Ion Beam Irradiation of Lanthanum Compounds in the Series La 2 O 3 -TiO 2
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Ion Beam Irradiation of Lanthanum Compounds in the Series La2O3-TiO2 Karl R. Whittle, Mark G Blackford, Robert D Aughterson, Katherine L Smith, Gregory R Lumpkin1, and Nestor J Zaluzec2 1 - Institute of Materials Engineering, Australian Nuclear Science and Technology Organisation, PMB1, Menai, NSW 2234, Australia 2 – Electron Microscopy Center, Argonne National Laboratory, Argonne, IL ABSTRACT Thin crystals of La2O3, La2/3TiO3, La2TiO5, and La2Ti2O7 have been irradiated in situ using 1 MeV Kr2+ ions in the Intermediate Voltage Electron Microscope-Tandem User Facility (IVEM-Tandem), at the Argonne National Laboratory (ANL). We observed that La2O3 remained crystalline to a fluence greater than 3.1 × 1016 ions cm-2 at a temperature of 50 K. The four binary oxide compounds in the two systems were observed through the crystalline-amorphous transition as a function of ion fluence and temperature. Results from the ion irradiations give critical temperatures for amorphisation (Tc) of 840 K for La2Ti2O7, 865 K for La2/3TiO3, and 1027 K for La2TiO5. The Tc values observed in this study, together with previous data for TiO2, are discussed with reference to the phase diagrams for La2O3-TiO2 systems and the different local environments within the crystal structures. Results suggest an observable inverse correlation between Tc and melting temperature (Tm) in the two systems. INTRODUCTION The behaviour of oxide compounds when used for the safe storage of nuclear waste undergoing radioactive decay has been studied as a means to improve current, and develop new waste form materials for future nuclear systems[1]. Examining how damage affects a structure is not always possible using radioactive isotopes, in many cases it is quicker to use ion-beam irradiation to simulate the short-term aspects of damage formation and recovery processes[2-4]. While such a technique may not give ‘real world’ results it can allow ‘model’ systems to be studied, whose results can then be used to understand currently relevant materials. One area of discussion in the literature is the effect of structure versus composition[4], as part of this process we have studied a number of compounds in the system La2O3-TiO2, primarily the binary oxide compounds, La2/3TiO3, La2Ti2O7 and La2TiO5. In these systems one must consider both the extent of phase stability field and detail of the different crystal structures as a function of composition. Of the oxide compounds being studied, ion irradiation data has been previously reported for TiO2 in the rutile (P42/mnm), brookite (Pbca), and anatase (I41/amd) modifications[4]. La2O3 is one of the five known A-type lanthanide oxide compounds (P3m1) with an atomic configuration that can be described as a rhombohedrally distorted CsCl type structure, e.g., La2O3, in which the La ions occupy a 7-coordinated "monocapped" polyhedron[5]. La2/3TiO3, a cation deficient perovskite, has previously been studied by X-ray and neutron diffraction and confirmed to adopt Cmmm symmetry, with ordered occupancy of one of t
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