Synthesis and Characterisation of Ln 2 TiO 5 Compounds
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Synthesis and Characterisation of Ln2TiO5 Compounds Robert D. Aughterson1, Gregory R. Lumpkin1, Katherine L. Smith1, Gordon J. Thorogood1 and Karl R. Whittle1,2 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 Bulk samples of six Ln2TiO5 compounds with Ln = La, Pr, Nd, Eu, Gd and Tb were prepared and characterised. Most of the samples have a phase purity of ~95% (based on BEI and EDS) with the predominant secondary phase primarily being Ln2Ti2O7. Using XRD, TEM selected area diffraction and high resolution imaging techniques, we have confirmed the results of previous studies which showed that at room temperature Pr2TiO5, Nd2TiO5, Eu2TiO5 and Tb2TiO5 have orthorhombic structures with Pnma symmetry. The structure of Tb2TiO5 was further monitored as a function of temperature. The relevance of Ln2TiO5 compounds to advanced nuclear fuel cycles is discussed. INTRODUCTION The Ln2TiO5 group of phases are of interest as potential constituents in waste forms due to their potentially high level of actinide incorporation and Gd2TiO5 is of specific interest as a burnable neutron absorbers in nuclear fuels. Fuel assemblies loaded at the beginning of a reactor cycle should have a certain excess amount of reactivity to compensate for reactivity loss and fission product build-up during use. This excess reactivity needs to be controlled and this can be achieved by inclusion of neutron absorbers [1] Previous studies indicated that the structure of these phases varies depending on both the radius of the Ln cation and also temperature. Results indicate that for elements from La to Sm the structure is orthorhombic (Pnma), between Er to Lu the structure is cubic (F-43m), and from Eu to Ho the structure is temperature dependent ranging from orthorhombic to hexagonal (P3/mmc) to cubic [2-5]. In the orthorhombic Ln2TiO5 compounds, the Ti4+ cation has a CN of 5, slightly offset from the centre of a pyramidal site (e.g., half of a normal octahedron). EXPERIMENTAL PROCEDURES Initial chemical mixing was via the nitrate-alkoxide route. Samples were shear mixed then dried on a hot plate. Dried, agglomerated samples were ground to a fine powder then calcined within an alumina crucible at 750˚C for 1 hour in air. The resultant material was removed from the furnace, and milled using yttrium stabilised zirconia milling balls, ~5mm, for 16 hours. The fine powders were then cold uniaxially pressed at 3tonne for 30 seconds and further condensed using a cold isostatic press at 400 MPa for 2 minutes. All samples were sintered at 1400˚C for 48 hours in air.
Figure 1. Crystal structures of the 3 phase types for the group Ln2TiO5 (Ln=lanthanide). The apices of the polyhedra indicate the positions of oxygen ions, while cations lying close to or at the centres of polyhedra. Ln is represented by the grey and Ti by the blue polyhedra. a) SG Pnma, Ti (CN 5), Tb (CN 7). The Ti is slightly displaced from the base of a square b
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