Simulating Radiation-Induced Defect Formation in Pyrochlores
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Simulating Radiation-Induced Defect Formation in Pyrochlores David S.D. Gunn1, John A. Purton1 and Ilian T. Todorov1 1 Scientific Computing Department, Science & Technology Facilities Council, Daresbury Laboratory, Sci-Tech Daresbury, Keckwick Lane, Daresbury, WA4 4AD, U.K. ABSTRACT The accuracy and robustness of new Buckingham potentials for the pyrochlores Gd2Ti2O7 and Gd2Zr2O7 is demonstrated by calculating and comparing values for a selection of point defects with those calculated using a selection of other published potentials and our own ab inito values. Frenkel pair defect formation energies are substantially lowered in the presence of a small amount of local cation disorder. The activation energy for oxygen vacancy migration between adjacent O48f sites is calculated for Ti and Zr pyrochlores with the energy found to be lower for the non-defective Ti than for the Zr pyrochlore by ~0.1 eV. The effect of local cation disorder on the VO48f ĺ VO48f migration energy is minimal for Gd2Ti2O7, while the migration energy is lowered typically by ~43 % for Gd2Zr2O7. As the healing mechanisms of these pyrochlores are likely to rely upon the availability of oxygen vacancies, the healing of a defective Zr pyrochlore is predicted to be faster than for the equivalent Ti pyrochlore. INTRODUCTION The disposal and safe storage of nuclear waste is a significant challenge for the global community. Several of the radionuclides generated through the nuclear fuel cycle, such as 239Pu 235 and U, have long half lives (24,100 years and 7x108 years respectively) and careful choice of suitable immobilisation matrices is crucial to prevent any environmental contamination. Such an immobilisation material must be able to withstand prolonged heavy ion particle bombardment while maintaining structural integrity. Pyrochlore-type compounds have been proposed as suitable host matrices for this purpose, and great attention has been paid to members of the series Gd2(ZrxTi2-x)O7 (0 x 2)1,2. The radiation tolerance of this series increases with increasing zirconium content, and the healing process in the zirconate is expected to be faster than for the titanate as it does not undergo an amorphous transition upon radiation damage and is a fast ion conductor. Devanathan et al. have suggested that one of the main factors in the titanate amorphization process is the accumulation of cation Frenkel pairs3, and we propose a new set of Buckingham potentials, specifically tailored for looking at radiation damage and defect formation in this Gd2(ZrxTi2-x)O7 series. THEORY The pyrochlore structure has Fd3m symmetry and is closely related to that of fluorite. The differences are that the pyrochlore structure has two distinct cation sites and one-eighth of the anion positions are vacant. One-eighth of a unit cell is shown in Figure 1, where the cation and anion sub-lattices are separated for clarity.
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Figure 1. Schematic of one-eighth of the unit cell of a pyrochlore structure. The cation (left) and anion (right) sublattices are separated for clarity. Gd
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