste forms provide attractive properties, in comparison with borosilicate glasses, for the direct disposal of actinides, due to their enhanced leach resistance and long term durability. A2B2O7 compounds (which include materials with pyrochlore and monoclinic slab perovskite structures), are constituent phases in the SYNROC system of waste storage, and have been proposed by the DOE as the basis for storing U and Pu high-level waste. Over geological time waste forms incorporating radioactive species will experience alpha decay. This can lead to swelling and cracking that might affect durability. The response of pyrochlores to radiation damage is complex and controlled by several factors. For example, La2Zr2O7 and La2Hf2O7 behave differently under identical conditions [1], La2Zr2O7 being more resistant to radiation damage than La2Hf2O7. However, the mechanism for the increased stability is not fully understood, and a proposed theory based on molecular orbitals is proposed in this paper for the Ln2Ti2O7 series of compositions. LN2TI2O7 – STRUCTURE TYPES The Ln2Ti2O7 compounds can exist in two different structural forms at room temperature: pyrochlore and a monoclinic layered perovskite, primarily dependent on the radius ratio of the A and B cations, e.g., La and Ti. As outlined below the predominant stability range for pyrochlore is Ra/Rb of 1.46-1.78, in these samples pyrochlore is found for Sm-Lu, while a monoclinic relative of perovskite is found for La, Pr and Nd. In the systems La2Ti2O7, Pr2Ti2O7 and Nd2Ti2O7, structurally similar to Ca2Nb2O7, the arrangement of the cations is different from pyrochlore and similar to perovskite[2,3]. The Lncations are located in four different irregular polyhedra with coordination number ranging from 7 to 9, while the Ti-cations remain octahedrally co-ordinated by oxygen. The pyrochlore structure type[4,5], A2B2X6Y is a superstructure of the fluorite (MX2) structure, and is based upon a 2*2*2 supercell containing M4O8, with ordered vacancies in the
oxide anionic lattice. An alternative description for pyrochlore is that of two interpenetrating 3D networ
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