Neutron and Resonant X-ray Diffraction Studies of Zirconolite-2M
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Neutron and Resonant X-ray Diffraction Studies of Zirconolite-2M Karl R. Whittle 1,2, Katherine L. Smith1, Neil C. Hyatt2, and Gregory R. Lumpkin1 1 2
Institute of Materials Engineering, ANSTO, Private Mail Bag 1, Menai 2234, NSW, Australia Department of Engineering Materials, University of Sheffield, Sheffield, S1 3JD, UK
ABSTRACT Zirconolite (nominally CaZrTi2O7) is a constituent phase of potential waste forms for the safe immobilisation of actinide wastes. Structural studies of such materials provide important information about cation ordering, lattice parameters, and strain effects, and provide input into the modelling of alpha decay damage and the development of future wasteform designs. A suite of zirconolites based on the replacement of Ti with Nb and Fe has been studied using high resolution neutron diffraction and resonant X-ray diffraction to determine the degree of disorder across the available cation sites. Resonant X-ray diffraction is a unique method which allows the location of certain cations to be determined accurately by taking advantage of the change in scattering power close to an absorption edge (e.g., Nb-K and Zr-K). Using standard X-ray diffraction alone this is not possible and there is little scattering difference between Nb and Zr. Raman spectroscopy and measured lattice parameters have shown that the exchange of Ti with Nb and Fe has a non-linear effect on the unit cell dimensions and Raman peak positions while retaining the 2M polytype. Mössbauer spectroscopy has shown that the Fe preferentially fills the Ti split (C2) site. The results from this study provide a more complete picture of the cation order-disorder problem and are generally consistent with the behaviour of lattice parameters across the series. INTRODUCTION Zirconolites 1-5, based on CaZrTi2O7 are an important class of accessory minerals which are able to accommodate a wide range cations within the structure. The chemical composition of natural zirconolite can vary extensively, with the main substitutions involving lanthanides (Ln), actinides (Act), Nb, and Fe. Zirconolite is one of the systems proposed to safely immobilise highly radioactive actinide waste. In natural samples, lanthanides and actinides often partially replace the Ca2+cations, with charge balance maintained by partial replacement of Ti4+ by lower charged cations, such as Al3+, Mg2+ and Fe3+. The zirconolite structure can be considered as a pyrochlore derivative formed by the contraction of the structure perpendicular to one of the pyrochlore (111) planes, resulting in a layered structure consisting of alternating HTB and Ca/Zr layers parallel to (001) in the new monoclinic space group C2/c, example images are shown in Figure 1. There is also more ordering of the cations in zirconolite compared to pyrochlore, giving a general formula of ABC2X7, where A = Ca, Ln, Act; B = Zr, Hf, Ln, Act; and C = Ti, Zr, Nb, Fe, Mg, Al, W, and other minor elements. The C site consists of three different sites, two octahedral sites (C1, and C3) which make up the HTB motif and
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