Topological connectivity analysis of accumulated radiation damage from multiple molecular dynamics recoil cascades
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Topological connectivity analysis of accumulated radiation damage from multiple molecular dynamics recoil cascades Henry R. Foxhall1, John H. Harding1 and Karl P. Travis1 1 Department of Materials Science and Engineering, University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield, South Yorkshire, S1 3JD, UK ABSTRACT The results of sequential large-scale molecular dynamics (MD) simulations of radiation damage cascades in Gd2Ti2O7 and Gd2Zr2O7 are presented. Twelve alpha recoil cascades, each due to a recoil atom with 40 keV of kinetic energy, are performed in both materials and a stark contrast in behaviour observed. Topological connectivity analysis is used to analyse the structural evolution of the two systems. Our results provide important insight into accumulation of disorder in pyrochlore-structured ceramics. INTRODUCTION At present, it seems likely that much of the stored plutonium in the UK is destined for reuse as MOx fuel, but some will be unsuitable for this purpose and require immobilisation. The current waste management option for these arisings is a ceramic phase material based on the SYNROC C [1] concept. SYNROC C is composed of titanate ceramic phases, most notably the monoclinic phase zirconolite 2M, CaZrTi2O7 and the related pyrochlore phase calcio-betafite, (Ca,U)Ti2O7. Zirconolite undergoes a structural transition to pyrochlore upon the addition of actinides[2], so the pyrochlore structure is of significant interest to radiation damage research. The phases have been shown to exist as stable metamict materials over geological timescales [36]. Amorphisation occurs because many atoms are displaced in successive collision cascades started by the action of an alpha-recoil atom[7]. Molecular dynamics (MD) is a suitable technique for studying this phenomenon. Simulations of radiation damage are often analysed by identification of vacancy, interstitial and anti-site defects as a measure of damage. However identifying point defects by reference to a static structure is not always sufficient as many crystal lattices show significant distortion during a damage cascade [8]. Topological connectivity analysis takes into account only connectivity, and so is suitable for crystalline, disordered or distorted materials [9]. Comparing Gd2Ti2O7 and Gd2Zr2O7, the former amorphises readily while the latter does not, forming a defective fluorite phase upon irradiation [10, 11]. Chartier et al. [12, 13] attribute this to differences in electronic structure using MD and Density Functional Theory (DFT), but there has been no observed phase transition on a large scale. Unfortunately, there are no natural analogues of Zr-pyrochlores and so it is hard to make the case for using them as waste-forms. Ti-pyrochlores are therefore still being proposed for immobilisation of Pu [14]. This work shows how topological analysis can give a more robust method of analysing for point defects.
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THEORY Pyrochlores (nominally A2B2O7) have the space group Fd3m with a 16d A-site and 16c Bsite. There are 7 tetrahedrall
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